Patent Publication Number: US-8541716-B2

Title: Heater control with high-limit thermal safety shutdown

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to power and control systems for heaters, for example, to apparatus and methods for powering and controlling multiple heaters used for heating pipes and other components in vacuum, process, delivery, transport, and other systems, developed by HPS Division of MKS Instruments, Inc., and Watlow Electric Manufacturing Company, parties to a joint research agreement (35 U.S.C. 103I(2)C)). 
     2. State of the Prior Art 
     Many vacuum, process, delivery, transport, and other systems used in industry for conducting or moving various gaseous, liquid, or solid materials from one point to another include pipes of various lengths, sizes, and shapes that have to be heated to maintain the pipes and/or materials in the pipes within certain temperature ranges. Pipe heaters for heating pipes for these and other purposes are well known to persons skilled in the art and have ranged from simple resistive wires and tape wrapped around the pipes to more sophisticated, insulated pipe heaters, such as those described in U.S. Pat. No. 5,714,738 (Hauschultz et al.), which is incorporated herein by reference, as well as many such heater products that are available commercially. 
     Along with the development of pipe heaters for various pipe heating applications, there was also a need for better pipe heater control systems for regulating heat output from the heaters along lengths of pipe and for monitoring and controlling such heater operations. There are many kinds and configurations of such heater control systems, such as the ones described in U.S. Pat. No. 6,894,254 (Hauschultz), which is also incorporated herein by reference. As good as such heater monitoring and control systems are, however, there are still problems that they have not solved. 
     For example, in higher temperature installations, the heat produced by the pipe heaters can be conducted to heat controller components that are mounted directly on the pipe heaters, thereby potentially raising the temperatures of such controller components to levels that can damage or destroy them or that can corrupt or degrade data in logic circuits or memories in the controller systems. Some power control systems are hard wired to heater components of the systems making it difficult to quickly replace them. Also, most industrial pipe heaters are equipped with thermal high limit fuses or thermal activated switches that cut the power to pipe heaters if the temperature reaches a maximum temperature threshold, regardless of the cause, for the safety of personnel, to prevent damage to capital equipment, and for safety agency certification. This function has been provided with a variety of thermal limit devices, none of which are entirely satisfactory for this application. 
     For example, standard, commercially available thermal switches are inaccurate and unreliable due to their wide set point tolerances and contact mechanisms, which can erode or, even worse, self-weld to a closed position that renders them totally inoperative and can allow a thermal runaway of the heater until either the heater element burns out or starts a fire. These problems are exacerbated when the thermal switches are placed in or on the heaters where they need to be for accurate response to the actual temperature of the heater and pipes, because the high heat at the heater is a major cause for such degradation of the thermal switches. Yet, the thermal switches cannot be placed off or away from the heaters, because they would not be able to respond to actual temperatures of the heaters or pipes. 
     Thermal fuses are more dependable and available commercially, but once they expire, i.e., “blow” or “burn out”, they cannot be reset. Since thermal fuses are typically embedded in the pipe heater structure near the heating element to be sure they are exposed to the heat near its source, they are not accessible without destructive mutilation of the heater components and materials. Therefore, a blown or burned out thermal fuse renders the heater completely useless so it has to be replaced. Also, thermal fuses age over time, and the higher the temperatures to which they are exposed, the faster they age. Such aging often causes thermal fuses to burn out at lower temperatures and eventually to burn out within the normal operating range of the pipe heaters, thus rendering the otherwise good pipe heaters unusable. Also, commercially available thermal fuses are bulky and difficult to install in pipe heaters. 
     There are sometimes circumstances that cause the temperatures of pipes, thus of the pipe heaters, to exceed such upper temperature limits that have nothing to do with a runaway or uncontrollable heater. For example, it is not uncommon to purge or clean process chambers upstream from the pipe systems by sending high temperature gases or reactive chemicals through them, which can cause the pipe temperature, thus also the pipe heater temperature, to temporarily exceed the upper temperature limit and thereby cause the thermal fuse to expire and open the power circuit to disable the heater. When the thermal fuse expires and cannot be reset or replaced, good heaters are ruined by such routine maintenance and other occurrences unrelated to the pipe heaters themselves. 
     Also, there is a need for more options and versatility in both connection and control configurations to accommodate a wider variety of piping configurations, applications, and user requirements. Each pipe installation is different and many operators need custom pipe heater and control systems to accommodate their particular requirements, but designing and manufacturing custom pipe heater systems is expensive, time consuming, and often not feasible for most applications. For example, some operators want a control mechanism for each heater in a heated pipe system, whereas other operators prefer to avoid the cost of individual controls on each heater and instead use a strategy wherein a single controller is used to operate an entire zone comprising a number of individual heaters. Such “zoning” or “single point” control heater systems often require complex wiring, which can create confusion and increases the probability of wiring errors, or it can require custom heaters to be designed and built to accommodate slaving and prevent wiring error, which adds costs and complexity to the system. 
     Another example is that some operators require remote communications with heater controllers and remote heater system control capabilities so that they can view operating status information and modify operating parameters from a remote location, whereas others want to be able to view such operating status information and to modify operating parameters locally at each heater within a system. Still others require only basic, pre-programmed control at each heater. Of course, there are also operators who want any combination or all of these functions for a group of heaters with only single point control. 
     These and other requirements in industrial and commercial use of pipe heaters creates a need for a more flexible system of pipe heater controls and wiring components that can be configured easily, neatly, and effectively to meet a wider variety of operator requirements. 
    
    
     
       BRIEF DESCRIPTION OF THE EXAMPLE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate several example embodiments and/or components that are presented to support the description, but not to limit the scope of the claims in any way. In the drawings: 
         FIG. 1  is an isometric view of an example individual heater control arrangement of the multiple heater control system; 
         FIG. 2  is an isometric view of some of the principal components utilized in an individual heater control arrangement such as that illustrated in  FIG. 1 ; 
         FIG. 3  is an isometric view of the principal components in  FIG. 2 , but from a different perspective to illustrate the connective components; 
         FIG. 4  is an isometric view of the individual heater control arrangement in  FIG. 1  in its assembled condition; 
         FIG. 5  is an isometric view of an example arrangement of the multiple heater control system in which a single point heater control is used for controlling a gang or zone comprising a master heater and one or more slave heaters; 
         FIG. 6  is an isometric view of some of the principal components utilized in a single point control arrangement for a master and slave heater combination such as that illustrated in  FIG. 5 ; 
         FIG. 7  is an isometric view of the principal components in  FIG. 6 , but from a different perspective to illustrate the connective components; 
         FIG. 8  is an isometric view of the single point control apparatus for the master and slave heater arrangement in  FIG. 5  in its assembled condition; 
         FIG. 9  is a cross-section view of a pipe heater mounted on a pipe for use with either the individual heater control arrangement or the single point control a master and slave heater arrangement of the multiple heater control system; 
         FIG. 10  is an isometric view of a T-type source power cable section; 
         FIG. 11  is a schematic circuit diagram of the T-type source power cable section of  FIG. 10 ; 
         FIG. 12  is an isometric view of a linear-type terminal source power cable section; 
         FIG. 13  is a schematic circuit diagram of the linear-type terminal source power cable section of  FIG. 12 ; 
         FIG. 14  is an isometric view of an example slave adapter cable; 
         FIG. 15  is a schematic circuit diagram of the slave adapter cable of  FIG. 14 ; 
         FIG. 16  is an isometric view of a T-type slave controlled power cable section; 
         FIG. 17  is a schematic circuit diagram of the T-type slave controlled power cable section of  FIG. 16 ; 
         FIG. 18  is an isometric view of a linear-type terminal slave controlled power cable section; 
         FIG. 19  is a schematic circuit diagram of the linear-type terminal slave controlled power cable section of  FIG. 18 ; 
         FIG. 20  is an isometric view of an example basic heater controller with an enhanced control expansion module installed on the base module to provide additional functionality to the heater controller; 
         FIG. 21  is an isometric view of the basic heater controller with the enhanced control expansion module in a position poised to be installed on the base module of the heater controller; 
         FIG. 22  is an isometric view of the enhanced control expansion module from a different perspective to illustrate the example expansion module contact pad and light transmissive boss components, which are enlarged for better definition of these features; 
         FIG. 23  is an isometric view of the basic heater controller with a substitute dust cover poised in position to be installed on the heater controller base module; 
         FIG. 24  is an isometric view of the heater controller base module from a different perspective to illustrate a module mounting apparatus; 
         FIG. 25  is an isometric view of the heater controller base module similar to  FIG. 24 , but with the mounting apparatus in a position poised for connection to the heater controller; 
         FIG. 26  is an isometric view of the mounting apparatus in  FIGS. 24 and 25 , but from a different perspective to illustrate the operative attachment components; 
         FIG. 27  is a schematic circuit diagram of an example multiple individual heater control configuration connected to an AC power source and to an alert/alarm signal circuit located, for example, at a remote monitoring station; 
         FIG. 28  is a schematic circuit diagram of the heater controller base unit and the enhanced control expansion module connected to the T-type source power cable and to multiple pipe heaters via a slave adapter cable, T-type slaved heater cable, and a terminal slave controlled power cable in an example single point control arrangement; 
         FIG. 29  is a schematic circuit diagram similar to  FIG. 28 , but with the controller connected to a terminal source power cable section; 
         FIG. 30  is a schematic circuit diagram of the heater controller base unit with the enhanced control expansion module, the T-type source power cable, and the pipe heater components connected directly to the heater controller base unit as could be done for a single pipe heater or for multiple local control configurations such as those illustrated in  FIGS. 1-4 ; 
         FIG. 31  is a schematic circuit diagram similar to  FIG. 30 , but with the high voltage power and the low voltage signal circuit connected to the controller directly with a terminated controlled power cable to illustrate an individual heater control arrangement where the heater is either the only heater or the last heater being controlled in a series of multiple, individually controlled heaters; 
         FIG. 32  is a logic flow diagram illustrating an example logic for the heater controller; 
         FIG. 33  is a schematic circuit diagram of an individual heater control arrangement similar to  FIG. 30 , but illustrating an example high-limit control circuit with a PTC thermistor temperature sensor; 
         FIG. 34  is a schematic circuit diagram similar to  FIG. 33 , but with another example high-limit control circuit with a PTC thermistor temperature sensor; 
         FIG. 35  is an isometric view of a slave adapter junction box; 
         FIG. 36  is an isometric view of the slave adapter junction box of  FIG. 35 , but from a different perspective; 
         FIG. 37  is a schematic circuit diagram of the slave adapter junction box of  FIGS. 35 and 36 ; 
         FIG. 38  is a schematic circuit diagram similar to  FIG. 28 , but with the slave adapter junction box of  FIGS. 35-37  replacing the slave adapter cable illustrated in  FIG. 28 ; 
         FIG. 39  is an isometric view of an example source power junction box; 
         FIG. 40  is an isometric view of the example source power junction box in  FIG. 39 , but from a different perspective; 
         FIG. 41  is a schematic circuit diagram of the source power junction box in  FIGS. 39 and 40 ; 
         FIG. 42  is an isometric view of a plurality of the heater controllers as they are daisy chain connected with a plurality of the source power junction boxes of  FIGS. 39 and 40 ; 
         FIG. 43  is an isometric view of another example variation of a source power junction box with multiple trunk outlet connectors; 
         FIG. 44  is an isometric view of the source power junction box in  FIG. 43 , but from a different perspective; 
         FIG. 45  is a schematic circuit diagram of the source power junction box of  FIGS. 43 and 44 ; 
         FIG. 46  is an isometric view of the controller base module and the expansion module with the branch outlet connector of a T-type source power cable poised for insertion into the inlet connector of the controller to illustrate a connector retainer feature comprising a resilient spring biasing tab; 
         FIG. 47  is a cross sectional view of the latch and resilient spring biasing tab for the branch outlet connector and controller inlet connector with the branch outlet connector plugged into the controller inlet connector; 
         FIG. 48  is a cross-sectional view similar to  FIG. 47 , but showing the latch lever pivoted against the bias force of the resilient spring biasing tab for release of the latch; 
         FIG. 49  is a cross-sectional view similar to  FIG. 47 , but showing a leaf spring for providing the securing bias force; 
         FIG. 50  is a cross-sectional view similar to  FIG. 47 , but showing a coiled compression spring for providing the securing bias force; and 
         FIG. 51  is a cross-sectional view similar to  FIG. 47 , but showing a resilient compressible material for providing the security bias force. 
     
    
    
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
     The multiple heater control system  10  illustrated generally in  FIGS. 1 and 5  is based on flexible and expandable modularity facilitated by the example components so that various components and combinations of components of the system can be assembled and connected in a variety of ways to serve a variety of heater monitoring and control configuration needs. The system  10  is best described in relation to two basic configurations—an individual local heater control configuration  12  illustrated, for example, in  FIG. 1 , and a single point control arrangement for multiple heaters in a zone or gang configuration  14  illustrated for example in  FIG. 5 . Other combinations and variations of these basic heater control system configurations  12 ,  14  can be created by using selected ones or all of the principal components of the system  10 , as will become apparent to persons skilled in the art as the description of these example embodiments and components continues. 
     The multiple heater control system  10  is designed primarily for pipe heaters  16 , as illustrated in  FIGS. 1 and 5 , although it can be used for other kinds of heaters as well. Therefore, for convenience, this description will proceed in the context of multiple pipe heaters  16  with the understanding that it can apply to other kinds of heaters as well. 
     Referring primarily to  FIG. 1  for the individual local heater control configuration  12 , a plurality of the pipe heaters  16  is shown. They are typically arranged and aligned for mounting on a pipe (not shown in  FIGS. 1-4 , but illustrated in  FIG. 9 ), as will be described in more detail below. In this embodiment, there is a separate controller  20  for each heater  16 . Therefore, as illustrated in  FIGS. 1-4 , each controller  20  is connected directly to each heater  16  in a manner that delivers and controls high voltage AC line (source) power to the heater element  32  ( FIG. 9 ) in the heater  16  as well as derives temperature information from the temperature sensors  50 ,  52  ( FIG. 9 ) in the respective heater  16  to which the controller  20  is connected. Therefore, each controller  20  responds to the temperature sensors  50 ,  52  in the individual heater  16  to which it is connected and turns the high voltage power on and off to that heater  16  according to settings in the controller  20 , as will be described in more detail below. Therefore, the high voltage AC power delivered to the heaters  16  by the controllers  20  is sometimes referred to herein as “controlled AC power,” whereas the high voltage AC power that is received by the controller from an AC power source, which is sometimes referred to herein as “source AC power” or “AC source power” or just “source power”. The term “high voltage” in this context means anything above thirty (30) volts. For example, typical heaters are often powered by ordinary 110-120 volts, 220-240 volts, 440-480 volts, or any other voltage that provides enough power to meet the heat production requirements of a particular installation. AC means alternating current, which can be 50 hertz, 60 hertz, or any other alternating current frequency that is used to power heating elements in heaters. 
     The plurality of controllers  20  in the individual local heater control configuration  12  are daisy chain connected to the high voltage AC power source  13  ( FIG. 27 ), which can be associated with remote monitor and/or control equipment  15  ( FIG. 27 ) by the T-type source power/signal cables  26 , which contain both high voltage power lines for carrying source AC power to the controllers  20  and low voltage signal lines, which comprise a low voltage temperature range alert signal that can be used for any purpose and will be described in more detail below. The term “low voltage” in this description generally means any voltage that does not exceed 30 volts. Also, as indicated in  FIGS. 1 and 4 , any number of additional heater  16  and controller  20  assemblies can be daisy chain connected together by additional T-type source power/signal cables  26  and with a linear-type power/signal cable  108  ( FIG. 27 ) (not shown in  FIGS. 1-4  but described below) connected to the last controller  20  in the daisy chain. 
     Also, any combination of individually controlled heaters  16  and slave heaters  16 ′ can be accommodated. For example, as indicated in  FIGS. 5-8 , additional individually controlled heaters  16  and/or additional single point control zones of slave heaters  16 ′ can be connected to the T-type source power cable  26 . 
     For more detailed descriptions of example embodiments and implementations, it is helpful to refer to the heater elements and to temperature sensors in example heaters, not for limitation, but to aid in understanding. In general, there are many varieties, materials, and structures of heaters that can be controlled by these systems. Therefore, this invention is not limited to any particular heater or heater structure. However, to facilitate the description, an example pipe heater  30  mounted on a pipe P is shown in cross-section in  FIG. 9 . This pipe heater  16  has many similarities to those described in U.S. Pat. Nos. 5,714,738 and 6,894,254, both of which are incorporated herein by reference, but there are several different or additional features that will also be described below. 
     In brief, the example pipe heater  16  in  FIG. 9  comprises a high density silicon rubber heater mat  30  with a heating element  32  comprising resistive wires or other resistive, heat producing material embedded in it. The heating element  32  creates heat when an electric current flows through it, usually at standard, high voltage levels, such as 110-120 volts, 220-240 volts, or any other voltage level that provides enough power to create the heat needed for a particular application. The heater mat  30  is surrounded by a thermally insulating heater jacket  34  comprised of low density, closed cell silicon rubber foam or any other suitable insulating material. A fastening halter  36  with straps  38  ( FIGS. 1-8 ) can be provided to secure the heater  16  in place on the pipe P or on other components that are to be heated. 
     The heater  16  has a cavity  40  where high voltage power lines  42 ,  44  are connected to leads  46 ,  48  from the heating element  32 . Two temperature sensors  50 ,  52 , such as thermocouples, thermistors, or any other suitable temperature sensing devices, are embedded in the foam insulating jacket  34  adjacent the heater mat  30  so that they can detect temperatures at or near the heater mat  30 . Signals from one of the temperature sensors, e.g., temperature sensor  52 , is used by the controller  20  for normal operational or process heater control functionality, and signals from the other temperature sensor, e.g., temperature sensor  50 , is used by the controller  20  for upper temperature limit control, as will be described in more detail below. One temperature sensor could be used for both of those functions, but it is better to provide the redundancy of two temperature sensors, especially for the high temperature limit function, which has to shut down the heater if the process temperature sensor and/or the process control circuit in the controller fails and causes a runaway heater situation. Some safety certifying agencies require such redundancy for safety certification. 
     The low voltage wires  54 ,  56  for the first (“high-limit”) temperature sensor  50  and  58 ,  60  for the second (“process”) temperature sensor  52  are routed through the cavity  40  and through a flexible cord  62  to a cable connector  64 , for example, a Molex™ connector. A boot  66  anchors the flexible cord  62  to the pipe heater  16  and covers the cavity  40 . The heater cord  62  can be any desired length. In some embodiments, the cord  62  is long enough to place the controller  20  ( FIGS. 1-8 ) and connector  64  far enough away from the heater  16  to avoid heat damage to the controller  20 , especially in high temperature applications. Of course, as shown in  FIGS. 1-8 , the controllers  20  are connected to the pipe heater  16  through the connector  64 , either directly as illustrated in  FIGS. 1-4  for the individual heater control configuration  14  or via a slave adapter  22  and slave heater cables  24  ( FIG. 5) and 184  ( FIG. 18 ), as will be described in more detail below. 
     Before proceeding further with structural details of individual component parts of the multiple heater control system, reference is made now to  FIG. 27 , which, in conjunction with  FIGS. 1 and 9 , provides an overview of some of the electrical components and functions of the system and is helpful for an understanding of other components and features that will be described below. Therefore, as shown in  FIG. 27  with secondary reference to  FIGS. 1 and 9 , a plurality of controllers  20  can be connected individually to respective heaters  16  primarily, but not exclusively, for providing controlled AC power to the heaters  16  in order to maintain the heaters  16  operating within certain desired temperature ranges. 
     A daisy chain connected series of cable sections  25 ,  26 ,  108  that connect together in daisy chain fashion to form a source power trunk line that delivers AC source power to the controllers  20 . The controllers  20  then switch the AC power on and off to deliver the controlled AC power to the heaters  16 , as necessary for the heaters  16  to produce the heat needed for maintaining the desired temperatures. The controllers  20  turn the AC power on and off with a process power switch arrangement  302 , which can be a solid state switch, such as a triac  303 , in parallel with a mechanical relay  305  to minimize arcing and heat production, or any other controllable switch to produce the controlled AC power. It is also possible to use a variable power controller, such as a variac transformer (not shown), for adjusting the controlled AC power up and down, but they are much larger, bulkier, and more expensive than switch devices. Control of the process power switch arrangement  302  using temperature signal feedback from the second (process) temperature sensor  52  in the heater  16  is described in more detail below. 
     A high temperature limit switch (also called a high-limit switch)  300  is also provided for shutting off the AC power to the heater  16  in the event the temperature of the heater  16  rises to an unsafe level as sensed by the first (high-limit) temperature sensor  50 . Such an unsafe temperature level could be due to a malfunction of the process power switch  302 , the process temperature sensor  52 , or the process control circuit  296  ( FIG. 28 ), or it could be due to some external cause, such as a high temperature purge or cleaning cycle in the pipe, or any other cause. Control of the high-limit switch  300  using temperature feedback or input from the temperature sensor  50 , including a latching function to keep the AC power turned off once it has been turned off pending an operator intervention, is described in more detail below. 
     An alert/alarm function is also provided, which signals an alert/alarm at the remote monitor station  15  when any one of the controllers  20  in the daisy chain connected series detects that the heater  16  which it controls is at a temperature above or below a desired or needed operating temperature range. For example, if it is necessary to keep the pipe P ( FIG. 9 ) within a certain temperature range for a chemical process, transport, or other activity to proceed, this alert/alarm function  17  can notify an operator at the remote monitor station  15  if any one of the controllers  20  detects a heater  16  temperature outside of that temperature range, and/or it can produce a signal to an equipment interlock  19  to prevent operation of, or shut down of, equipment until the heaters  16  are all producing temperatures in the desired range, as will be understood by persons skilled in the art. The depiction of the monitor station  15  in  FIG. 27  as a defined block is only schematic. The various components and functions, e.g., DC power supply  21 , continuity detector  31 , signal circuit  23 , and alert/alarm  17 , can be in one location or in divers locations, so the use of the term “remote monitor station” in this description is for convenience only and does not limit the components or functions described or depicted to being together at one location or in any unitary configuration or assemblage. 
     To implement this alert/alarm function (also sometimes called the temperature range signal), a low voltage DC power supply  21  at the remote monitoring station  15  provides a low voltage DC potential on a signal circuit  23  comprising a pair of conductors  27 ,  29  that runs via the daisy chain cable sections  25 ,  26 ,  108  to all of the controllers  26 . Low voltage is generally considered to not exceed 30 volts, which is how the term is used herein. Therefore high voltage is anything above 30 volts. One of the conductors, e.g., conductor  29 , extends through each controller  20 , where it is connected in series to opposite terminals of a relay switch  310 . Therefore, any of the series connected relay switches  310  in any of the controllers  20  can open the circuit  23 , i.e., prevent current from flowing in the signal circuit  23 . Conversely, all of the relay switches  310  in all of the controllers  20  have to be closed in order for the signal circuit  23  to be closed. The term “relay switch” as used herein can mean any switch, mechanical or solid state, in which a control signal input can be applied to open and/or close the switch, i.e., to block and/or allow current flow through the switch. 
     A continuity detector  31  associated with the remote monitoring station  15  detects whether the signal circuit  23  is opened or closed. Upon detection that the signal circuit is open, which can be caused by any of the relay switches  310  being opened or by any disconnect or break in the daisy chain cables  25 ,  26 ,  108 , the continuity detector  31  generates a signal to the alert/alarm  17  and/or to an equipment interlock  19 , or to any other device or function desired by the operator. In other words, the signal from the continuity detector  31  can be used to initiate an alert or alarm, or it can be used to stop equipment in any use, as will be apparent to persons skilled in the art upon reading this description. A variety of continuity detectors that can perform this function, e.g., current detector circuits, voltage detector circuits, and the like, are readily available and well known to persons skilled in the art or can easily be constructed by persons skilled in the art, so no further description is required for an understanding of this feature. For convenience, but not for limitation, the signal circuit  23  is sometimes called the “alert/alarm signal circuit” or “temperature range signal circuit”, even though the signal can also be used for equipment interlock and other purposes. 
     The relay switch  310  in each controller  20  is controlled to open and close by a process control circuit  296  ( FIG. 28 ) in the controller  20 , which uses temperature information from the process temperature sensor  52  to determine if the sensed temperature at the heater  16  connected to that controller  20  is within the desired operating range. If not, it outputs a signal to open the relay switch  310 , which opens the signal circuit  23 . The open signal circuit  23  is detected by the continuity detector  31 , which generates the alert/alarm signal. The relay switch  310  can be a mechanical relay or a solid state relay, as is well known to persons skilled in the art. 
     The daisy chain connection components for connecting the controllers  20  electrically to the AC power source  13  and to the temperature alert/alarm circuit  23  at the remote monitoring station includes at least one T-type source power/signal cable section  26  (“T-type source power/signal cable” or “T-type source power cable” or just “T-type source cable” for short) and at least one linear-type terminating linear source power/signal cable section  108  (“linear-type power/signal terminating cable” or “linear-type terminating source power cable” or just “terminating source cable” for short) as shown in  FIGS. 1-4  and  27 . The T-type source cables  26  are used to connect the first and intermediate controllers  20  in the daisy chain connected series to the AC power source  13  and to the alert/alarm signal circuit  23 . The terminating source cable  108  is used to connect the last controller  20  in the daisy chain connected series to the AC power source  13  and to the alert/alarm signal circuit  23  via the T-type source cable(s)  26 , as shown in  FIGS. 1-4  and  27 . The first T-type source cable  26  can be connected directly to the monitor station  156  if it is close enough, or, as shown in  FIG. 27 , an optional linear-type source power/signal extension cable (“source power/signal extension cable” or just “source extension cable” for short)  25  of any necessary length, can be used to connect the first T-type source cable  26  to the monitor station  15 , as indicated schematically in  FIG. 27 . 
     To implement the functions of providing AC source power to the series of daisy chain connected controllers  20  and routing the alert/alarm signal circuit  23  through the relay switches  310  in each of the controllers  20 , as described above, the T-type source cables and the linear-type source cable  108  (and optional source extension cable  25 , if needed) are constructed and configured not only to perform those electrical functions, but also to provide a neat, tidy appearance. The structure and configuration also makes it almost foolproof to connect the AC power source and alert/alarm signal circuit  23  with as many controllers  20  as desired. As shown in  FIG. 27 , each terminating source cable  108  is fairly straight forward in that one pair of high voltage wires  114 ,  116  (“AC power wires” or just “power wires” for short) and one pair of low voltage wires  118 ,  120  (“signal wires” for short) extend all the way straight through the terminating source cable  108  from the inlet connector  110  to the outlet connector  112 . 
     Any type of connector that can connect four wires from one cable to four wires of another cable can be used. Molex™ connectors work well because they are available in configurations that accommodate four, six, or more high voltage and low voltage wire pairs in a manner that mates with corresponding connectors on other components in only one orientation so that they cannot be improperly connected. Also, both the male and female pins are sheathed so it is difficult to accidentally short them. In this description, for convenience and not for limitation, the term “inlet” is used to designate the connector or cable end that receives AC source power and the term “outlet” is used to designate the connector or cable end that delivers AC source power, regardless of whether those connectors or cable ends also receive and/or deliver low voltage signals. 
     For the linear-type terminating source cable  108 , the inlet connector  110  has at least two power pins for the AC source power wires  114 ,  116  and at least two signal circuit pins for the signal circuit wires  118 ,  120  and is configured to mate with a trunk outlet connector  86  on the T-type source cable  26 . The outlet connector  35  at the remote monitoring station  15 , which delivers source power to the daisy chain components  25 ,  26 ,  108  and connects the signal circuit  23  to those components, is also configured the same as the trunk outlet connector  86  on the T-type source cable  26 . Therefore, the inlet connector  110  of the terminating source cable  108  could be plugged directly into the monitoring station outlet connector  35  in situations where there is only one controller  20  in a heater system. 
     As will be discussed in more detail below, the terminating source cable  108  has to be used to connect the last controller  20  in a daisy chain connected series or the only controller  20 , if there is only one, to the remote monitoring station  15  so that the signal circuit  23  can be closed. A daisy chain terminated with a T-type source cable  26  would leave the signal circuit  23  open, regardless of whether all of the relay switches  310  in all of the controllers  20  are closed, which would render signal circuit  23  inoperative for its intended purpose as described above. 
     The outlet connector  112  of the terminating source cable  108  also needs at least two power pins for the source power wires  114 ,  116  and at least two pins for the signal circuit wires  118 ,  120 , and it is configured to mate with the inlet connector  140  of the controller  20 . The inlet connector  140  of the controller  20  has a different configuration than the inlet connectors  82 ,  110  of the T-type source cables  26  and terminating source connectors  108 , respectively, so the outlet connector  112  of the terminating source cable  108  also has to be different than the trunk outlet connectors  86  of the T-type source cables and different than the outlet connector  35  at the remote monitoring station  15 . This different configuration for the inlet connectors  140  of the controllers  20  is provided for the purpose of orderly use of one AC source power cable section per controller, which is easy for users. Of course, the inlet connector  140  of the controller  20  could have the same configuration as the inlet connectors  82 ,  110 , if desired. 
     The T-type source cables  26  are used for connecting the first and any intermediate controllers  20  to the source power circuit  33  and the signal circuit  23  at the remote monitoring station  15 , as mentioned above. Each T-type source cable  26  has a trunk section  83  extending between the inlet connector  82  and the trunk outlet connector  86  and a branch section  85  extending from the trunk section  83  to the branch outlet connector  78 . As best seen in  FIG. 27 , with secondary reference to  FIG. 1 , the trunk power wires, comprised of power wires  87 ,  88  of the inlet trunk segment  70  and the power wires  90 ,  92  of the outlet trunk segment  72  extend uninterrupted between the trunk inlet connector  82  and the trunk outlet connector  86 . The branch power wires  89 ,  91  are connected electrically in parallel to the trunk power wires  87 ,  88  and to the branch connector  78  so that, when the controller  20  is connected to the branch section  85 , the power circuit comprising the power conductors  290 ,  292  in the controller  20  are in parallel electrically with the trunk power wires  87 ,  88  and with the power circuits comprising the power conductors  290 ,  292  in the other daisy chain connected controllers  20 . In the T-type source cable  26  shown in  FIGS. 10 ,  11 , the branch wires are very short jumpers within the connector  78  itself, and, alternatively, they could even be eliminated by joining wires  87 ,  88  and  90 ,  92  together at or adjacent the pins  2 ,  1 , all of which are equivalents as will be understood by persons skilled in the art. 
     The signal wires  98 ,  102  in the inlet trunk segment  70  and outlet trunk segment  72  are connected together to extend electrically uninterrupted through the trunk  83  of T-type source cable  26  from the inlet connector  82  to the outlet connector  86 , electrically bypassing the branch segment  85  and the branch outlet connector  78 . The other signal wires  100 ,  104 , of the signal wire pairs in the T-type source cable  26 , however, detour from the trunk section  83  to extend through the branch section  85  to respective separate pins in the branch outlet connector  78 . Therefore, when the branch outlet connector  78  is connected to the controller  20 , the signal circuit  23  extends in series through the relay switch  310  in the controller  20 . With multiple controllers  20  daisy chain connected in this manner, all of the relay switches  310  of all the controllers  20  are connected in series in and to the extended signal circuit  23 , so all of the relay switches  310  in all of the controllers  20  have to be closed in order to have a closed signal circuit  23 , as explained above. The branch outlet connector  72  is configured to mate with the inlet connector  140  of the controller  20  and the trunk outlet  86  is configured to mate with the inlet connector  82 , so that any number of the T-type source cables  26  can be daisy chain connected together to deliver source power to any number of controllers  20 , while maintaining continuity in the signal circuit  23 , as explained above. 
     As also mentioned above, the extension source cable  25  shown in  FIG. 27  can be provided in any length needed to connect the daisy chain components  26 ,  108  to the source power  33  and the signal circuit  23  at remote monitoring station  15 . The power wire pair  93 ,  95  and the signal wire pair  97 ,  99  extend electrically uninterrupted from the inlet connector  101 , which is configured to mate with the outlet connector  35  at the remote monitoring station  15 , to the outlet connector  103 , which is configured to mate with the inlet connector  82  of the T-type source cable  26  and with the inlet connector  110  of the terminating source cable  108 . 
     Referring now primarily to  FIG. 10  in conjunction with  FIGS. 1-9 , the T-type source cable  26  can, but does not have to, comprise two coiled trunk cable segments  70 ,  72  fastened together with a band  74  to form a neat, T-shaped, coiled, source power cable section  26 . Both of the trunk cable segments  70 ,  72  have respective ends  74 ,  76  that are terminated in the common branch cable connector  78 . The other end  80  of the inlet trunk cable segment  70  is terminated in the inlet cable connector  82 , and the other end  84  of the outlet trunk cable segment  72  is terminated in the outlet trunk connector  86 . Any suitable cable connectors can be used, for example, Molex™ connectors, as discussed above. 
       FIG. 11  is a schematic circuit diagram of the T-type source cable  26 . Each trunk segment  70 ,  72  contains at least two power wires, e.g., the power wires  87 ,  88  in trunk segment  70  and the power wires  90 ,  92  in the trunk segment  72 , for carrying source power to the controllers  20 . The power wires  87 ,  88  in the inlet trunk cable segment  70  are terminated in pins  1 ,  4  in trunk inlet connector  82  and in pins  1 ,  2  in the common branch outlet connector  78 . The power wires  90 ,  92  in the outlet trunk cable segment  72  are terminated in pins  1 ,  4  in trunk outlet connector  86  and in pins  5 ,  6  in the common branch outlet connector  78 . Source power from a source, for example, the AC power supply  13  ( FIG. 27 ), is usually connected to the inlet trunk segment  70  via the trunk inlet connector  82 , and both trunk segments  70 ,  72  are connected to a controller  20  via the common branch outlet connector  78  (see  FIGS. 1-8 ), so source power is supplied to the controllers  20  via pins  1 ,  2  in the common connector  78 . However, by-pass connections  94 ,  96  are provided to connect the power wires  87 ,  88  to the power wires  90 ,  92  in the outlet trunk cable segment  72  in order to supply source power to the pins  1 ,  4  in the trunk outlet connector  86  for other controllers  20  and pipe heaters  16  that may be daisy chain connected to the trunk outlet connector  86  as described above. 
     One of the low voltage signal wires, e.g., wire  98 , in the inlet trunk segment  70  is connected directly to a corresponding signal wire  102  in the outlet trunk segment  72  so that pin  3  in connector  82  of the inlet trunk segment  70  is at a common potential with pin  3  in the trunk outlet connector  86  of the outlet trunk segment  72 . However, those signal wires  98 ,  102  by-pass the branch outlet connector  78 , so they do not get connected to the controllers  20 . The other signal wire  100  in inlet trunk segment  70 , however, does connect the pin  6  in the trunk inlet connector  82  to a pin  4  in the branch connector  78 . Likewise, the other signal wire  104  in the outlet trunk segment  72  connects pin  6  in the trunk outlet connector  86  to pin  8  in the common branch outlet connector  78 . Therefore, the controllers  20  can either close or open the signal circuit comprising the two signal wires to either maintain or interrupt a closed circuit comprising the signal wires, for example, to cause the circuit continuity detector  31  ( FIG. 27 ) to detect that the signal circuit  23  is opened and to trigger the alert/alarm  17  at the remote monitoring station  15  ( FIG. 27 ) in the event the controller  20  detects a heater problem or to trigger some other function, as mentioned above. The unused pins  2 ,  5  in the trunk inlet connector  82 , the unused pins  3 ,  7  in the common branch outlet connector  78 , and the unused pins  2 ,  5  in the trunk outlet connector  86  are optional and can serve the function of maintaining a spatial distance between high and low voltage connections to avoid electrical noise or interference in the low voltage signals by the high voltage AC power. 
     The linear-type terminated source power cable  106  shown in  FIG. 12  is used to connect AC source power and the signal circuit  23  to the last controller  20  in a daisy chained plurality of controllers  20  or optionally to a sole controller  20  in a heater system that has only one controller  20 , as mentioned above. It comprises one cable  108 , preferably, but not necessarily, coiled to maintain a neat structure. It is terminated at one end with the inlet connector  110  that mates with the trunk outlet connector  86  of the T-type source cable  26  and at the other end with the outlet connector  112  that, like the branch outlet connector  78  of the control power cable  26 , mates with the inlet connector  140  ( FIGS. 20-21  and  29 ) on the controllers  20 . As shown in the schematic circuit diagram in  FIG. 13 , this terminated source cable  106 , like the T-type source cable  26 , contains at least two power wires  114 ,  116  and at least two signal wires  118 ,  120 . The power wires connect the pins  1 ,  4  of the inlet connector  110  to pins  2 ,  1  of the outlet connector  112 , and the signal wires connect pins  3 ,  6  of the inlet connector  110  to the pins  8 ,  4  of the outlet connector  112 . The terminated source cables  106  are used to provide source power from an AC power source  13  ( FIG. 27 ) and the signal circuit  23  from the remote monitor station  15  ( FIG. 27 ) to the last controller  20  in a series of daisy chain connected controllers  20 , instead of using a T-type source cable  26 , because the T-type source cables  26  at the end of a daisy chain would leave the two signal wires unconnected, thus always an open circuit voltage situation that would prevent operation of the temperature range alert/alarm signal function, which will be described in more detail below. 
     In the single point control configuration  14  for zoned master  16  and slave heaters  16 ′ shown in  FIGS. 5-8 , a single controller  20  is connected via a slave adapter, for example, the slave adapter cable  22  or a slave adapter junction box  324  described below in relation to  FIGS. 33-36 , to one or more T-type controlled slave cables  24  to control a plurality of heaters  16 ,  16 ′ in a ganged group or zone of heaters with the single controller  20 . The first heater  16  in the zone, which is connected to the single controller  20  by the slave adapter cable  22 , is considered to be the master heater for the zone because the controller  20  responds to temperature sensors  50 ,  52  ( FIG. 9 ) in that first heater  16  to control both that master heater  16  and the rest of the slave heaters  16 ′ in the zone. The rest of the heaters  16 ′ in the zone, other than the master heater  16 , are called the slave heaters, because they simply heat or not heat as the AC power is switched on and off, i.e., controlled, by the controller  20  without providing any temperature feedback to the controller  20 . For convenience, the T-type controlled power slave cables  24  are so designated because they carry controlled AC power from the controller  20  to the slave heaters  16 ′, as opposed to the T-type source power cables  26  described above, which carry AC source power to the controllers  20 . 
     The master heater  16  and the slave heaters  16 ′ in typical installations are usually identical for convenience and standardization, which is how they are shown and described herein as an example, although identical master and slave heaters is not a requirement for every embodiment of the invention. The slave heaters are designated  16 ′ instead of  16  just for convenience in this description for indicating their slaved functions as distinct from the master functions of the master heater  16 . As will be explained in more detail below, the temperature sensors  50 ,  52  ( FIG. 9 ) in the slave heaters  16 ′, if they exist, are not used. Therefore, the slave heaters  16 ′ could be made without temperature sensors, if desired, and still be used with this invention. However, as mentioned above, the slave heaters  16 ′ can be the same as the master heater  16 , in which case the slaved heater cables  22 ,  24 ,  184  used for connecting the slave heaters  16 ′ to the controller  20  are configured in a way that isolates the temperature sensors  50 ,  52  ( FIG. 9 ) in the slave heaters  16 ′ and that does not route the signals from those temperature sensors to the controller  20 , thereby rendering the temperature sensors  50 ,  52  of the slave heaters  16 ′ effectively inoperative in the system, as will be described in more detail below. 
     As indicated in  FIGS. 5 and 8 , there can be any number of slave heaters  16 ′ in the grouping or zone controlled by the one controller  20 . Subsequent slave heaters  16 ′ in the zone can simply be connected in a daisy chain manner to the last T-type controlled slave cable  24  shown in  FIGS. 1 and 4  by additional T-type controlled slave cables  24  and a terminating controlled cable  184  ( FIG. 18 ), which is not shown in  FIGS. 5-8  but will be described in more detail below. 
     To summarize, the T-type controlled slave cables  24  only conduct electricity to the heater coils ( FIGS. 9 and 32 ) in the slave heaters  16 ′. The electricity for powering the slave heaters  16 ′ is controlled by the controller  20 , so when the controller  20  switches on electric power to the slave heaters  16 ′, they produce heat. When the controller  20  switches off the electric power to the slave heaters  16 ′, they stop producing heat. No temperature information is derived by the controller  20  from any of the slave heaters  16 ′. 
     The master heater  16  also produces heat when the controller  20  switches on the electric power, and it stops producing heat when the controller  20  switches off the electric power. However, the controller  20  also receives temperature information from temperature sensors  50 ,  52  ( FIGS. 9 and 32 ) in the master heater  16  and turns the power on and off in response to sensed temperature levels in the master heater  16 . Therefore, when the sensed temperature in the master heater  16  is low, based on settings in the controller  20 , the controller  20  will turn on the power, and all of the master and slave heaters  16 ,  16 ′ in the zone will be turned on in unison. Likewise, when the temperature sensed in the master heater  16  is high, based on settings in the controller  20 , the controller  20  will turn off the power, and all of the master and slave heaters  16 ,  16 ′ in the zone will be turned off in unison. 
     Electric power is provided to the controller  20  in  FIG. 5  via a T-type source power cable  26 . The T-type source cable  26  looks similar to the T-type controlled power slave cables  24  from the outside, but it also has at least a pair of low voltage signal wires in addition to the pair of high voltage power wires, as described above, whereas the T-type controlled power slave cables  24  have the pair of high voltage power wires for powering the heater elements in the slave heaters  16 ′ but not the signal circuit wires for the alert/alarm circuit described above. 
     The slave adapter cable  22  shown in  FIG. 14  is used, as shown in  FIGS. 5-8 , to connect a controller  20  to the master heater  16  and to one or more slave heaters  16 ′, as explained above. The slave adapter cable  22  is comprised of two cable segments, a master controlled power cable segment  126  and a slave controlled power cable segment  128 , which are so designated for convenience because they carry controlled (e.g., switched on and off) power from the controller  20  as opposed to source power to the controller  20 . One end  127  of the master controlled power cable segment  126  is terminated at an inlet connector  130 , which, like the inlet connector  64  on the heater cord  62  ( FIGS. 1-9 ), has at least six pins to handle at least two high voltage power wires to conduct AC controlled power to the heating element  32 , and two pairs of signal wires for the two temperature sensors  50 ,  52  ( FIG. 9 ) in the master heater  16  ( FIGS. 5-9 ). In some embodiments, the signal wires can be low voltage, while in other embodiments at least one of the pairs of signal wires may also be high voltage, depending on the kind of temperature sensor used for the high-limit control, as will be described in more detail below. Therefore, the inlet connector  130  can be the same configuration as the inlet connector  64 , which provides the option of connecting a heater cord  62  directly to the output connector  142  in the controller  20 , as is shown in  FIGS. 1-4  for the individual local heater control configuration  12 , or of connecting the heater cord  62  to a controller  20  via a slave adapter  22 , as is shown in  FIGS. 5-8  for the single point control configuration  14  for a zone comprising master and slave heaters  16 ,  16 ′. The other end  129  of the master controlled power cable segment  126  is terminated at a common outlet connector  132 , which is configured like the outlet connector  142  on the controller  20  ( FIGS. 3 ,  7 ,  20 ,  21 ) so that it can mate with the inlet connector  64  of the heater cord  62 , which, again, provides the option of connecting the heater  16  directly to a controller  20  for individual heater  16  control or to the slave adapter  22  for a single point control configuration  12 . The slave cable segment  128  of the slave adapter  22  contains two high voltage power wires for powering the slave heaters  16 ′, but it does not have to have wires for the temperature sensors  50 ,  52 , as will be explained in more detail below. One end  136  of the slave cable segment  128  is terminated in the common outlet connector  132  and the other end  138  is terminated in a slave outlet connector  134 . 
     As shown in the schematic circuit diagram in  FIG. 15  for the slave adapter cable  22 , and as mentioned above, the master cable segment  126  has at least two power wires  144 ,  146 , which connect pins  1 ,  5  of the inlet connector  130  to pins  1 ,  5  of the outlet connector  132  for providing high voltage AC power to the heater elements  32  in the master heater  16  ( FIGS. 1-4  and  9 ). The master cable segment  126  also has two pairs of signal wires, e.g., a first pair of wires  148 ,  150  and a second pair of wires  152 ,  154 , for connecting the two temperature sensors  50 ,  52  ( FIG. 9 ), respectively, in the master heater  16  to the single point controller  20  ( FIGS. 5-8 ). The signal wire pair  148 ,  150  connect pins  4 ,  8  of inlet connector  130  to pins  4 ,  8  of the outlet connector  132 , and the other signal wire pair  152 ,  154  connect pins  3 ,  7  in the inlet connector  130  to pins  3 ,  7  in the outlet connector  132 . However, as explained above, the controller  20  in the single point control configuration  14  ( FIGS. 5-8 ) gets temperature information only from the master heater  16 , not from the slave heaters  16 ′. Therefore, the slave cable segment  128  of the slave cable adapter  22  does not need any signal wires. Its only function is to provide controlled high voltage power to the slave heaters  16 ′, so the slave cable segment  128  contains two high voltage power wires  156 ,  158 , as shown in  FIG. 11 . Also, by not having signal wires in the slave cable segment  128 , use of the slave cable adapter  22  automatically isolates the temperature sensors  50 ,  52  of subsequent heaters in a daisy chain, which makes them function as slave heaters  16 ′. Also, since there does not have to be any signal wires in the slave cable segment  128 , the outlet connector  134  can be simpler with fewer pins than the connectors  130 ,  132 . Also, this smaller outlet connector  134  with its different configuration prevents mistaken connection of a source power cable  26  or a terminated source power cable  106  to the slave adapter cable  22 , which could inadvertently connect the temperature sensors  50 ,  52  of more than one heater  16  to the single point controller  20 . Of course, the smaller, differently configured connector  134  also requires a smaller mating connector  172 ,  190  on subsequent slave heater cables  24 ,  184 , which will be discussed in more detail below. Those smaller connectors  172 ,  190  also prevent those slave cable sections  24 ,  184 , which do not have signal wires, from being inadvertently connected into the power/signal trunk line, which does have signal wires, as described above. 
     As shown in  FIG. 15 , the high voltage power wires  156 ,  158  of the slave adapter cable  22  connect pins  1 ,  5  of the connectors  130 ,  132  to the pins  1 ,  3  of the outlet connector  134  so that high voltage source power provided from the controller  20  ( FIGS. 5-8 ) through the inlet connector  130  is also provided to the outlet connector  132  for the master heater  16  and to the outlet connector  134  for the slave heaters  16 ′. Again, the pins  2 ,  6  in the connectors  130 ,  132  are unused and provide space between the high voltage connections and the signal connections. Pins  2 ,  4  in the outlet connector  134  are not used. 
     The T-type controlled power slave cable  24  is best seen in  FIG. 16 , and its schematic circuit diagram is shown in  FIG. 17 . This T-type controlled power cable  24  comprises two trunk segments  160 ,  162 , preferably, but not necessarily, coiled and banded together with a band  164  to create and maintain a neat structure. Since this T-type controlled power cable  24  only provides high voltage controlled power to the slave heaters  16 ′ ( FIGS. 5-8 ) as discussed above, these first and second slaved trunk segments  160 ,  162  contain high voltage power wires  166 ,  168 , but they do not have to contain any signal wires. Further, with no signal wires in the T-type controlled power slave cable  24 , the selection and use of these T-type controlled power slave cables  24  to get controlled AC power to a heater, instead of connecting a controller  20  directly to the heater, automatically isolates the temperature sensors  50 ,  52  of the heater, thus makes the heater function as a slave heater  16 ′ instead of as master heater  16 . Also, since the branch outlet connector  78  of the T-type source power cable  26  described above is configured different from the branch outlet connector  170  of the T-type controlled power slave cable  24  in the example embodiment described above, the T-type source power cable  26 , which does have signal wires, cannot be connected to the heater. 
     One end of each trunk segment  160 ,  162  of the T-type controlled power slave cable  24  is terminated in a common branch outlet connector  170 , and the opposite end of the inlet trunk segment  160  is terminated in a inlet daisy chain connector  172  while the opposite end of the slave outlet trunk segment  162  is terminated in a trunk outlet slave daisy chain connector  174 . The slave inlet daisy chain connector  172  is configured to mate with the slave outlet daisy chain connector  134  of the slave adapter cable  22  ( FIGS. 5-8  and  14 ). The trunk outlet slave daisy chain connector  174  is configured the same as the daisy chain outlet connector  134  of the slave adapter cable  22  so that any T-type controlled power cable  24  can be connected either to the slave adapter cable  22  or to another T-type controlled power cable  24 . 
     The common slaved heater outlet connector  170  is configured to mate with the inlet connector  64  of the heater cord  62  so that it can deliver high voltage power to the slave heaters  16 ′ ( FIGS. 5-8 ). Therefore, even though the T-type controlled power cable section  24  does not have to have any signal wires, the common slave branch outlet connector  170  is the same configuration as the outlet connector  132  of the slave adapter cable  22  and as the outlet connector  142  in the controller  20  so that it can mate with the inlet connector  64  of the heater  16 ′. As shown in  FIG. 17 , the high voltage controlled power wires  176 ,  178  in the slave inlet trunk segment  160  connects the pins  1 ,  3  of inlet connector  172  to the pins  1 ,  5  of the common branch outlet connector  170 , which is the same as the high voltage power connections to pins  1 ,  5  in the outlet connector  132  of the slave adapter cable  22 . The high voltage power wires  176 ,  178  of the inlet trunk segment  160  are also connected to the high voltage power wires  180 ,  182  of the outlet trunk segment  162  in order to provide high voltage power at the pins  1 ,  3  of the outlet slave daisy chain connector  174 . As shown in  FIG. 17 , there are numerous unused pins  2 - 4  and  6 - 8  in the branch outlet connector  170 , but having no signal wires connected to the pins  3 ,  7  and  4 ,  8  isolates the temperature sensors  50 ,  52  in the pipe heater  16 ′ and prevents them from being connected to the controller  20 , which makes the heater function as a slave heater  16 ′. 
     It should be apparent from this description, therefore, that the same heaters can be used as either: (i) individually controlled heaters  16  in an individual local heater control configuration; (ii) a master heater  16  in a single point heater control configuration; or (iii) a slave heater  16 ′ in a single point control configuration. No modification or change is needed in either the controller  20  or the heater  16  to make this selection or to implement these functions. The desired function of the heater—individually controlled, master, or slave—is implemented merely by choosing to either: (i) connect the heater directly to a controller  20  for an individually controlled heater  16 ; (ii) connect the heater to controller  20  via a slave adapter, e.g., a slave adapter cable  22 , for a master heater  16 ; or (iii) connect the heater to a controller  20  via a slaved heater controlled power cable section  24  for a slave heater  16 ′. 
     The selection of a heater to function as a slave heater  16 ′ can also be made for the last slave heater  16 ′ in a zone of heaters in a single point heater control configuration by using a terminated controlled power cable  184 , which is best seen in  FIG. 18  with its schematic circuit diagram in  FIG. 19 . Essentially, the terminated controlled power cable  184  is substantially the same as the inlet trunk segment  160  of the T-type controlled power cable  24  ( FIG. 16 ). It only has to have two high voltage power wires  186 ,  188 , a inlet connector  190  that is the same as the inlet connector  172  in the T-type controlled power cable  24 , and an outlet connector  192  that is the same configuration as the outlet connector  170  of the T-type controlled power cable  24 . The high voltage power wires  186 ,  188  connect the pins  1 ,  3  in the inlet connector  190  to the pins  1 ,  5  in the outlet connector  192 . In use, the outlet connector  192  is connected to the inlet connector  64  on the heater ( FIG. 9 ), which makes it a slave heater  16 ′ because there are no signal wires connected to the pins  3 ,  7  and  4 ,  8  of outltet connector  192 , which isolates the temperature sensors  50 ,  52  in the heater ( FIG. 9 ), as explained above. The inlet connector  190  can be connected to the outlet connector  134  of the slave adapter cable  22  ( FIGS. 5-8  and  14 ), if there is only one slave heater  16 ′, or to a connector  174  of the T-type controlled power cable  24 , if the heater  16 ′ is the last in a series of more than one slave heater  16 ′. 
     The controller  20  is modular so that it can be used in a simpler arrangement with factory-preset parameters or so that it can be expanded, if desired, to accommodate more user interface and settable parameter options. As best seen in  FIGS. 20-23 , the controller  20  has a base module  200 , which includes circuit components that are necessary for the basic functions of the controller  20  with factory-preset parameters, including, but not limited to: (i) Monitoring the temperature sensors  50 ,  52  in the heater  16  ( FIG. 9 ); (ii) Turning the high voltage power to the heater elements  32  on and off according to factory-preset temperature parameters and hysteresis; (iii) Disconnecting the high voltage power in the event of an over-temperature event according to a factory-preset upper temperature limit; (iv) Initiating an alarm signal to a remote monitoring station if the high voltage power is disconnected due to a high-temperature event; and (v) Displaying several status indicators, e.g., low temperature, high temperature, in-range, high voltage power to the heater(s) on or off, and high voltage power disconnected due to a high temperature event. 
     Additional functionality and user interface capabilities, such as re-settable parameters, data communications, system monitoring, alpha-numeric visual display capabilities, and others can be added to the controller  20  by attaching an expansion module  202  to the base module  200 , as shown by  FIGS. 21 and 22 , as well as in  FIGS. 2 ,  3 ,  6 , and  7 . The example expansion module  202  shown in  FIGS. 21 and 22  includes a circuit (not shown in  FIGS. 21 and 22 ) that processes user inputs either from inputs on the expansion module  202  itself or from a remote location via the communication components or other communications implementations as explained below. It also communicates with process control and, in some embodiments, with high temperature limit control circuits  296 ,  298  ( FIG. 29 ) in the base module  200  to view, set, reset, and monitor some or all functions of the base module  200  depending on the level of adjustability built into the base module  200  and the level of capabilities built into a particular expansion module  202 . As shown in  FIGS. 20 and 21 , the expansion module  202  has an alpha-numeric display  204  that is visible through a transparent front face portion  206  of a housing  208 , user input buttons  210 ,  212 ,  214 , and status LED display nubbins  216 ,  218 ,  220 , all of which will be discussed in more detail below. The expansion module  202  can also have data line communication ports  222 ,  244  to transmit and receive data to and from a remote station and/or to and from another controller  20  in a daisy chain connected system. It should also be noted that different expansion modules  200  can also be made with fewer than or more than these features so that users can select and install particular expansion modules with a particular package of capabilities and features, depending on what they want or need for their particular heater control systems. Also, wireless communication components, such as infrared, RF, or other wireless communications implementations and components for such implementations (not shown) can also be included in the expansion module, if desired, as is understood by persons skilled in the art. Therefore, the communications ports and components shown in the drawings are examples—not exclusive or limiting embodiments. 
     The expansion module  202  attaches very easily to the base module  200  as best seen in  FIGS. 21 and 22  by simply aligning a plurality, e.g., three, latch dogs  222 ,  224 ,  226  protruding from the back side  234  of the expansion module  202  with a plurality, e.g., three, corresponding or mating latch holes  228 ,  230 ,  232  in the front panel  288  of the base module  200  and snap it into place. It can be removed just about as easily by simply pulling the expansion module  202  apart from the base module  200 . 
     The circuit board in the base module  20  has a set of electric contacts, for example, the pad of contacts  236 , or any other suitable plug receptacle, and a plurality, e.g., three, LEDs  240 ,  242 ,  244  adjacent an opening  246  in the front panel  238 . A correspondingly aligned and mating contact assembly  248 , or a suitable plug, protrudes from a circuit board in the expansion module  202  through the rear panel  256 , which, when the expansion module  202  is snapped into place on the base module  200 , protrudes through the opening  246  and into contact with mating electrical contacts on the contact pad  236  or into the plug receptacle (not shown) in the base module  200  in order to connect the expansion module  202  electrically to the base module  200  to receive power and to communicate data. Also, there are a plurality, e.g., three, transparent or at least translucent bosses or wave guides  250 ,  252 ,  254  mounted in the circuit board in the expansion module  202  that are aligned with and extend from the display nubbins  216 ,  218 ,  220  on the front face  206  to protrude out the back panel  256  toward the base module  200 . These protruding bosses  250 ,  252 ,  254  align with the LEDs  240 ,  242 ,  244  in the base module  200 , so that, when the expansion module  202  is snapped into place on the base module, the bosses  250 ,  252 ,  254  are positioned adjacent the LEDs  240 ,  242 ,  244  so that they transmit light from the LEDs  240 ,  242 ,  244  to the display nubbins  216 ,  218 ,  220  on the front face  206 . 
     When the base model  200  is operated alone, without the expansion module  202 , a dust cover  258  is provided to snap into place on the base module  202  in place of the expansion module  200 , as best seen in  FIG. 23 , in order to prevent dust and debris from entering the base module  200  through the opening  246 . The dust cover also has latch dogs similar to those on the expansion module  202  that align with and snap into the latch holes  228 ,  230 ,  232  to hold the dust cover  258  in place on the base module  200 . The dust cover  258  has three bosses  260 ,  262 ,  264  similar to the bosses  250 ,  252 ,  254 , but shorter, that extend from the front of the dust cover  258  into the hole  246  to the LEDs  240 ,  242 ,  244  so that they transmit light from the LEDs to the front of the dust cover for status displays. 
     Of course, more or fewer LED status displays can be provided for either the expansion model display or the dust cover display. The three LED status displays  216 ,  218 ,  220  on the expansion module  202  and the three LED status display  260 ,  262 ,  264  on the dust cover  258  in the example embodiment described herein may be, for example, an “Alert/Alarm” when the controller  20  detects a condition that needs attention, such as a heater not working so that the sensed temperature, e.g., from the process temperature sensor  52 , is too hot or too cold, an “In Range” mode to indicate the temperature of the heater is in the preset desired operating range, and an “Output” mode, which shows that the controlled AC power to the heater is turned on, i.e., being output to the heater. 
     As mentioned above, the expansion module  202  can be equipped or programmed to provide more or fewer of the functions, capabilities, and/or features described herein. Also, some expansion modules  202  can be made with more or fewer of these functions, capabilities, and/or features than other expansion modules  202 . Also, one of the expansion modules  202  can be moved from one base unit  200  to another base module  200  to check and/or reset parameters in the first controller and then to check and/or reset parameters in the second and/or any number of additional base modules  200 . Therefore, if desired, a single expansion module  202  can be used on one or more base modules  200 , if desired. 
     To help hold the controller  20  and associated wiring away from hot heaters, which could damage its electronic components, and to help maintain a neat, daisy chained connection layout, the controller  20  is provided with a convenient wall mount bracket  270  and mating locking socket  272  in the back panel  274 , as best seen in  FIGS. 24-26 . The bracket  270  has a plurality of radially extending ears  276 , which are sized to slip through mating radially extending slots  278  between adjacent sector plates  280  in the socket  272 . Then, when the controller  20  is rotated, the ears  276  are captured under the sector plate guides  280  so that the bracket  270  cannot be withdrawn from the socket  272 . Several backing plate guides  282  on the bracket that are recessed axially behind the ears  276  contact the sector plate guides  280  when the bracket  270  is inserted into the socket  272 , so when the controller  20  is rotated about an axis  284  of the socket  272 , the sector plate guides  280  get captured between the ears  276  and the backing plate guides  282  to hold the bracket  270  firmly and securely in the socket  272 . 
     In use, the wall bracket  270  can be fastened to a wall or other structure (not shown) by screws or other fasteners (not shown) through the holes  286  in the cross piece  288 . Alternatively, the bracket  270  can be fastened to an object, e.g., to a heater  16 , with a strap, wire, tape, or other material (not shown) wrapped around the cross piece  288  and around the object. The controller  20  is then positioned adjacent the bracket  270 , axially aligned with the bracket  270  on axis  284 , and axially pushed toward the bracket  270  to pass the ears  276  through slots  278  into the socket  272 . The controller  20  is then rotated about the axis  284  to lock the controller  20  in place on the bracket  270 , as shown in  FIG. 24 . The controller  20  can be easily removed from the bracket  270  by reversing those steps. 
     The functions and control logic in one embodiment can be described by reference primarily to the schematic circuit diagrams in  FIGS. 28-31  in conjunction with the logic flow diagram in  FIG. 32 . The schematic circuit diagram in  FIG. 28  depicts the multiple heater control system  10  of the present invention in a single point heater control configuration  14  as illustrated in  FIGS. 5-8  and described above. In summary, the master heater  16  is connected to the controller  20  via a slave adapter cable  22  to the base module  200  of controller  20 . The controller  20  is connected to a high voltage power source, e.g., an AC power supply, by the T-type source power cable  26  connected to the controller  20 . The high voltage source power is delivered to the controller  20  by the high voltage wires  87 ,  88  in the T-type source power cable  26  and is represented in the controller  20  by high voltage conductors  290 ,  292 . In the controller  20 , the high voltage source power is tapped by a DC power supply  294  which supplies low voltage DC power to the process control chip  296 , to a high limit control chip  298  in the embodiment shown in  FIGS. 28-31 , and to the contact pad  236  or plug receptacle (not shown), where it is available to the expansion module  202 , if the expansion module  202  is installed. The high voltage controlled power is also routed to the outlet connector  142 , where it is available to heaters  16 ,  16 ′ via the slave adapter cable  22 , T-type slave controlled power cable  24 , and slave terminating controlled power cable  184 . In the master heater  16  and slave heaters  16 ′, the high voltage controlled power from conductors  290 ,  292  in the controller is conducted to the heater elements  32  by the high voltage wires  42 ,  44 . 
     In the example controller  20  embodiment shown in the schematic diagrams in  FIGS. 28-31 , the high-limit control circuit  298  is depicted as including a digital logic circuit, such as a microprocessor, which can be programmed to perform the high-limit cutoff functions. In such a digital logic, high-limit control circuit, one high voltage conductor  292  is routed directly to the outlet connector  142 , from where it connects directly to the high voltage wire  44  in each heater  16 ,  16 ′. However, the other high voltage conductor  290  is routed through two switch devices  300 ,  302 . The first switch device  300  is in front of the second switch device  302  and is controlled by a microprocessor or other logic circuit in high-limit control circuit  298  to disrupt and turn off the high voltage power to everything behind the first switch device  300 , including all the heaters  16 ,  16 ′ and the second switch device  302 . Therefore, when the high limit control  298  opens the first switch  300 , such as due to an excess temperature event, nothing downstream from the relay switch can operate until the first switch  300  is reset. In this description, “upstream” and “in front of” refers to the side, direction, or relative position from which the electricity comes, e.g., from the AC power source or supply  13  ( FIG. 27 ). In complementary fashion, “downstream” or “in back of” or “behind” refers to the side, direction, or relative position away from the source, e.g., the direction in which the power goes away from a component, etc. 
     The first high voltage power switch  300  is preferably, but not necessarily, a mechanical relay that is normally open, so power (current through the relay coil) is required to close it. Also, once the power switch (relay)  300  is opened, it is preferred, although not essential, that the power switch  30  cannot be reset (closed) without some operator or user intervention. In other words, when the temperature at the heater recedes, the relay switch  300  does not reset or close automatically. Instead, an operator or user has to actively do something to reset (close) the relay switch  300  in order to restart the controller  20  to deliver controlled power to the heaters. A mechanical relay switch is preferred, although not essential, for the high-limit switch  300 , because a solid state switch, such as a triac, has more resistance, thus would produce more unnecessary heat and would be an unnecessary power drain. 
     A conventional latching relay device could perform the functions described above, but conventional latching relay devices that could be used in these kinds of heater control applications are large, bulky devices that require a second coil and substantial power to operate. Therefore, an embodiment of this invention includes a high-limit control circuit  298  that is configured to cause an ordinary, normally open mechanical relay switch to remain open, even after the heater temperature recedes below the upper temperature limit, until an operator or user intervenes. Several example high-limit control circuits  298 , one digital and two analog, that enable an ordinary, normally open mechanical relay switch to function in this manner in the heater control system  10  are included in this description. 
     An ordinary, normally open mechanical relay switch is a relay switch with at least one set of electrical contacts that are spring biased to an open mode or position and a coil, which, when powered, generates a magnetic field or bias that overcomes the spring bias to close the contacts. When the power to the coil is turned off so that no current or not enough current flows through the coil to create a strong enough electromagnetic field or bias to overcome the spring bias, then the spring bias re-opens the contacts. 
     One example high-limit control circuit  298  for controlling the high-limit mechanical relay switch  300  to function as described above includes a digital logic microprocessor or other logic circuit as indicated diagrammatically in the schematic circuit diagram of the controller  20  in  FIGS. 28-31 . In this example, the microprocessor or other digital logic circuit of the high-limit circuit  298 , upon startup, is programmed to progress through a series of startup logic steps, which include: (i) comparing the temperature sensed by the first (upper-limit) temperature sensor  52  to a preset high temperature limit, and (ii) if the sensed temperature does not equal or exceed the preset high temperature limit, generating a signal to close the normally open relay switch  300 . For example, but not for limitation, the signal can be applied to the gate of a low voltage, solid state switch, e.g., a transistor (not shown) to turn on a flow of low voltage DC electric current through the coil of the mechanical relay switch  300  to cause it to close. If the sensed temperature does equal or exceed the preset high temperature limit, the startup logic does not generate the signal that would cause the relay power switch  300  to close. Therefore, in one example implementation, if the relay power switch  300  is not closed, the DC power that powers the high-limit control circuit has to be turned off and then turned on again to make it go through its reboot or restart logic when the sensed temperature does not exceed the preset high temperature limit in order to close the relay power switch  300  after it has been opened. Such turning off or removal of DC power to the high-limit control circuit  298  can be accomplished in a number of ways. For example, but not for limitation, since the DC power supply  294 , which provides DC power to operate the high-limit control circuit  298  in the example implementation in  FIG. 28  is tapped into the AC power in the AC power leads  290 ,  292 , the removal of DC power from the high-limit control circuit  298  can be accomplished simply by unplugging or disconnecting the controller  20  from the AC source power, which also cuts off power to the DC power supply  294 , thereby removing power from the high-limit control circuit  298 . Then, reconnecting the controller  20  to the AC source power will re-power the high-limit control circuit  298 , thereby causing it to reboot and go through its startup logic again, which will close the relay power switch  300  if the startup logic determines that the sensed temperature does not equal or exceed the preset high temperature limit, as explained above. Of course, other ways of turning the DC power to the high-limit circuit  298  on and off could also be provided, for example, a manually operated switch (not shown) in front of the DC power supply  294  or between the DC power supply  294  and the high-limit circuit  298  could also be provided. A suitable logic circuit for the high-limit control circuit  298  can include, for example, an ATmega168 microprocessor manufactured by Amtel Corporation, San Jose, Calif., although other integrated circuit chips that can be programmed to perform the described functions are readily available commercially and are well known to persons skilled in the art. 
     Again, a purpose of this example digital implementation is to require an operator or user to actively intervene in order to restart a heater that has been turned off by the upper-limit control circuit  298 , and thereby make it more likely that the operator or user will check on the cause of the high-limit shutoff of the heater before turning it back on and leaving it unattended. At the same time, the use of the mechanical relay switch  300  controlled in the manner described above, i.e., to open and shut off AC power to the heater in a reliable manner at or near a predetermined high temperature limit and then being closable again by a simple operator intervention, avoids the disadvantages of a thermal fuse in the heater that either has to be replaced or renders the heater unusable. It also avoids the disadvantages of a conventional latching relay, e.g., large, bulky, and a power drain, and it avoids the disadvantages of a solid state switch, e.g., resistance, heat production, and power drain. Also, in the digital implementation described above, the upper temperature limit or parameter is adjustable, which provides additional options and flexibility for users. 
     As persons skilled in the art know, there is little, if any, substantive difference between a logic step that generates an action if a parameter is “equal to or greater than” a value or just “greater than” the value, other than the particular logic statement that the programmer chooses to use. Likewise, there is little, if any, substantive difference between a logic step that generates an action if a parameter is “equal to or less than” a value or just “less than” the value. In other words, for example, if the logic step of the high-limit circuit is described or claimed as generating a signal to open the relay  300  when the sensed temperature equals or is greater than a preset upper temperature limit parameter, it is considered equivalent to generating a signal to open the relay  300  when the sensed temperature exceeds, i.e., is greater than, the upper temperature limit parameter. Therefore, unless specified otherwise, &gt;= is considered to be equivalent to &gt; and vice versa, and &lt;= is considered to be equivalent to &lt; and vice versa. 
     As long as the temperature in the master heater  16  remains below the high temperature limit set in the high-limit control  298 , the first switch remains closed, and the heaters  16 ,  16 ′ are controlled by the process control  296  in the controller  20  based on temperature signals from the second temperature sensor  52  in the master heater  16 , which can be, for example, a thermocouple or thermistor. As shown in  FIG. 28 , the signals from the second temperature sensor  52  are fed by the low voltage wires  58 ,  60  in the heater  16  and by a low voltage wire pair through the slave adapter cable  22  ( FIG. 15 ) to an amplifier  306  in the controller  20 , where they are conditioned and amplified for use by the process control  296 . 
     Essentially, the process control  296  operates the second power switch assembly  302  to turn on and off the high voltage AC power to the heaters  16 ,  16 ′ in order to maintain the temperature sensed by the second temperature sensor  52  within a predetermined range that is set in the process control  296 , as is shown in more detail in  FIG. 32 . The switch assembly  302  in the example embodiment illustrated in  FIG. 28  comprises two switches, e.g., a mechanical relay switch  303  and a solid state triac switch  305 , in parallel to minimize arcing and heat. The triac  305  turns on just before, e.g., about 20 milliseconds before, the relay switch  303  closes to minimize arcing in the relay switch  303  during the initial closing of the contacts in the mechanical relay switch  303 . The triac  305  then turns off, e.g., about 20 milliseconds after the mechanical relay switch is closed, i.e., to avoid heat production in the triac  305  while the relay switch  303  is closed and conducting the controlled AC power to the heaters  16 ,  16 ′. Then, the triac  305  turns on again just before the relay switch  303  opens to minimize arcing in the relay switch  303  as it opens. These functions are controlled by the process control circuit  296 , as is understood by persons skilled in the art. Mechanical relay switches and triac power switches are readily available commercially in many sizes and configurations from numerous manufacturers, as is well-known by persons skilled in the art. 
     The process control  296  also provides a number of other functions shown in more detail in  FIG. 32 , including, but not limited to, processing information to operate the display of, for example, green, amber, and red LED light displays  240 ,  242 ,  244 , communicating information back and forth between the expansion module  202  and the base module  200 , and receiving signals from the high-limit control for processing for displays and output relating to the status of the first switch  300 . The process control circuit can also comprise an ATmega168 manufactured by Amtel Corporation, although myriad other microprocessors that could also serve these and other functions are well known and readily available to persons skilled in the art. 
     One of the functions provided by the process control  296  is processing temperature input information for producing temperature range signals (sometimes also called “alert/alarm signals”) to be delivered to a remote monitoring location to confirm that the heater or heaters  16  are operating within a desired temperature range. This function can serve a number of uses. For example, if the heater temperature is outside of a certain desired operating range, which may or may not be related to the high temperature limit discussed above, this electronic temperature range signal can be used to trigger a mechanism ( FIG. 27 ) for equipment interlock, i.e., preventing or interrupting an industrial process that depends on the heaters  16  operating properly to maintain the heat within a particular temperature range. Another use for such an electronic temperature range signal may be to generate a notice or alarm function for operators at a remote location to notify them that a heater or group of heaters is outside of a desired operating range, i.e., either too cold or too hot. Of course, the uses for such an electronic temperature range or “out-of-range” signal are not limited to these examples. 
     To implement an electronic temperature range signal (also called “alert/alarm signal”) in this invention, an electronic relay device  310 , which can be operated by the process control  296 , is provided in the controller  20 . A desired temperature range for the heater  16 , either factory-preset or user determined, is programmed into the process control  296 . The range can be set in absolute degrees or upper and lower limits, or it can be in incremental values around some operating temperature setting that can be either fixed or floating, depending on the operator&#39;s requirements. 
     A low voltage, such as thirty (30) volts or less, supplied by a remote monitoring device  15  ( FIG. 27 ), is delivered to the controller  20  via the low voltage wires  98 ,  100  and/or  102 ,  104  provided in the T-type source power cable sections  26  and/or via the low voltage wires  118 ,  120  in a terminated source power cable section  106 , as explained above and shown in  FIGS. 10-13 , depending on whether the controller  20  is or is not either the last controller  20  in a daisy chained series of controller  20  or the only controller  20  in a system. 
     In the controller  20 , one of the low voltage signal conductors is routed through the relay device  310 , as shown by the traces  312 ,  314  in  FIG. 28 , before it is routed back into the T-type source power cable  26  or terminated source power cable  106  (not shown in FIG.  28 —see  FIGS. 12-13 ). A remote monitor device ( FIG. 27 ) at the remote location  15  is connected to the low voltage wires  98 ,  100  and/or  102 ,  104  in the T-type source power cable  26  and/or  118 ,  120  in the terminated source power cable  106  for monitoring the voltage and/or current on these low voltage wires. For example, if all the relay devices  310  in all the controllers  20  connected to the remote monitoring device at  15  via one or more of the T-type source power cables  26  or the terminating source power cable  106  are closed, then a current will flow and/or the voltage will drop. On the other hand, if any one of the relay devices  310  in any of the controllers  20  is open, no current will flow in the low voltage lines in any of the source power cables  26 ,  106  and/or the voltage will be the highest, i.e., the open circuit voltage that is applied to the low voltage wires by the remote monitoring device  15 . Such voltage and/or current conditions are monitored by the continuity detector  31  in the remote monitoring station  15 , which can thereby detect whether all the relay devices  310  of all the controllers are closed, thus indicating that all of the heaters  16  are operating within the desired temperature range (closed signal circuit condition), or it can show that at least one of the heaters  16  is not operating within the desired temperature range (open signal circuit condition). Therefore, it becomes apparent from this description why the last or only controller  20  in a daisy chain connected series has to be connected to the remote monitoring device via terminated control power cable  106 , as shown in  FIGS. 12-13 , and not with a T-type source power cable  26 , as shown in  FIGS. 10-11  and  28 . Specifically, if there is no controller  20  connected to the last T-type source power cable  26 , the low voltage signal circuit will always be open at the unconnected connector  112 , e.g., unconnected wires  102 ,  104  in  FIG. 28 , thus falsely indicating a heater  16  operating outside the desired range. The linear-type terminated source power cable  106  prevents that problem, as shown in  FIG. 29 . 
     In summary, each controller  20  in a series that is daisy chain connected with the T-type source power cables  26  and the last controller  20  in the series that is connected with the terminated source power cable  106 , has programmed in it a desired temperature operating range. As long as the control process  296  of a controller  20  determines that its temperature sensor  52  or both temperature sensors  50 ,  52  do not indicate a temperature outside the desired temperature range, the control process circuit  296  keeps the relay device  310  closed. However, if the controller  20  determines from the sensed temperature information that the heater  16  is not operating within the desired temperature range, it will open the relay device  310 , thereby opening the low voltage signal circuit, which is detectable by the continuity detector  31  at the remote monitoring location  15  ( FIG. 27 ). In response, the signal from the continuity detector  31  can then trigger some alarm, notice, and/or control or interlock signal for whatever purpose is desired, as discussed above. 
     As also discussed above and as can be seen in  FIG. 28 , the temperature sensors  50 ,  52  in the master heater  16  are connected to the controller  20  by the slave adapter cable  22 , and the controller  20  uses signals from those temperature sensors  50 ,  52  in master heater  16  in the process described. However, even though the slave heaters  16 ′ are identical in structure to the master heater  16  in some embodiments, including having the same temperature sensors  50 ,  52 , those temperature sensors  50 ,  52  of the slave heater  16 ′ are not connected to the controller  20 . With no low voltage wires in the slave cable segment  128  of the slave adapter cable  22 , and no low voltage conductors in either the T-type slaved heater cable  24  or the terminated controlled power slave cable  184 , the controller  20  does not get any temperature signals from the sensors  50 ,  52  in the slave heaters  16 ′, which is what makes them function as slave heaters  16 ′. Whatever the controller  20  determines to do, whether it is turning on and off the high voltage power, operating the temperature range relay  310 , or other functions based on heater temperature, it is based on the temperatures sensed by the sensors  50 ,  52  in the master heater  16 . 
     As mentioned above, all of the parameters needed by the high-limit control  298  and the process control  296  to operate as described can be preprogrammed or preset into the process control  296  and the high-limit control  298 , which is built in the base module  200  of the controller  20 . However, if more control, functionality, monitoring, or other capabilities are desired, such additional control functionality, monitoring or other capabilities can be provided in the expansion module  202  that attaches to the base module  200  ( FIGS. 20-22 ) as explained above. The example expansion module  202  shown schematically in  FIG. 28  includes a display/adjust microprocessor  316 , an alpha-numeric display  204 , user interface buttons  210 ,  212 ,  214 , digital communications input/output portals  222 ,  224 , and a communications microprocessor  318 . The display/adjust microprocessor  316  can also be an Atmega168 manufactured by Amtel Corporation, although myriad other microprocessor circuits can also be used. 
     The display/adjust microprocessor  316  is connected to the user interface buttons  210 ,  212 ,  214 , which can be used to retrieve and reset various parameters and information, which the display/adjust microprocessor  316  gets from, and inputs to, the process control  296  and/or the high-limit control  298 , which it also sends to the display  204 . Such information that can be retrieved, displayed, and reset with the microprocessor  316  can include, but is not limited to, desired operating temperature set point, high temperature safety limit, high temperature alert set point, low temperature alert set point, hysteresis, output PID (proportional band, integral, and deviation), cycle time, ambient temperature (read only), modbus device address, modbus band rate, and temperature units (Celsius or Fahrenheit). Other read only information such as base release version, base build number, interface release version, interface prototype version, and interface build number can also be retrieved and displayed. 
     The communications microprocessor  318  enables external data communications with a remote monitoring or control station, service computers, and the like to input and output information, make adjustments, modify programming, and the like, via the input/output ports  222 ,  224 . The communications microprocessor  318  can be, for example, a MAX3157 manufactured by Maxim Integrated Products, Sunnyvale, Calif., which has a transmitter and a receiver, although myriad other microprocessors could also be used for this function, as is known by persons skilled in the art. 
     The schematic circuit diagram in  FIG. 30  illustrates a heater  16  connected directly to a controller  20 , as is done in the multiple local heater control configuration of  FIGS. 1-4 . All of the connections and functionalities described for the controller  20 , base module  200 , process control  296 , high-limit control  298 , first switch  300 , second switch  302 , temperature range relay  310 , heater  16 , temperature sensors  50 ,  52 , heating element  32 , expansion module  202  and other components are the same as explained above for  FIG. 28 , except that heater  16  is connected directly to the controller  20 . Therefore, there is no slave adapter cable in this configuration, thus no slave heaters. 
     The schematic circuit diagram in  FIG. 31  is also for a controller  20  connected directly to a heater  16 , thus no slave adapter cable and no slave heaters. Therefore, the circuit in  FIG. 31  is the same as the circuit in  FIG. 30 , except that it is either the last controller  20  in a series or the only controller  20 , so it has the terminated source power cable  106  instead of the T-type source power cable  26  for supplying the high voltage source power and the low voltage electronic temperature range circuit to that controller  20  shown in  FIG. 31 . 
     An example operating logic for implementing the present invention is shown in  FIG. 32 . The logic as well as the values and parameters in  FIG. 32  and used in this description are examples and not intended to be limiting. The illustrated logic starts in the upper-limit control  298 . From start  320 , a temperature measurement is taken from the first (high-limit) temperature sensor  50  at  322  and compared to the upper temperature limit parameter. If the actual measured temperature from the high-limit temperature sensor  50  is less than the high-limit parameter in step  322 , then the next step  324  tests whether that actual temperature from sensor  50  is within 20° C. of the second (process) temperature sensor  52 . This comparison  324  is done as a test to determine if the temperature sensors  50 ,  52  are measuring reasonably accurate in relation to each other. If yes, then the temperature of the controller itself is measured at  326  to be sure it is not overheated, i.e., is less than 85° C. Overheating could occur, for example, if the controller  20  is too close to the heater  16 , and it could damage the electronic components in the controller  20 . If the controller  20  is found at  326  to not be overheated, then the high-limit control  298  keeps the relay switch  300  closed, as indicated at  328 , so that the high voltage AC power remains available for control by the process control  296  to power the heater(s)  16 ,  16 ′. 
     On the other hand, if any of the tests at  322 ,  324 ,  326  are negative, i.e., the sensed temperature is over the high temperature limit, then the high-limit control  298  opens the relay switch  300  at  330 , which interrupts the AC power to the heater(s)  16 ,  16 ′. It also sends a signal to the process control  296  that indicates the relay switch  300  is opened, and, in response, the process control  296  activates an alarm signal and/or flashes the appropriate (red) LED  240 . 
     Continuing with the process control  296 , a temperature measurement from the second (process) temperature sensor  52  is compared at  332  with the programmed set point (desired operating temperature) minus the set hysteresis parameter (e.g., about 3° C.). If the actual process temperature measured by the process sensor  52  is at or below the set point minus hysteresis, then the actual temperature is compared at  334  to the programmed low temperature alert (LTA) parameter, i.e., to see whether the temperature is below the desired operating range. If the temperature is at or below the LTA (e.g., about 20° C. below the set point temperature), then the process control  296  closes the second (process) switch  302  at  336  to provide AC power to the heater(s)  16 ,  16 ′, and it turns on the output LED  244  (e.g., green) to indicate that the heater(s)  16 ,  16 ′ are turned on. On the other hand, if the temperature at  334  is not at or below the low temperature alert (LTA) parameter, then the process switch (relay)  302  is closed as indicated at  338 , but the LED  242  (e.g., amber) is turned on to indicate that the actual process temperature is in the proper operating range. 
     If the comparison at  332  shows that the actual process temperature measured by the process temperature sensor  52  is not at or below the set point minus the hysteresis, then the temperature is checked at  340  to see if it is at or above the set point plus the hysteresis parameter. If it is, then the temperature is checked at  342  to see if it is at or above the programmed high temperature alert (HTA) parameter (e.g., about 20° C. above the set point temperature). If so, then the control relay switch  302  is opened at  344  to turn off the AC power to the heater(s)  16 ,  16 ′, and the “Alert/Alarm” LED (red)  240  is turned on. 
     On the other hand, if the temperature at  340  is not at or above set point plus hysteresis, then the temperature is within the control hysteresis range, so the control relay switch  302  is kept open at  346  pending changes in the thermal condition, and the “In Range” LED is turned on or left on. 
     If the temperature from the process temperature sensor  52  is not found at  342  to be at or above the programmed HTA parameter, then the control relay switch  302  is open, as indicated at  348 , and the “In Range” LED is on. 
     These and other functions are shown in the example drawings and described above as being performed by several control processors, e.g.,  296 ,  298 ,  316 ,  318 . However, these functions and others can be performed by one or more processors in various combinations and with various allocations of the functions among one or more microprocessors, as is understood by persons skilled in the art. Therefore, there can be more or fewer processors than shown in the drawings to perform these example functions. 
     Another example implementation of the high-limit control circuit  298  is illustrated in  FIG. 33 , wherein a switching positive temperature coefficient (PTC) thermistor semiconductor device is used for the upper-limit temperature sensor  50 . Switching PTC thermistors are semiconductor devices that exhibit a very small negative temperature coefficient of resistance until the device reaches a critical temperature, often referred to as the switch or transition temperature, whereupon the device exhibits a sharp rise in the temperature coefficient of resistance as well as a large increase in resistance, e.g., a resistance change of as much as several orders of magnitude within a temperature span of a few degrees. Such switching PTC thermistors are readily available commercially with transition temperatures in ranges from 60° C. to 160° C. and can be manufactured with transition or switch temperatures at least as low as O° C. and at least as high as 200° C. With the switching function inherent in the switching PTC thermistor device used as the high-limit temperature sensor  50 , the high-limit control circuit  298  can be analog, as shown in  FIG. 33 , and still provide the desired features and functions of operating the normally open mechanical relay high-limit power switch  300  to open and shut off the AC power to the heater(s), whenever the heater temperature equals or exceeds an upper temperature limit and then not close and turn on the AC power again without an operator intervention or manual input when the heater temperature recedes below the upper temperature limit. 
     As shown in  FIG. 33 , the PTC thermistor temperature sensor  50  is positioned in the heater  16  adjacent the AC powered heating element  32  in order to sense temperatures caused by the heat produced by the heating element  32  as described above for the upper-limit temperature sensor  50  in previously described example implementations shown in FIGS.  9  and  27 - 30 . The high-limit mechanical relay switch  300  is also positioned in the AC power circuit in the controller  20  to open and close at least one of the AC power conductors, e.g., the AC power conductor  290 , as also described above and shown in  FIGS. 27-30 , so that it shuts off the AC power to the heater  16  when the normally open contact  307  is closed and turns off the AC power to the heater  16  when the normally open contact  307  is open. The other AC power conductor  292  passes through the controller  20  to the outlet connector  142 , where it connects with the switched AC power conductor  290  to the heater  16 , as described above. 
     The switching PTC thermistor used as the high-limit temperature sensor  50  is connected in series with a rectifier circuit  301  that powers the coil of the relay switch  300  so that current has to flow through the switching PTC thermistor of the temperature sensor  50  in order to power the coil to close the normally open contact  307  of the relay switch  300 , i.e., to turn on the AC power to the heater  16 . Therefore, in normal temperature operation, i.e., when the temperature sensor  50  is under the upper temperature limit, which is set by the switching or transition temperature of the PTC thermistor of the temperature sensor  50 , the PTC thermistor has a low resistance that easily conducts enough AC current that, when rectified, flows through the coil of the relay switch  300  to create the magnetic field required to close the contact  307 . Consequently, in such normal temperature operation, the AC power circuit in the controller  20 , comprising the AC power conductors  290 ,  292 , is closed and can conduct AC power to the heater  16 , subject, of course, to the closed or open status of the process relay switch arrangement  302 , as described above. However, if the temperature of the switching PTC thermistor of the high-limit temperature sensor  50  rises to or exceeds its switching or transition temperature, its resistance increases sharply and effectively turns off the rectified current to the coil of the relay switch  300 , thereby allowing the normally open contacts  307  to open and the normally closed contacts  308  to close. Consequently, the open contacts  307  opens the AC power circuit of AC conductors  290 ,  292 , thereby turning off the AC power to the heater  16 . Rectifier circuits, for example, full-wave bridge rectifier circuits, are well known to persons skilled in the art, thus need no further description for an understanding of this circuit. 
     Then, when the temperature of the PTC thermistor of the upper limit temperature sensor  50  recedes back down to a temperature below the high temperature limit, i.e., below the switching or transition temperature of the PTC thermistor, and the current then again flows through the PTC thermistor, the high-limit control circuit  298  still prevents the coil of the relay  300  from re-closing the contacts  307  to turn the AC power back on to the heater  16  until there is an operator intervention. In the example upper-limit control circuit  298  shown in  FIG. 33 , there is a drain circuit comprising a switch  309 , e.g., a triac as shown in  FIG. 33  or other solid state or mechanical relay switch, and a drain resistor  311  connected parallel to the rectifier circuit  301  and coil of the relay switch  300 . The drain resistor  311  has much less impedance than the coil of the relay switch  300 , for example, an order of magnitude less, so that when the triac or other relay switch  309  is turned on, the current that flows through the PTC thermistor of the temperature sensor  50  is drained away from the rectifier  301  and coil of the relay  300 , which prevents the coil from generating the electromagnetic field that is necessary to close the contacts  307  in the relay switch  300 . 
     The triac  309  is turned on by AC current that flows through the relay switch  300 , which is applied to the gate  313  of the triac  309  via the normally closed contacts  308  of the relay switch  300 . Therefore, when the PTC thermistor of the temperature sensor  50  turns off the rectified current to the coil of the relay switch  300  upon the occurrence of a high temperature event at the heater  16 , the normally open contacts  307  in the relay switch  300  open to turn off the AC power to the heater  16 , as described above, and the normally closed contacts  308  close, as shown in  FIG. 33 , to apply the AC power to the gate  313  of the triac  309  to activate (close) the drain circuit. Consequently, when the temperature at the heater  16  recedes so that the temperature of the PTC thermistor of the temperature sensor  50  falls below its switching or transition temperature and again conducts current, the current is diverted away from the coil of the relay switch  300  and is instead drained through the drain resistor  311 . With the current conducted by the PTC thermistor being drained away from the coil of the relay switch  300 , the coil cannot create the electromagnetic field required to close the normally open contacts  307 , so the AC power to the heater  16  remains turned off, even though the temperature at the PTC thermistor of the temperature sensor  50  has receded, and it is again conducting electric current. 
     To turn the AC power back on to the heater  16 , therefore, a manually operated switch  315  is provided to break or open the gate power circuit and thereby to turn off the triac  309 . As soon as the triac  309  is turned off by the manually operated switch  315 , the drain circuit through the drain resistor  311  is deactivated, so the current from the PTC thermistor of the temperature sensor  50  again is rectified by the rectifier circuit  301  and flows through the coil of the relay switch  300 . Therefore, the coil creates the electromagnetic field required to open the contacts  308  and to close the contacts  307  to thereby reactivate the AC power to the heater  16  and to remove the AC power from the gate circuit. Consequently, when the manually operated switch  315  returns to the closed mode, the triac  309  does not turn back on, because the current from the PTC thermistor keeps the contacts  308  open as long as the temperature at the temperature sensor  50  remains below the switching or transition temperature of the PTC thermistor. For the reasons described above, therefore, the provision of the drain circuit, which is disabled by the manually operated switch  315 , an operator intervention, i.e., to operate the switch  315 , is required to reactivate AC power to the heater  16  after it has been turned off due to a high temperature event in the heater  16  that equals or exceeds the switching or transition temperature of the PTC thermistor of the high-limit temperature sensor  50 . 
     It should be noted that it is primarily heat from an external source, e.g., heat from the heating element  32  or from hot purge or other gases or liquids in the pipe itself, that causes the temperature of the PTC thermistor in this example implementation to rise to its switching or transition temperature to turn off the AC power to the heater  16 . In contrast, temperature rises in PTC thermistors used in conventional thermal fuse or thermal circuit breaker applications are caused primarily by I 2 R heat generated internally in the PTC thermistors. In other words, PTC thermistors have inherent resistance (R) to current flow (I), and excessive current (I) flow in the PTC thermistor will cause substantial heat production in the PTC thermistor itself, and, if the temperature reaches the transition or switching temperature, the PTC thermistor will substantially shut off current flow. 
     Any of a variety of status signals from the high-limit circuit  298  can be provided to the process control circuit  296  for use in generating status and/or alert/alarm signals, or for use in process logic, and the like. For example, but not for limitation, a sensor  317 , such as a current detector, can be used to indicate that the relay switch  300  is activated to provide AC power to the heater  16  or deactivated to shut off AC power to the heater  16 . Also, for example, but not for limitation, a sensor  319 , such as a current detector, can be used to indicate whether the temperature at the high-limit temperature sensor  50  is either (i) below the switching or transition temperature of the PTC thermistor, i.e., current is detected, or (ii) at or above the switching or transition temperature of the PTC thermistor, i.e., current is not detected. These and other status signals can be used by the process control circuit  296 , for example, to generate status and/or alert/alarm signals to the LED display  321  and/or to the display/adjust microprocessor  316 . 
     Another example implementation of the high-limit control circuit  298  utilizing a PTC thermistor for the high-limit temperature sensor  50  is shown schematically in  FIG. 34 . In this example implementation, the triac  309  of the  FIG. 33  example is replaced by a second switch mechanism  325  in the relay switch  300 ′, which is normally closed and is activated by the same coil that activates the first or primary switch mechanism  323  of the relay switch  300 ′. This second switch  325  could also be provided by a separate relay switch (not shown), but dual switch relays, such as the dual switch relay  300 ′ shown schematically in  FIG. 34  are readily available and more compact than two separate relay switches. In this  FIG. 34  example, the relay switch is labeled  300 ′ instead of  300 , not for limitation, but only to distinguish this example relay switch  300 ′ from the previously described relay switch  300 . In other words, while the primary function of both of these example relay switches  300 ,  300 ′ is to turn the AC power to the heaters off if there is a high temperature event, the relay switch  300 ′ has the additional second switch  325  for the drain circuit in this implementation. 
     In the  FIG. 34  example, the contact  307  of first switch  323  in the relay  300 ′, which turns on and off the AC power to the heater  16 , is normally open, as is the contact  307  of the  FIG. 33  example, so current has to flow through the coil of the relay  300 ′ to close the contact  307  so that the AC power can be provided to the heater  16 , subject, of course, to the opening and closing of the process switch assembly  302  as described above. The coil of the relay  300 ′ is powered by rectified current derived by the rectifier  301  from AC current that flows through the PTC thermistor of the high-limit temperature sensor  50  whenever the temperature of the PTC thermistor is below its switching or transition temperature, which defines the upper temperature limit. However, if the temperature at the temperature sensor  50  reaches or exceeds the switching or transition temperature of the PTC thermistor, the current flow through the PTC thermistor, thus also the rectified current through the coil of the relay  300 ′, is stopped. With no current flow through the coil, the normally open first switch  323  opens the contacts  307 , thereby turning off the AC power to the heater  16 , and the normally closed second switch  325  closes the contacts  308 ′, thereby closing or activating the drain circuit comprising the drain resistor  311 . 
     When the temperature at the temperature sensor  50  recedes below the switching or transition temperature of the PTC thermistor so that it again conducts electric current, the closed drain circuit drains the current through the drain resistor  311 , thereby depriving the coil of the relay  300 ′ of the current required to re-close the AC power (first) switch  323 . Again, as mentioned above, the drain resistor  311  has much smaller resistance than the coil, so, when the drain circuit is closed, the current will flow preferentially through the drain circuit instead of through the coil, which is connected electrically in parallel to the drain circuit. Therefore, even though the temperature at the temperature sensor  50  has receded below the upper temperature limit, the drain circuit prevents the relay  300 ′ from providing AC power to the heater  16 . 
     To restore AC power to the heater  16 , an operator can open the drain circuit with the manually operated switch  315 . By even momentarily opening the manually operated switch  315 , the drain circuit is deactivated, so rectified current is restored to the coil of the relay  300 ′. With current flowing again through the coil, the contacts of the first switch  323  close to turn on the AC power to the heater  16 , and the contacts of the second switch  325  open to disable the drain circuit. Therefore, when the manually operated switch  315  closes again, the drain circuit stays deactivated. 
     The manually operated switch  315  can be any of a variety of switch types, but the normally closed, push button switch illustrated schematically in  FIGS. 33 and 34  is a convenient example switch type for this application. Depression of the button  327  causes the switch  315  to momentarily open. Then, when manual force is removed from the button  327 , the spring  329  re-closes the switch. 
     While the embodiments of the invention described above have the source power and signal circuit distributions made with cables sections, e.g., the T-type source power cable  26  with its branch  85  branching from the trunk  83  ( FIG. 10 ), the slave cable adapter  22  with its slave cable segment  128  branching away from its master cable segment  126  ( FIG. 14 ), and the T-type controlled power cable  24  with its branch  163  branching away from its trunk  161  ( FIG. 16 ), these distributions can also be made with junction boxes. For example, but not for limitation, the function of the slave cable adapter  22  can also be provided by the slave adapter junction box  322  shown in  FIGS. 35-37  and illustrated in use position in the schematic circuit diagram of  FIG. 38  for a single point control system with two slave heater branches controlled by a single controller  20 . 
     The slave adapter junction box  322  has a housing  324  with an inlet connector  330  and a master outlet connector  332  in opposite top and bottom walls  325 ,  326 , of the housing  324  and two slave outlet connectors  333 ,  334  in opposite lateral side end walls  327 ,  328  of the housing  324 . The inlet connector  330 , like the inlet connector  130  of the slave adapter cable  22 , is configured to mate with the outlet connector  142  of the controller  20  ( FIG. 20 ). The master outlet connector  332 , like the master outlet connector  132  of the slave adapter cable  22 , is configured to mate with the heater input connector  64  on the heater cord  62  of a heater  16 . Therefore, the pair of power conductor leads  336 ,  338  carry controlled power from the controller  20  to the master heater  16 , while the two pairs of signal conductors  340 ,  342  and  344 ,  346  carry signals from the high-limit temperature sensor  50  and the process temperature sensor  52 , respectively, to the controller  20 . 
     The two slave outlet connectors  333 ,  334  of the slave adapter junction box  322  are configured the same as the slave outlet  134  of the slave adapter cable  22  so that they can mate with the inlet connectors  172  of the T-type slave controlled power cables  24  and the inlet connectors  190  of the linear-type terminating controlled power slave cables  194 . One pair of power conductors  348 ,  350  connect the outlet connector  333  electrically in parallel to the controlled power conductors  336 ,  338 , and another pair of power conductors  352 ,  354  connect the outlet connector  334  electrically in parallel to the controlled power conductors  336 ,  338 . 
     When the slave adapter junction box  322  is connected to the controller  20 , as shown in  FIG. 38 , with one heater  16  connected to the master outlet connector  332  and other heaters  16 ′ connected into the slave outlet connectors  333 ,  334 , the slave adapter junction box enables the heater that is connected to the master outlet connector  332  to function as the master heater  16  and disables the temperature sensors  50 ,  52  of the heaters that are connected to the slave outlet connectors  333 ,  334  so that those heaters function as slave heaters  16 ′. Therefore, the controller receives temperature information from the master heater sensors  50 ,  52  and uses it to provide controlled power to the heating elements  32  of both the master heater  16  and the slave heaters  16 ′. 
     There can, of course, be more than two slave outlet connectors in the slave adapter junction box  322  to accommodate more than two daisy chain connected series of slave heaters. Also, while it is not shown in the drawings, split slave cables or additional junction boxes can be connected to the slave outlet connectors  333 ,  334  of the slave adapter junction box or to the slave outlet connector  134  of the slave adapter cable  22  to power additional daisy chain connected series of slave heaters  16 ′ if desired or needed. 
     A source power junction box, for example, the source power junction box  350  shown in  FIGS. 39-42 , can be used in place of the T-type source power cable  26  in the assemblies shown in  FIGS. 1-8 . In the example source power junction box  350 , a source power junction branch outlet connector  352  protrudes from the bottom surface  354  of the source power junction box  350  and is configured for mating connection to the inlet connector  140  of the controller  20  ( FIGS. 20 and 21 ) so that the source power junction box  350  can be mounted directly on the controller  20  by plugging the source power junction branch outlet connector  352  to the inlet connector  140 . 
     A source power junction inlet connector  356  on a first lateral side surface  358  of the source power junction box  350  receives source power into the source power junction box  350  from, for example, a source power extension cable  25  as described above in relation to  FIG. 27  and shown, for example, in  FIG. 42 . Therefore, the source power junction inlet connector  356  in  FIGS. 39 and 40  can be configured the same as the inlet connector  82  of the T-type source power cable  26  for substitutable modular connectivity to the AC power source  13  ( FIG. 27 ). 
     A trunk outlet connector  360  on a second lateral side surface  362  of the source power junction box  350  in  FIGS. 39 and 40  is provided for daisy connection of one or more additional controllers  2 , as shown in  FIG. 42 , and can be configured the same as the trunk outlet  86  of the T-type source power cable  26  ( FIGS. 1-8 ) for substitutability with the T-type source power cable  26 . Therefore, any of the following can be plugged into the trunk outlet  360  of the source power junction box  350 : (i) another source power extension cable  25 ; (ii) a T-type source power cable  26 ; or (iii) a linear-type terminating source power cable  108 . 
     An example schematic circuit diagram for the example source power junction box  350  is shown in  FIG. 41 . A pair of trunk source power connectors  364 ,  366  extends uninterrupted from the inlet connector  356  to the trunk outlet connector  360 , and a pair of source power branch conductors  368 ,  370  extend from a parallel connection with the trunk source power conductors  364 ,  366  to the branch outlet connector  352 . Therefore, the branch outlet connector  352  is connected electrically in parallel to the source power conductors in relation to the trunk outlet connector  360 . 
     The branch outlet connector  352  is connected electrically in series, however, between the inlet connector  356  and the trunk outlet connector  360  with respect to the signal circuit conductors in the source power junction box  350 . Therefore, as shown in  FIG. 41 , one of the signal circuit conductors, e.g., the trunk signal circuit conductor  372 , extends straight through the junction box  350  from the inlet connector  356  to the trunk outlet connector  360 . The other signal circuit conductor comprises an inlet branch signal circuit conductor  374  extending from the inlet connector  356  to the branch outlet connector  352  and an outlet branch signal circuit conductor  376  extending from the branch outlet connector  352  to the trunk outlet connector  360 . 
     Therefore, while a plurality of controllers  20  can be daisy chain connected electrically in parallel via the power source junction box  350  to the AC power source  13  ( FIG. 27 ), as shown in  FIG. 42 , they will be connected electrically in series via the power source junction box  350  to the signal circuit  23  ( FIG. 27 ) in the same manner as described above for the T-type power source cables  26 . Of course, any number of source power junction boxes  350  can be daisy chain connected together, with source power extension cables  25 , as shown in  FIG. 42 , for any number of controllers  20 . 
     Another example power source junction box  380  illustrated in  FIGS. 43 and 44  has more than one trunk outlet. For example, but not for limitation, in addition to the inlet connector  384  and the branch outlet connector  382 , which are substantially the same as the inlet connector  354  and branch outlet connector  352  described above for the junction box  350 , the power source junction box  380  is shown in  FIGS. 43 and 44  with two trunk outlet connectors  386 ,  388  in respective opposite sides  387 ,  389 . Both of the outlet connectors  386 ,  388  are configured the same for daisy chain connectivity to additional power source extension cables  25 , T-type source power connectors  26 , and linear-type terminating source power cables  108  so that two separate daisy chain connected sets of controllers (not shown) can be connected to the AC power source  13  and to the signal circuit  23  via the source power junction box  380 . 
     As shown in the example schematic circuit diagram in  FIG. 45  for the example power source junction box  380 , the trunk source power conductor pairs  390 ,  392 ,  394  and the branch source power conductor pair  396  connect both of the trunk outlet connectors  386 ,  388  and the branch outlet connector  382  electrically in parallel to the inlet connector  384 . The signal circuit conductors  398 ,  400 ,  402 ,  403 , however, connect the trunk outlet connectors  386 ,  388  and the branch outlet connector  382  electrically in series to the inlet connector  384 . 
     Of course, more than two trunk outlet connectors can be provided in the source power junction box  380 , if desired, with substantially the same kinds of parallel source power and series signal circuit conductor connections as described above for each additional trunk outlet connector. Also, if desired, the branch outlet connector  382  could be eliminated so that the junction box  380  would then function only to connect a plurality of daisy chain connected series of controllers (not shown) to an AC power source  15  and to a signal circuit  23 , but it would not be connectable directly to a controller inlet connector  140  without an intervening T-type source power cable  26 , an intervening linear-type terminating source power cable  108 , or a source power extension cable  25  (if the inlet connector  140  is configured for connection of a source power extension cable  25  as discussed above). 
     A conventional connector latch feature on some commercially available connectors, such as Molex™ connectors include a latch lever, such as the latch lever  410  shown on the male connector  78  in  FIG. 46 , with a dog  412  on its distal end that is sized and shaped to engage a latch protrusion on the female connector, such as the protrusion  414  shown on the controller inlet connector  140  in  FIG. 47 . Such engagement of the latch protrusion  414  by the dog  42  on the latch lever  410  is intended to secure the male connector to the female connector until it is disengaged by pivoting the latch lever  410  on an elastic hinge  416 , as shown in  FIG. 48 , which releases the male connector from the female connector and allows them to be disconnected or unplugged from each other. However, in some applications, such conventional latches are not secure enough, and it is too easy for the connectors to be unplugged unintentionally, for example, by bumping or rubbing past them in tight spaces, and the like. 
     Therefore, to provide further security and resistance to unintentional disconnection of the connectors, for example, of the connectors  78 ,  140  shown in  FIGS. 46-48 , a cantilevered resilient spring biasing tab  420  is positioned adjacent the distal end  418  of the latch lever  410 . The biasing tab  420  bears against the distal end  418  of the latch lever  410  and has a resilient spring bias force that resists movement of the latch lever  410  in a manner that would disengage the dog  412  from the latch protrusion  414 . However, when a user forces the latch lever  410  to pivot about the elastic hinge  416 , which also acts as a fulcrum for the latch lever  410 , as indicated by pivot arrow  422 , the distal end  418  of the latch lever  410  pushes outwardly against the spring bias force of the biasing tab  420  and forces the biasing tab  420  to pivot outwardly, as indicated by pivot arrow  424  in  FIG. 48 . The elastic resilient spring bias of the tab  420  does yield under enough force to allow the dog  412  on the latch lever  410  to disengage from the latch protrusion  414  so that the branch outlet connector  78  can be unplugged from the controller inlet connector  140 . 
     There are myriad ways to provide a spring biasing force to bear on the latch protrusion  414 . One example implementation of this feature is to mold the biasing tab  420  as cantilevered part of the housing  201  of the base unit  200 , as illustrated in  FIGS. 46-48 . Depending on how much bias force or how yieldable a particular application requires for the tab  420 , a portion of the housing  201  at the cantilevered joint of the tab  420  to the rest of the housing  201  can be thinner to function as a resilient elastic hinge  426 , as shown in  FIGS. 47 and 48 . A slot  428  can be provided in the housing to accommodate movement of the latch lever  410  into and out of the housing  201 . A tapered cam surface  421  can be provided on the tab  420  to facilitate camming the tab  420  out of the way when the latch lever  410  is being inserted into the housing  201  as the outlet connector  78  is plugged into the inlet connector  140 . 
     As mentioned above, this bias force feature can also be implemented in other ways. Several examples are shown in  FIGS. 49-51 . In  FIG. 49 , the biasing force is provided by a compressible leaf spring  430  mounted in a bracket  432  on the inside of the housing  201 . In  FIG. 50 , a coil compression spring  434  provides the bias force, and, in  FIG. 51 , an elastically compressible material  436 , such as rubber, silicon rubber, a foamed elastomer, or other foamed material is shown to provide the bias force against the distal end  418  of the lever  410 . 
     While the biasing tab  420  has been described above in relation to the branch outlet connector  78  and the controller inlet connector  140 , it is also applicable to the controller outlet connector  142  and whatever interfacing inlet connector is plugged into the controller outlet connector, e.g., the heater inlet connector  64 , slave adapter inlet connector  130 , slave junction box inlet connector  330 , etc., as described above. It can also be used in relation to the slave junction box outlet  322 , as indicated by tab  420 ′ in  FIGS. 35 and 36 . 
     Since these and numerous other modifications and combinations of the above-described method and embodiments will readily occur to those skilled in the art, it is not desired to limit the invention to any of the exact construction and process shown and described above. While a number of example aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions, and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, and sub-combinations as are within their true spirit and scope. The words “comprise,” “comprises,” “comprising,” “has,” “have,” “having,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features or steps, but they do not preclude the presence or addition of one or more other features, steps, or groups thereof.