Patent Publication Number: US-6218647-B1

Title: Method and apparatus for using direct current to detect ground faults in a shielded heater wire

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 09/008,770, entitled “METHOD AND APPARATUS FOR USING DIRECT CURRENT TO DETECT GROUND FAULTS IN A SHIELDED HEATER WIRE”, filed Jan. 19, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the detection of ground faults, and, more particularly, to the detection of ground faults in electric heaters used to melt and thus remove snow and ice from pavement, roofs, gutters, down spouts, satellite dishes and the like. 
     2. Description of the Related Art 
     Electric heaters may be utilized to supply heat used in snow and ice melting systems. Typical melting applications include but are not limited to satellite dishes, roofs and gutters, pavement, building and garage entrances and facilities accommodating the physically challenged. Efficient operation requires embedding the electric heaters in or attaching the electric heaters to satellite dishes, pavement and other structures which may sometimes become covered with snow and ice. 
     Heater cable construction may employ one of several methods. For example, self-limiting heaters typically consist of two parallel stranded copper bus wires separated by a semiconducting polymer enclosed in one or more concentric layers of organic insulating material. Other common heater cable construction methods involve extruding a thermoplastic insulating compound over a single conductor or a pair of parallel conductors. Another construction method, the oldest, involves packing mineral insulation, commonly magnesium oxide, over a single conductor or a pair of parallel conductors enclosed within a copper or stainless steel tube. Current practice as dictated by the U.S. National Electric Code requires covering the heating cable with a grounded conductive copper braid or shield that serves as a return path for any ground current. Mineral insulated heaters accomplish this requirement by way of their outer stainless steel or copper tubular jackets. 
     Ground current is the difference between the outbound and return heater currents. The U.S. National Electric Code requires using a ground fault circuit interrupter (GFCI) on all snow and ice melting circuits. The GFCI interrupts heater current if the ground current exceeds a predetermined limit; usually 30 milliamperes. The GFCI requires manual reset after tripping. This preserves safety by not restarting heater operation during intermittent ground leakage current that may occur in wet locations. 
     Independent of the heater fabrication method, ground current can flow due to a heater failure caused by a manufacturing defect, corrosion, wear and tear or mechanical damage. Excessive ground current causes the dual safety problems of fire and shock hazard. 
     The fire hazard is variously referred to as a wet fire or heater burn-back. Although this can occur with heaters of any construction, it is more likely to occur in heaters with parallel conductors in the presence of moisture. Conductors exposed to the ambient due to mechanical damage are the starting point for the fire hazard. Moisture acting as an electrolyte on the cable in the area of the damage forms a conductive path between parallel conductors or between a conductor and a surrounding shield. Current flows through a small area and strikes an arc which creates a high temperature plasma. This carbonizes a portion of the polymer insulation and creates a conductive carbon arc track in the polymer. Flames and high temperatures occurring during the burn-back can ignite combustible materials in proximity to the heating cable. The burn-back mechanism in mineral insulated cable is similar except that magnesium hydroxide forms by mixing moisture with the magnesium oxide insulation to form a conductive electrolyte. 
     Aside from the fire hazard described above, an electrical shock hazard can also occur whenever ground current flows since its path to earth ground is usually not predictable. Thus, a GFCI is required to be incorporated into snow and ice melting electrical circuits. 
     Snow and ice melting systems commonly employ automatic controls that operate heaters only while required to minimize energy consumption and operating costs. Typically, the automatic controls sense ambient moisture and temperature. Heaters operate at ambient temperatures below a threshold—usually 38° F. while ambient moisture is present and for a period of time thereafter to clear accumulated snow and ice. Optionally, the automatic control may inhibit heater operation at temperatures too low for effective melting, e.g., below 17° F. 
     Current practice is to use a GFCI circuit breaker external to the automatic control of the snow and ice melting system. Such a self-contained GFCI circuit does not provide an output signal indicative of a ground fault condition. The automatic control may or may not require an external contactor for controlling heater operation. 
     It is also known from U.S. Pat. No. 5,710,408, assigned to the Assignee of the present invention, to sense a ground fault condition by inductively measuring both the current flowing into the heating element and the current flowing out of the heating element. Any difference between these two current levels represents ground leakage current. If the ground leakage current exceeds a preset value, then a ground current interface sends a signal to a microcontroller. The microcontroller may shut down power to or otherwise control the heater based upon this signal. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for automatically controlling snow and ice melting electric heaters while continuously monitoring the ground leakage current in the shield surrounding the heater wire. In addition to the alternating current (AC) power voltage which heats the heater wire, a constant or direct current (DC) voltage, referred to herein as a tracer signal, is applied to the heater wire. Upon detecting the tracer signal in the shield, indicating a ground leakage current, the automatic controller interrupts electrical power provided to the heater until reset by operating personnel. Thus, the present invention combines the GFCI function with the automatic controller associated with the snow melting heaters. The automatic controller performs these tasks within norms established by the U.S. National Electrical Code and the testing requirements established by Underwriters&#39; Laboratories. 
     The invention comprises, in one form thereof, an ice and snow melting system including at least one sensor configured for sensing a temperature or moisture associated with an ambient environment and providing a signal indicative thereof. A heater for melting the ice and snow includes a heater wire, a layer of insulation substantially surrounding the heater wire, and a conductive shield substantially surrounding the layer of insulation. A ground fault circuit interrupter is coupled with the shield of the heater. The ground fault circuit interrupter detects a ground fault condition between the heater wire and the conductive shield and provides a signal indicative thereof. An automatic controller is connected to the at least one sensor. The controller includes heater control circuitry receiving each of the sensor signal and the ground fault circuit interrupter signal. The heater control circuitry selectively controls operation of the heater dependent upon the sensor signal and the ground fault circuit interrupter signal. 
     An advantage of the present invention is that the automatic control and GFCI functions are combined together into a single automatic controller, thereby reducing installation cost and complexity. 
     Another advantage is that by combining the automatic control and GFCI functions, an automatic controller costing less than individual GFCI and automatic snow and ice melting controls is realized. 
     Yet another advantage is that the direct current signal introduced into the heater wire can be easily monitored in the shield, thereby eliminating the circuitry that is needed to detect an AC voltage in either the heater wire or the shield. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic illustration of an embodiment of the overall snow and ice system of the present invention, showing each of the subsystems and their interconnections; 
     FIG. 2 is a sectional, perspective view of one embodiment of a heater cable of the present invention and the associated ground fault circuit interrupter; 
     FIG. 3 is a plot of one embodiment of voltage signals provided by the power supply of the overall snow and ice system of FIG. 1; 
     FIG. 4 is a block diagram of one embodiment of the ground fault circuit interrupter of the overall snow and ice system of FIG. 1; 
     FIG. 5 is a sectional, perspective view of another embodiment of a heater cable and an associated ground fault circuit interrupter of the present invention; and 
     FIG. 6 is a schematic diagram of the ground fault circuit interrupter of FIG.  5 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to FIG. 1, there is shown an overall view of an embodiment of a snow and ice melting system  10  of the present invention. Snow and ice system  10  generally includes an automatic controller  11 , heater  14 , power supply  16 , moisture sensor and interface  18 , ambient temperature sensor and interface  24  and ground fault circuit interface  29 . 
     Unless otherwise noted, details familiar to persons skilled in the electronic arts will be omitted since they are extraneous detail and thus have no bearing on reducing the invention to practice. Where in this application the terms “control”, “controlling” or the like are used, it is to be understood that such terms may include the meaning of the terms “regulate”, “regulating”, etc. That is, such “control” may or may not include a feedback loop. Moreover, it is also to be understood, and it will be appreciated by those skilled in the art, that the methodology and logic of the present invention described herein may be carried out using any number of structural configurations such as electronic hardware, software, and/or firmware, or the like. 
     A line voltage  12  supplies power to system  10  including heaters  14 . Power supply  16  derives its power from the line voltage  12  and supplies all circuits with appropriate AC and DC operating voltages. 
     Automatic controller  11 , in the embodiment shown, is constructed as an integral unit which includes a number of separate subsystems or modules. In the particular embodiment shown in FIG. 1, controller  11  includes an electrical processor or microcontroller  20 , supervisor  36 , indicators and drivers  38 , switches and interface  42 , potentiometer and interface  46 , and contactor driver  50 . Such modules or subsystems are preferably incorporated into a single housing, shown schematically in FIG.  1 . However, it will also be appreciated that any of the individual subsystems or modules making up automatic controller  11  may also be separate or remotely located from automatic controller  11 , if desirable for a particular application. 
     The moisture sensor and interface  18  uses an on-board temperature regulated heater to convert snow and/or ice to liquid water. Water on the surface of a sensing grid is detected as a change in conductivity. An interface circuit incorporated within moisture sensor and interface  18  converts the conductivity change into a low-impedance analog signal which is transmitted to an electrical processor such as a microcontroller  20  via conductor  22 . 
     The ambient temperature sensor and interface  24  converts the ambient temperature sensor signal into an analog signal which is appropriate for inputting to the microcontroller  20  via a conductor  26 . 
     In the embodiment of ice and snow melting system  10  shown in the drawings, moisture sensor and interface  18  and ambient temperature sensor and interface  24  are shown as separate subsystems. However, it is also possible to combine moisture sensor and interface  18  and ambient temperature sensor and interface  24  into a single subsystem. An example of a single sensor which may combine the moisture sensing and ambient temperature sensing into a single unit is known, e.g., from a model CIT-1 Snow Sensor and a model GIT-1 Gutter Ice Sensor, each of which are manufactured by the Assignee of the present invention. 
     Heater  14 , shown in more detail in FIG. 2, includes a heater wire  27  surrounded by an electrically conductive heater shield  31 . Power supply  16  provides current to heater wire  27  which acts as a resistive heating element. Heater shield  31  is electrically insulated from heater wire  27  by a layer of insulation  33  which surrounds heater wire  27 . In the embodiment shown, heater shield  31  is surrounded by a layer of extruded polyvinylchloride plastic insulation  35 . 
     It is possible for an ohmic contact to be established between heater wire  27  and heater shield  31  in the event that heater  14  is damaged or the integrity of layer of insulation  33  is otherwise compromised. This can occur if an air gap forms between heater wire  27  and heater shield  31 , such as through a crack  37  in insulation  33 . Arcing in the air gap between heater wire  27  and heater shield  31  can create a carbonized track or ohmic contact through which current can leak from heater wire  27  to heater shield  31 . Although heater shield  31  can be grounded in order to carry away such ground leakage current, it is clearly not desirable for melting system  10  to continue to operate with ground leakage current between heater wire  27  and heater shield  31 . The present invention monitors heater shield  31  and shuts down power to or otherwise controls heater  14  when ground leakage current in heater shield  31  is detected. 
     Power supply  16 , in the embodiment shown, energizes heater  14  with both an alternating current voltage  39  (FIG. 3) and a direct current voltage  41  provided by AC voltage supply  43  and DC voltage supply  45 , respectively. AC voltage  39  sources substantially all of the energy used to heat heater wire  27 . DC voltage  41  is more easily detectable than an AC signal, and thus is used by the present invention to trace ground leakage current from heater wire  27  to heater shield  31 , as will be described in more detail hereinafter. In the event that a ground fault occurs between heater wire  27  and heater shield  31  through the above-described ohmic contact, both AC voltage  39  and DC voltage  41  leak into heater shield  31 . 
     The ground fault circuit interrupter  29  converts any ground leakage current in heater shield  31  into a signal appropriate for inputting to the microcontroller  20  via conductor  34 . Referring now to the embodiment shown in FIG. 4, ground fault circuit interrupter  29  includes a filter  47  which allows alternating current from AC voltage  39  in heater shield  31  to pass to ground. Ground fault circuit interrupter also includes a low pass filter  49  which allows DC voltage  41  in heater shield  31  to pass to microcontroller  20  of automatic controller  11 . Microcontroller  20  thereby senses the presence of DC voltage  41  in heater shield  31 , which presence indicates that ground current is leaking from heater wire  27  to heater shield  31 . Upon sensing DC voltage  41 , microcontroller  20  instructs contactor driver  50  to shut off power to heater  14 . In this way, DC voltage  41  functions as a tracer signal to indicate the presence of the less easily detectable power AC voltage in heater shield  31 . 
     DC voltage  41  can be set to a level such that, after factoring in voltage drops along heater wire  27 , layer of insulation  33 , heater shield  31  and low pass filter  49 , an appropriate DC voltage, e.g. 5 volts, is input to microcontroller  20 . In some applications, however, it may be desirable for ground fault interrupter  29  to include circuitry (not shown) between low pass filter  49  and microcontroller  20  for sensing a DC voltage above some threshold value. The threshold value may be substantially less or greater than a voltage level appropriate for inputting to microcontroller  20 . The circuitry, upon sensing a DC voltage in heater shield  31  above the threshold voltage, would send a signal to microcontroller  20 , the signal having a voltage level compatible therewith. The signal would indicate that ground current is leaking from heater wire  27  to heater shield  31  and that power to heater  14  should be shut off. The circuitry could include one or more comparators. Rather than inputting a signal to microcontroller  20 , it is also possible for the circuitry to send a signal directly to power supply  16 , as indicated by dotted line  51  in FIG. 1, shutting off power to heater  14  in the event that ground fault leakage current is sensed. 
     In an alternative embodiment, instead of a DC voltage, another AC voltage waveform can act as the tracer signal for power AC voltage  39 . Such an AC voltage tracer signal is shown in FIG. 3 as the dotted waveform  53 . AC tracer signal voltage  53  has a lower amplitude and a different frequency than power AC voltage  39 . Although AC voltage tracer signal  53  is shown as having a particular amplitude and frequency, it is also possible for tracer signal  53  to have a different amplitude and frequency than as shown. For example, tracer signal  53  can have the same frequency as power AC voltage  39  but be out of phase therewith. 
     Snow and ice melting system  10  is particularly suited for use with a heater on a non-conductive substrate, such as an antenna. Such a non-conductive substrate would not electrically interfere with voltages on either heater wire  27  or heater shield  31 . 
     The supervisor  36  controls the restarting of microcontroller  20  upon the initial application of power and under brown-out conditions. Supervisor  36  holds the microcontroller  20  in its reset condition so long as its supply voltage is too low to permit reliable operation. Supervisor  36  asserts reset until the supply voltage has been reliable long enough for the microcontroller  20  to initialize itself. 
     The microcontroller  20  in combination with its firmware form the primary subsystem of snow and ice melting system  10 . Microcontroller  20  provides one time programmable program memory, data memory, program alterable permanent memory (i.e., electrically erasable read only memory (EEROM), an 8-bit analog to digital (A/D) converter, timers, counter, a fail-safe (i.e., watch dog) timer and digital inputs and outputs. If the fail-safe timer is not reset frequently enough, it restarts the microcontroller  20 . This prevents microcontroller  20  from latching due to electrical transients from lighting and similar causes. An example of a microcontroller which has been found suitable for use within automatic controller  11  is a PIC16C84 manufactured by Microchip Corporation, Chandler, Ariz. 
     The indicators and drivers  38  provide status information for operating personnel. Typical status information includes but is not limited to the presence of electric power, snow, operation of heater  14  and a ground fault condition. In the particular embodiment shown, the indicators are visible light emitting diodes (LED&#39;s), and the associated drivers consist of bipolar or metal oxide field effect transistors used as saturating power amplifiers for the low power microcontroller  20  outputs received over conductor(s)  40 . However, some microcontrollers have sufficient current capacity to drive the LED&#39;s directly. 
     The switches and interface  42  provides an interface between operating personnel and the automatic snow and ice melting control. Switches and interface  42  is connected to microcontroller  20  via conductors  44 . Switch functions include but are not limited to test/reset of the GFCI, testing of heater  14 , cycle heater  14  and abort heater operation. Typically, the interface consists of a pull-up resistor for each active switch contact. 
     The potentiometer and interface  46  converts a potentiometer shaft azimuth into a proportional analog signal for input via conductor(s)  48  to an analog to digital (A/D) converter associated with microcontroller  20 . Since the particular microcontroller  20  described with reference to FIG. 1 includes an A/D converter, interface circuitry is not required. Counter-clockwise potentiometer terminals are grounded and clockwise terminals are connected to the A/D converter reference voltage—typically the positive supply voltage for microcontroller  20 . The potentiometer&#39;s wiper is connected directly to an A/D converter input. If the microcontroller used does not provide the analog to digital converter function, the potentiometer shaft position can be directly inputted to a digital input through the use of a resistor-capacitor network and a digital output using techniques well known to persons skilled in the electronic arts. 
     Depending upon the application, it may take several hours for the system to heat to ice melting temperature, thus causing an accumulation of snow and ice. Removing the accumulation requires heater operation for a period of time after precipitation stops. Automatic controls usually provide an adjustable hold-on timer for this purpose. An analog potentiometer associated with potentiometer and interface  46  provides a calibrated hold-on time adjustment. 
     The contactor driver  50  is a saturated power amplifier employing either a bipolar or metal oxide field effect transistor to drive the solenoid coil of a contactor  52 . Microcontroller  20  output port(s)  54 , in the particular embodiment shown, lack the voltage and current capacity to do this directly. Contactor  52  provides two normally open contacts that control power applied to the heater  14 . More particularly, contactor  52  is connected to one side of respective relays including relay contacts  56 ,  58 . Thus, microcontroller  20  and contactor driver  50  define heater control circuitry within automatic controller  11  for selectively controlled operation of heater  14 , through contactor  52  and relay contacts  56 ,  58 . The U.S. National Electrical Code requires breaking both line leads of 208/240/480 volt circuits. Only the line side and not the neutral side of the power line needs to be broken in 120/277 volt circuits. 
     As apparent from the foregoing description, the present invention combines the functions of automatic snow and ice melting control with ground fault circuit interruption. Automatic controller  11  causes melting by operating control contactor  52  to close relay contacts  56  and  58  thus applying the line voltage  12  to heater  14 . Operation continues while moisture is present and the ambient temperature is in the operating range and for the hold-on time thereafter. 
     Operationally, the GFCI function has a higher priority than of automatic control. That is, unless a ground fault current occurs, controller  11  performs an automatic snow and ice melting control function. Upon detecting a ground current above a threshold value, the invention terminates its automatic control function and acts as a GFCI. Once tripped, operating personnel must operate a reset switch to cancel GFCI operation even though power may have been removed from the automatic controller. An indicator displays GFCI operation. 
     In addition to the normal control functions of automatic controller  11 , operation of an override switch forming a part of switches and interface  42  causes heaters  14  to operate for the hold-on time independent of environmental conditions. This permits clearing tracked snow in doorways and parking garage entrances that go undetected by the sensor. Another switch permits testing heaters for a brief interval. This permits nondestructive heater testing during the summer months. 
     It is possible for ground leakage current to enter heater  14  from other parts of ice and snow melting system  10  or from an external source. In another embodiment (not shown) heater  14  is electrically isolated or can be isolated in order to determine whether a ground fault is in heater  14  or originates in some other source. 
     In another embodiment (FIG.  5 ), a DC voltage supply is not applied to heater wire  27 , but rather a DC voltage supply  60  is applied to heater shield  31  through GFCI  62 . In this embodiment, heater shield  31  is not grounded, and, consequently, no current will flow through heater shield  31  in the absence of a ground fault condition, even though a DC voltage is being applied to heater shield  31 . If a ground fault does occur, however, DC current may flow from heater shield  31  to heater wire  27 . GFCI  62  then senses the DC current in heater shield  31  and transmits a signal to microcontroller  20  indicative thereof. 
     DC voltage supply  60  and GFCI  62  are shown in more detail in FIG.  6 . DC voltage supply  60  includes an AC voltage supply  64  whose AC voltage is rectified and converted into a DC voltage. Diode  66  is a half-wave rectifier. Capacitor  68  charges to approximately the peak value of AC voltage supply  64 . Resistor  70  discharges capacitor  68 , thus eliminating a shock hazard that could occur when the supply voltage is not applied, such as when the snow and ice melting system is turned off. Diode  72  prevents reverse current flow in the event that heater shield  31  is inadvertently shorted to another voltage source, thus ensuring proper operation. Resistor  74  limits the current that can flow through heater shield  31 , possibly to less than four milliamperes. 
     GFCI  62  includes an opto-isolator  76  which senses DC current flow and transmits an optical signal indicative thereof with a light emitting diode (LED)  78 . Opto-isolator  76  also includes a photo-transistor  80  which receives the optical signal from LED  78  and transmits a signal indicative thereof to microcontroller  20 . Opto-isolator  76  can have a current transmission ratio of 100% or greater. Thus, a current of 10 milliamperes through LED  78  can produce a current of 10 milliamperes in photo-transistor  80 . In this circuit, the saturation current of photo-transistor  80  is approximately 0.5 milliamperes. Diode  82  protects LED  78  from at destructive reverse voltage transients. 
     If heater insulation  33  fails, creating a current path between heater shield  31  and heater wire  27 , the DC current in heater shield  31  increases through a threshold value. Photo-transistor  80  then charges a capacitor  84  through a current-limiting resistor  86  in order to reduce electrical noise. The voltage of capacitor  84  is applied to microcontroller  20  through line  34  and current-limiting resistor  88 . Current-limiting resistor  88  limits the worst-case discharge current into line  34  and microcontroller  20  to a safe value. An optional resistor  90  can be used to discharge capacitor  84  when photo-transistor  80  turns off. 
     It is possible for AC voltage supply  43 , in addition to supplying heater power to heater wire  27 , to function in the place of AC voltage supply  64 . It is also possible for DC voltage supply  60  to be in the form of a DC voltage battery. 
     Where in this application the terms “ground fault circuit interrupter”, “GFCI” or the like are used, it is to be understood that such terms may encompass a more generic type of current detecting device. That is, the leakage current detected by the GFCI may not necessarily pass directly to “ground”, or to a grounded device. Rather, the current detected by GFCI  62  can flow from shield  31  to heater wire  27 , with heater wire  27  being otherwise electrically connected only to AC voltage source  43 . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.