Abstract:
A railroad signaling method includes measuring a track current on a first track section, measuring a local voltage on the railroad line wires, determining whether a magnitude of the track current meets or exceeds a threshold value independent of the local voltage, and determining whether the track current and the local voltage are nominally in phase with one another in a manner that is independent of the track current measuring step. A first signal is provided only if the magnitude is determined to meet or exceed the threshold and the track current and local voltage are determined to be nominally in phase. The first signal is an indication that a train is not present in the track section, that the track section does not have a broken rail, and that the insulated joints that bound the track section do not include a fault condition. An associated relay arrangement is also provided.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to signaling system for railroads, and in particular to an electronic vital relay used to detect the presence of a train or a broken rail in particular track sections and/or track fault conditions between track sections.  
         [0003]     2. Description of the Prior Art  
         [0004]     Railroad systems, such as electrified railroad systems that employ power frequently track circuits, utilize signaling systems to provide signals for train operators and to control the operation of railroad crossing gates and warning lights. In such systems, each track circuit is bounded by insulated joints so that the presence of a train can be defined to a unique section of track. Railroad signaling systems use what is termed a vital relay or track relay to detect the presence of a train or a broken rail in particular track sections and/or track fault conditions between track sections (i.e., a fault in the insulated joint connecting two track sections). The signaling system only permits trains to pass and/or crossing gate and warning light systems to be in an off condition when the relay is in an operated condition. The relay will drop out of an operated condition if a train or a broken rail is in the particular track section being controlled or if there is otherwise a fault on the system which prevents an accurate detection of the presence of a train.  
         [0005]     A typical use of a vital relay in a track circuit is shown in  FIG. 1 . As seen in  FIG. 1 , a track section T of a stretch of electrified railroad is shown with its rails  1  and  2  illustrated by conventional single line symbols. The rails of section T are electronically insulated from the rails of the adjoining sections by the insulated joints  3 , also illustrated by conventional symbols. In order to provide a return circuit for the propulsion current, impedance bond windings  4  are connected across rails I and  2  at each end of section T and the associated ends of the adjoining sections. The center taps of each associated pair of bond windings  4  are connected by a lead  5  to provide a conventional circuit path through section T for propulsion current.  
         [0006]     The signaling system for this stretch of railroad is based on continuous train detection using a track circuit for each track section such as section T. Signaling energy for the track circuits is provided from a central source S having a frequency of, for example, 50, 60 Hz or 100 Hz, and is distributed along the stretch of railroad by the line wires  6  and  7 . Energy is supplied across the rails of section T at the left or transmitting end through a track transformer  8  from the line wires  6  and  7 . Even though AC energy is used, the supply connections are such that the instantaneous polarity of the rails on each side of the insulated joints  3  are opposite, as indicated by the polarity markings at the rails  1  and  2 . The supply connections include a selected resistor  11  which limits the current flow when a train shunts the rails  1  and  2  at the transmitting end. At the other or receiving end of section T, a vital relay circuit  12  is connected across the rails  1  and  2  through a track transformer  13  and a control coil  14  (alternatively, they may be directly connected) and to the line wires  6  and  7  through a local coil  15 . As discussed above, the vital relay circuit  12  detects the presence of a train or a broken rail and/or an insulated joint track fault condition in the section T, which detection is in turn used by the signaling system to provide signals for train operators.  
         [0007]     As will be appreciated, if a train is not present and no rail is broken in the section T, a substantial amount of current will be present in the control coil  14 . However, if a train is present in the section T, it will shunt the rails  1  and  2 , thereby resulting in little or no current in the control coil  14 . Similarly, there will be little or no current in the control coil  14  if a rail is broken in the section T. In addition, if the insulating joints  3  are intact, the current in the control coil  14  and the current in the local coil  15  will be substantially in phase. However, if a fault condition develops at the insulating joints  3 , the current in the control coil  14  and the current in the local coil  15  will be out of phase. These principles are utilized by vital relays to detect the presence of trains and fault conditions in track sections.  
         [0008]     A common type of vital relay in use in electrified railroad systems is what is known as a vane relay. A vane relay operates by a principle similar to that of a watt-hour meter. A vane relay includes a vane positioned in the magnetic gap between two coils (i.e., the control coil  14  and the local coil  15 ). The vane is responsive to the product of: (i) the current in the control coil, (ii) the current in the local coil, and (iii) the cosine of the angular difference of the current in the two coils. Maximum torque in a first direction is produced in the vane if current of a certain magnitude is present and the angular difference is zero (cosine of 0°=1), and maximum torque is produced in a second, opposite direction if current of a certain magnitude is present and the angular difference is 180° (cosine of 180°=−1). In addition, as the level of current decreases, the level of torque in either direction will decrease. The vane is fitted with a ladder structure so that a multiplicity of electrical contacts will close only when the vane rotates with sufficient torque in the first direction, which proves that the current in the two windings is nominally in phase and the current is of sufficient magnitude, i.e., there is not a train or a broken rail in the track section and there is no insulated joint fault condition. Just as important for railway train detection purposes, the contacts will not close if the phase relationship is reversed (indicates a fault at insulating joints  3 ), regardless of the magnitude of the product of current and angular difference, or if the current in one of the windings is not of sufficient magnitude (i.e., substantially zero) (indicates the presence of a train or broken rail).  
         [0009]     The problem with vane relays is that, as an electromechanical device with moving parts, they require considerable preventive maintenance to assure reliable operation. In addition, because vane relays are a product based device, the control current required to close the contacts is inversely effected by the local voltage. If local voltage regulation is poor, safety or reliable operation can be impacted. For example, if the local voltage decreases, track current decreases proportionally, but the control current required to maintain the track relay energized is increased. In such a situation, the potential exists for the track relay to drop and falsely indicate that the track circuit is occupied. Alternatively, at increased local voltage, the rail current is greater, but the current that is required to maintain the track relay energized is decreased. This increases the risk of the track relay&#39;s failure to drop in the presence of a broken rail.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention relates to a signaling method for a railroad system having a plurality of track sections that are connected to one another. Each of the track sections is bounded on either side by an insulated joint. Signaling energy is provided to the track sections by a source, and the source has line wires connected thereto. The method includes measuring a track current on a first one of the track sections at a first location thereon, wherein the signaling energy is provided to first one of the track sections at a second location thereon. The method further includes measuring a local voltage on the line wires, determining whether a magnitude of the track current meets or exceeds a threshold value independent of the local voltage, and determining whether the track current and the local voltage are nominally in phase with one another in a manner that is independent of the step of determining whether the magnitude of the track current meets or exceeds the threshold value. Finally, the method includes providing a first signal only if the magnitude is determined to meet or exceed the threshold value and the track current and the local voltage are determined to be nominally in phase with one another. The first signal is an indication-that a train is not present in the first one of the track sections, that the first one of the track sections does not have a broken rail, and that the particular insulated joints that bound the first one of the track sections do not include a fault condition. The first signal may cause a relay of the railroad system to be in an operated condition, or, alternatively, the first signal may be provided directly to a signaling system of the railroad system. In the preferred embodiment, the step of determining whether the track current and the local voltage are nominally in phase with one another includes synchronously rectifying the local voltage in response to the phase relationship between the local voltage and the track current.  
         [0011]     The present invention also relates to a vital relay arrangement for a railroad system having a plurality of track sections connected to one another. Each of the track sections are bounded by insulated joints. Signaling energy is provided to the track sections by a source having line wires connected thereto. The vital relay arrangement includes a control transformer for generating a track voltage signal that is proportional to a track current on a first one of the track sections at a first location thereon, wherein the signaling energy is provided to the first one of the track sections at a second location thereon, and a local transformer for generating a local voltage signal that is proportional to a local voltage on the line wires. The vital relay arrangement also includes a means for converting the track voltage signal to a DC voltage, a means for determining whether the DC voltage meets or exceeds a voltage threshold value, and a means, independent of the means for determining whether the DC voltage meets or exceeds a voltage threshold value, for determining whether the track voltage signal and the local voltage signal are nominally in phase with one another. The vital relay arrangement provides a first signal only if the DC voltage is determined to meet or exceed the voltage threshold value and the track voltage signal and the local voltage signal are determined to be nominally in phase with one another. The first signal is an indication that a train is not present in the first one of the track sections, that the first one of the track sections does not have a broken rail, and that the particular insulated joints that bound the first one of the track sections do not include a fault condition. Preferably, the means for determining whether the track voltage signal and the local voltage signal are nominally in phase with one another uses synchronous rectification and therefore includes a synchronous rectifier. The synchronous rectifier may include a solid state relay coupled, for example, optically, to a synchronous switching element, wherein the solid state relay receives the track voltage signal and controls the switching of the synchronous switching element based thereon, and wherein the synchronous switching element receives the local voltage signal. The vital relay arrangement also preferably includes an averaging filter that receives the synchronous output voltage signal from the synchronous switching element and produces an averaged output voltage signal that is used to indicate whether the track voltage signal and the local voltage are nominally in phase with one another. The vital relay arrangement may include a relay, wherein the first signal causes the relay to be in an operated condition. Alternatively, the first signal may be provided directly to a signaling system of the railroad system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and other advantages of the present invention will become readily apparent upon consideration of the following detailed description and attached drawings, wherein:  
         [0013]      FIG. 1  is a schematic diagram showing a typical use of a vital relay in a track circuit;  
         [0014]      FIG. 2  is a schematic diagram of an electronic vane relay architecture according to the present invention;  
         [0015]      FIGS. 3A and 3B  are schematic diagrams of example half-wave doubler circuit implementations; and  
         [0016]      FIGS. 4A and 4B  are a schematic diagram of one particular embodiment of the electronic vane relay of  FIG. 2 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]      FIG. 2  is a schematic diagram of an electronic vane relay architecture  20  according to the present invention. The electronic vane relay architecture  20  may be used in connection with the track configuration shown in  FIG. 1 , wherein it is the vital relay circuit  12 . As described above, a prior art electromechanical vane relay operates by producing torque that drives mechanically linked metallic contacts wherein the torque is proportional to the product of: (i) the current in the local winding, (ii) the current in the control/track winding, and (iii) the cosine of the angular difference of those currents. The current portion of the product is an indication of whether a train or a broken rail is present in the track section being monitored, and the phase portion of the product is an indication of whether a fault condition exists between track sections. In contrast, in electronic vane relay architecture  20 , the determination of sufficient current and phase difference for operation of the relay are determined independently from one another in an electronic manner and combined as a logical (digital) AND function to deliver a DC output. In addition, the current determination is based solely on the control/track current, entirely independent of the local voltage.  
         [0018]     As seen in  FIG. 2 , a control transformer  25  (connected to a track transformer such as track transformer  13  in  FIG. 1  (not shown)) operates as a current transformer, i.e., it translates the primary track current into a proportional voltage. The transfer function (secondary voltage/primary current) is controlled by the turns ratio of the control transformer  25  and the secondary load provided by resistor  30 . To achieve a particular input impedance, such as the input impedance of the vane relays used in many systems (i.e., 62 Ω), a resistor  35  is connected in series with the control transformer  25 . Diodes  40  and  45  take the output of the control transformer  25  and produce a DC voltage proportional to the track current. The DC voltage is input into level detector  50 . As is known in the art, a level detector is a device that takes a voltage as an input and outputs an oscillating signal, e.g., a square wave, if the input voltage is greater than or equal to a predetermined threshold voltage value, and outputs nothing if the input voltage is less than a predetermined threshold voltage value. Example level detectors are described in U.S. Pat. Nos. 4,384,250, entitled “Vital Vehicle Movement Detector.” and U.S. Pat. No. 4,056,739, entitled “Fail-Safe Electronic Polarized Relay,” the disclosures of which are incorporated herein by reference. In the particular embodiment shown in  FIG. 2 , the predetermined threshold voltage value of level detector  50  is the voltage that is produced by control transformer  25  and diodes  40  and  45  in response to a control current of approximately 50 mA, and the output of level detector  50  when the predetermined threshold value is met or exceeded is a square wave switching at approximately 20 kHz. The output of level detector  50  is provided as one input into optical isolator/vital AND gate  55 . The second input into optical isolator/vital AND gate  55  and the output thereof will be described in greater detail herein. However, the important point to note is that the level detector  50  will output an oscillating signal only when the voltage output by control transformer  25  and diodes  40  and  45  meets or exceeds the predetermined threshold voltage value, i.e., when the control/track current meets or exceeds the predetermined threshold voltage value. Those values are chosen to be the values that indicate that a train is not present and there is no broken rail in the track section being monitored by the electronic vane relay architecture  20 . Thus, according to this aspect of the invention, the level detector  50  will output an oscillating signal only if there is sufficient current to indicate that neither a train nor a broken rail is present in the track section in question. This is determined by the control/track current and voltage only, and independent of the local voltage.  
         [0019]     In addition, the AC voltage output of the control transformer  25  (which is proportional to the control/track current) is input into high speed solid state relay  60 . The solid state relay  60  in turn optically transmits a control signal based on the received input voltage signal to synchronous switching element  65 . As seen in  FIG. 2 , electronic vane relay architecture  20  also includes a local transformer  70  (connected to the track supply line wires such as line wires  6  and  7  in  FIG. 1  (not shown)) that operates as a step down transformer, i.e., it translates the local voltage into a lower proportional AC voltage. That AC voltage is input into the synchronous switching element  65 . Thus, the synchronous switching element  65  has input thereto a control signal that controls the switching of the synchronous switching element  65  based on the AC voltage that is proportional to the control/track current and an AC voltage that is proportional to the local voltage. The solid state relay  60  and the synchronous switching element  65  together operate as a synchronous rectifier. As used herein, the term synchronous rectifier shall refer to a device that takes two AC signals having the same frequency as inputs and outputs a first signal (e.g., half wave rectified) having a first polarity if the signals are in phase and a second signal (e.g., half wave rectified) having a second, opposite polarity if the signals are 180° out of phase. Other phase relationships (e.g., 90° out of phase) will result in a signal of mixed polarity (wherein the average voltage for a 90° out of phase condition will be zero). Thus, synchronous switching element  65  outputs a signal having a first polarity (e.g., positive) if the AC voltage that is proportional to the control/track current and the AC voltage that is proportional to the local voltage are in phase with one another, and a signal having an opposite polarity (e.g., negative) if the AC voltage that is proportional to the control/track current and the AC voltage that is proportional to the local voltage are  1800  out of phase with one another. In particular, synchronous switching element  65  includes a switching mechanism that turns on and gates the AC voltage that is proportional to the local voltage through during the positive half cycle of the AC voltage that is proportional to the control/track current, and turns off and gates nothing through during the negative half cycle of the AC voltage that is proportional to the control/track current.  
         [0020]     The output of synchronous switching element  65  is input into averaging filter  75 . Averaging filter  75  produces a DC output signal that is an average of the AC voltage signal that is input thereto. It should be noted that the inductance value of the inductor forming a part of the averaging filter  75  is extremely high, e.g., on the order of 50-150 Henries. The use of averaging filter  75  is preferred to ensure that level detector  80 , described below, responds only if the synchronous switching element  65  is operating properly. If a peak filter (simple capacitor) were used instead, a sequence of component failures could satisfy the level detector  80  regardless of phase differential between the control/track current and the local voltage. Of chief concern is a series of component failures, each of which is undetectable, followed by a shorted synchronous switching element  65 , the consequence being continued operation regardless of phase reversal and therefore continued operation of the output in the event of a failed insulated joint.  
         [0021]     The voltage signal output by the averaging filter  75  is input into level detector  80 . As described above in connection with level detector  50 , level detector  80  will output an oscillating signal, e.g., a square wave, only when the voltage output by averaging filter  75  meets or exceeds a predetermined threshold voltage value, and will output nothing if the voltage output by averaging filter  75  is less than the predetermined threshold voltage value (including negative values). The predetermined threshold voltage value is chosen to correspond to the voltage that would be output by averaging filter  75  in cases where the AC voltage that is proportional to the control/track current and the AC voltage that is proportional to the local voltage are nominally in phase, meaning they are in phase within a particular predetermined amount. For example, the AC voltage that is proportional to the control/track current and the AC voltage that is proportional to the local voltage may be considered to be nominally in phase when they are at least within 45° of one another. Such tolerance to phase shift is necessary because in operation, the control input will typically lag the local input due to the inductance of the rails. Thus, as will be appreciated, level detector  80  will output an oscillating signal only when the AC voltage that is proportional to the control/track current and the AC voltage that is proportional to the local voltage are nominally in phase, because it is only then that output of synchronous switching element  65  and averaging filter  75  will output a positive polarity signal of sufficient magnitude.  
         [0022]     Diodes  85 ,  90 ,  95  and  100  are coupled to local transformer  65  as shown in  FIG. 2 . Diodes  85 ,  90 ,  95  and  100  produce positive and negative rectified (DC) voltage sources (+L) and (−L).  
         [0023]     The output level detector  80  and positive source +L are input into a half-wave doubler circuit  105 . As is known in the art, a half-wave doubler circuit receives a DC source voltage and produces a DC output voltage of polarity opposite (the magnitude may differ) to that of its source voltage only if it also receives a second, oscillating input such as a square wave input. If either the source voltage or the oscillating input is missing, the half-wave doubler circuit outputs nothing. Essentially, a half-wave doubler circuit functions as a fail-safe AND gate, i.e. it produces an opposite polarity output only with an oscillating input AND an applied source voltage. Both inputs, oscillating and source voltage, are required for an opposite polarity output. With only one of the two inputs, no component or combination of component failures can produce an opposite polarity output. Examples of suitable half wave doubler circuits for use in the present invention are shown in  FIGS. 3A and 3B .  
         [0024]     Thus, in  FIG. 2 , half-wave doubler circuit  105  will output a negative polarity voltage signal only if it receives an oscillating signal from level detector  80 . As noted above, level detector  80  will output an oscillating signal only when the AC voltage that is proportional to the control/track current and the AC voltage that is proportional to the local voltage are in phase. Thus, the output of half-wave doubler circuit  105  is an indicator of the phase relationship between the control/track current and the local voltage, i.e., if a negative polarity voltage signal is present at the output of the half-wave doubler circuit  105 , it means the two currents are nominally in phase.  
         [0025]     The output of half-wave doubler circuit  105  is input into optical isolator/vital AND gate  55 . As noted above, optical isolator/vital AND gate  55  also receives the output of level detector  50 . The optical isolator/vital AND gate  55  will output an oscillating signal only if: (i) the output of level detector  50  is an oscillating signal (meaning that the control/track current has been determined to be of sufficient magnitude to indicate that no train is present and not rail is broken), AND (ii) the output of half-wave doubler circuit  105  is a negative polarity voltage signal (meaning that the control/track current and local voltage are nominally in phase and thus there is not an insulated joint fault condition); if either signal is missing, then optical isolator/vital AND gate  55  outputs nothing.  
         [0026]     The output of optical isolator/vital AND gate  55  is input into half-wave doubler circuit  110 . Also input into the half-wave doubler circuit  110  is the negative source (−L) generated by diodes  85 ,  90 ,  95  and  100 . The half-wave doubler circuit I  10  will output a positive polarity signal only if the input it receives from optical isolator/vital AND gate  55  is an oscillating signal. Thus, a positive polarity signal output from the half-wave doubler circuit  110  proves that the control/track current is greater than the predetermined threshold value (e.g., 50 mA) and that the local AND control/track currents are nominally in phase. Because control/track input current magnitude and phase comparison are independently determined according to the present invention, sensitivity to control/track current is not influenced by phase or magnitude of the local input.  
         [0027]     The output of the half-wave doubler circuit  110  may be used directly as an input to the railroad signaling system in question, wherein it acts as the vital relay output. However, in most applications, the output of the half-wave doubler circuit  110  is not of sufficient power for that purpose. Thus, as an alternative, as shown in  FIG. 2 , the output of the half-wave doubler circuit  110  may be input into output circuitry  115  that acts as a power conversion circuit for driving relay  120 , such as a PN- 150  relay manufactured by the assignee of the present invention. The output circuitry  115  includes pulse width modulator  125  and power switching FETs  130  that drive a transformer  135 . Output of half-wave doubler # 1  furnishes the control voltage to the 1524 control chip; it produces the gate signals that alternately turn on the FETs driving the transformer. In this case, the output power of the half-wave doubler circuit  110  is insufficient to also act as the source for the power switching FETs  130 , and therefore the positive source +L is used as the source for the power switching FETs  130 .  
         [0028]      FIGS. 4A and 4B  are a schematic diagram of the electronic vane relay architecture  20  according to one particular embodiment of the present invention. Specifically,  FIGS. 4A and 4B  show one particular implementation of the solid state relay  60  and the synchronous switching element  65 . The elements identified as K 1  and K 2  are solid state relays, such as the model AQV225 relay sold by Aromat Corporation of New Providence, New Jersey. Switching speed is preferably on the order of about 100 microseconds. A diode symbol bounded by terminals a and b is intended as a representation of the control element, and the mechanical switch enclosed in the dashed box is intended as a representation of the switching element (pseudo contacts). Current through terminals a and b effectively produces a low resistance connection from terminals c to d. The control element and the switching element (pseudo contacts) share no common connection; they are isolated, being optically coupled.  
         [0029]     On each positive excursion at A of the control transformer  25 , the pseudo contact of K 1  closes and a 200 microsecond negative going pulse is delivered to terminal e of the 555 timer  140 . In response, its output on pin f switches high for a much greater time, controlled by the resistor and capacitor connected to pins g &amp; h. Additionally, the pseudo contacts of K 2 , coupled to 12V to 5V DC to DC converter  143 , close and gate the triac  145  ON. The time is set to approximately 75% of one half the period of the operating frequency. For example, for 50 or 60 Hz applications, the time is set to approximately 6 milliseconds, and for 100 Hz applications, the time is set to approximately 3 milliseconds. It is expected B of the local transformer  75  is nominally in phase with A of the control transformer  25  and therefore the triac  145  is turned ON for a substantial portion of time during which A and B are simultaneously positive. It is important that the gate of the triac  145  be switched OFF before the negative half-cycle of A is initiated otherwise it will remain ON continuously and disrupt the synchronous rectification process. Limiting ON time of the gate of the triac  145  also allows for a moderate amount of phase shift of local with respect to control. By limiting the ON time of the gate of the triac  145  to 75% of one-half the period of the operating frequency, a phase shift of approximately 45 degrees can be tolerated. In track circuit operation, the control input will lag local due to inductance of the rails, and therefore, tolerance to phase shift is necessary. Thus, if A and B are nominally in phase, a positive voltage the level detector  80  and the half-wave doubler  105 , thereby producing a negative polarity voltage signal that, as discussed above in connection with  FIG. 2 , confirms the phase relationship of A and B.  
         [0030]     If A and B are out of phase, meaning they are not nominally in phase, which can occur resulting from a failed insulated joint  3 , synchronous rectification will produce a negative voltage. The level detector  80  will not respond to a negative voltage and thus the failed insulated joint  3  is detected because the relay  120  will de-energize.  
         [0031]     It is shown that synchronous rectification is critical to the task of ensuring that local and control are essentially in phase but it is of great importance to ensure no component failures can mask the in phase relationship. For example, if the triac  145  or gate control thereof fails and the triac  145  is either shorted or ON continuously, the voltage at A 1  will revert to a sine wave instead of half-wave rectification. In that event, the averaging filter  75  will average the input to the level detector  80  to zero volts; the level detector  80  will not respond and the relay  120  will de-energize. The averaging filter  75  was selected instead of a peak filter because its failure modes preclude the possibility of continued operation with a series of component failures.  
         [0032]     Peak filtering is realized with a single capacitor. Under normal conditions, a peak filter will produce a much greater voltage than an averaging filter like averaging filter  75 . However, the level detector  80  can be scaled to operate at a significantly greater voltage and, therefore, a peak filter could operate just as well. Sequential component failures are undetectable, however, and can result in inability to detect phase reversal as the result of a failed insulated joint  3 . With a single capacitor, if either connection opens, the level detector  80  input will revert to positive half sine wave DC pulses. The level detector  80  will respond with an output approximately one-half the time corresponding to each input pulse. The pulsating response will ripple through to the final output stage (I  15 ), producing reduced voltage to the relay  120 , but still sufficient to retain the relay  120  in an energized state. Thus, an open lead to the peak filter capacitor is undetectable. Subsequently, if the triac  145  or control thereof fails and it is continuously in conduction, the level detector  80  will continue its pulsating response with the relay  120  held energized. If phase reversal then occurs as a result of a failed insulated joint  3  it will be undetected and jeopardize integrity of the signal system.  
         [0033]     With the averaging filter  75 , disconnection of either capacitor thereof decreases the voltage to the level detector  80  and response to an AC input averages an output to zero. If both capacitors open, the net DC that is produced will decrease, but the level detector  80  will marginally continue to function. Thereafter, if the triac  145  shorts, it will provide an AC input to the averaging filter  75 , which in turn will produce an output that is well blow the level required by level detector  80  (it will be at or near zero). The failure of triac  145  is thus detectable.  
         [0034]     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the breadth of the claims appended in any and all equivalents thereof.