Abstract:
A system is disclosed for verifying the operativeness of a crowbar circuit which normally protects an electrical device in a series circuit with a power supply. The crowbar circuit includes means for sensing fault current in the series circuit and a crowbar switch responds to the sensing means for protecting the electrical device by directing the fault current away from the electrical device. The system includes verification testing means including a fuse and actuatable pneumatic switch connected together in series for, when said pneumatic switch is actuated, providing a short circuit across the crowbar switch to verify the operativeness of the crowbar circuit which, if not operative, causes the fault current to flow through and blow out the fuse.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the art of crowbar circuits and, more particularly, to verification of the operativeness of such a circuit. 
     BACKGROUND OF THE INVENTION 
     Crowbar circuits are known in the art and are typically employed for protecting an electrical device from damage by high currents resulting from fault conditions. For example, an inductive output tube (IOT) is frequently employed in UHF television transmitters. Such an IOT is connected to a high voltage (HV) power supply and may suffer adverse damage from a sudden high current resulting from internal tube arcing. Under such conditions, an unprotected IOT will draw excessive current from the HV power supply causing possible damage to the tube. 
     A crowbar circuit serves to detect a sudden rise in current drawn from the HV supply due to fault conditions. This will cause an electronic switching device, such as a deuterium thyratron, to be turned on and it serves to direct the fault current from the supply away from the IOT to prevent damage. When the crowbar switching device, thyratron, is turned on it informs an amplifier controller and the controller causes a circuit breaker to open disconnecting the HV power supply from its AC line voltage source. 
     The thyratron is connected directly across the HV supply and, hence, when it is turned on it provides essentially a short circuit across the IOT. An electrode, such as the anode, of the thyratron is connected to earth ground. In order to determine whether the crowbar circuit is operative, a fuse wire may be connected between the cathode of the thyratron and a point that may be shorted to ground quickly. 
     A crowbar circuit verification device known in the prior art is illustrated in FIG.  1  and it includes a fuse wire together with a vacuum switch, which, when closed, provides a short circuit around the thyratron to direct fault current to ground. The fuse wire together with the vacuum switch are disclosed in detail in FIG. 2 which will be described in greater detail hereinbelow. This vacuum shorting switch of the prior art has several shortcomings including the fact that it requires an external power source to operate the switch. Additionally, this form of a shorting switch cannot normally be installed inside a typical HV compartment because it requires transmitter interlocks to be defeated. The shorting switch does not provide positive provision for connecting the fuse wire. The switch is bulky and difficult to transport and is relatively expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide means for verifying the operativeness of a crowbar circuit which does not require an external power source to operate the crowbar shorting switch. 
     In accordance with the present invention, a system is provided for verifying the operativeness of a crowbar circuit that normally protects an electrical device located in a series circuit with a power supply. The crowbar circuit includes means for sensing fault current in the series circuit and a crowbar switch that responds to the sensing means for protecting the electrical device by directing the fault current away from the electrical device. The system includes verification testing means having a fuse and actuatable pneumatic shorting switch connected together in series so that when the pneumatic switch is actuated it provides a short circuit across the crowbar shorting switch to verify the operativeness of the crowbar circuit which, if not operative, will cause the fault current to flow through and blow out the fuse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and advantages of the invention will become more readily apparent from the following description of the preferred embodiment of the invention as taken in conjunction with the accompanying drawings, which are a part hereof, and wherein: 
     FIG. 1 is a schematic-block diagram illustration of a crowbar circuit including a prior art crowbar test fixture; 
     FIG. 2 is a schematic-block diagram of the crowbar test fixture of FIG. 1 in greater detail; 
     FIG. 3 is an end view illustrating one embodiment of the present invention; 
     FIG. 4 is an elevational view taken generally along line  4 — 4  looking in the direction of the arrows in FIG. 3; and 
     FIG. 5 is an end view looking generally along the line  5 — 5  in FIG. 4 looking in the direction of the arrows. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing the preferred embodiment, reference is first made to the crowbar circuit illustrated in FIG. 1, followed by a description of the prior art vacuum shunting switch shown in FIG.  2 . 
     Reference is now made to FIG. 1 which illustrates a crowbar circuit known in the art. This circuit includes a high voltage power supply  10  which may provide 36 kilovolts (kv) direct current from an AC three phase source  12  interconnected with the supply by means of a typical circuit breaker  14 . An energy storage capacitor  16  is connected across the output of the power supply  10 . A current limiting resistor  18  is connected in the series circuit with a device to be protected taking the form of an inductive output tube (IOT) which is used as an RF amplifier in a UHF transmitter. The IOT tube  20  may be driven from an RF driver source  22  for supplying power to an RF load  24 . The current limiting resistor  18  is connected to the cathode circuit of the IOT  20 . The crowbar circuit includes a current sensor taking the form of a current transformer  30  located in the series circuit for sensing the level of the current flowing therethrough. This transformer is coupled with a controller  32  that monitors the magnitude of the current sensed by the transformer  30  and if the current is sufficiently high, then it is deemed to be a fault current representative of fault in the series circuit. This fault may be an arcing taking place in the IOT  20  or an HV cable fault. The crowbar circuit serves upon detection of this condition to actuate a shunt switch which may take the form of a deuterium thyratron  50  having its anode connected to earth ground  52  and its cathode connected to the cathode of the IOT  20 . Whenever the thyratron  50  is triggered on as result of sensed fault current, the fault current is diverted away from the IOT  20  and instead flows through the thyratron to earth ground. 
     Whenever the thyratron is turned on by the control circuit  32 , the control circuit notifies an amplifier controller  33  that the thyratron has been turned on and the amplifier controller  33 , in turn, opens the circuit breaker  14  to disconnect the power supply  10  from the AC voltage source  12 . 
     In order to verify that the crowbar circuit is operative, the prior art is provided a verification circuit  60  which is connected between earth ground  52  and the cathode of the thyratron  50 . This is a series circuit and includes a fuse  62  and a vacuum shunt switch  64 . Closure of this shunt switch causes a short circuit across the thyratron. This results in a sudden increase in current flowing through the series circuit. This is sensed as a fault current by the current transformer  30  and the controller  32  attempts to turn on the thyratron  50 . If the thyratron  50  does not turn on, then the fault current will flow through fuse  62  and cause the fuse to blow. 
     Reference is now made to FIG. 2 which illustrates the prior art verification testing circuit  60  in greater detail. Circuit  60  includes a fuse  62  and a vacuum enclosed switch  64  connected together in series. The series circuit has clip connectors  80  and  82  at either end which may be used to clip the circuit between circuit ground  52  and the cathode of tubes  20  and  50 . The vacuum switch  64  takes the form of a vacuum enclosed high voltage direct current contactor which may obtained from Jennings Corporation of San Jose, Calif. and known as their model No. RP101F. This device may be represented as shown in FIG.  2  and includes a sealed vacuum enclosure  100  connected to a closed actuator housing  102  by means of an insulated support post structure  104 . The vacuum enclosure  100  serves as a housing for a pair of spaced stationary electrical contacts  110  and  112  together with a movable contact  114  which, when actuated, completes a short circuit between stationary contacts  110  and  112 . The movable contact  114  is connected by a drive post  116  to a vacuum sealed bellows arrangement  120  and, thence, to the actuator housing  102 . The drive post  116  may be of insulated material such as plastic, although the contact  114  is of electrically conductive material. The drive post  116  is of magnetic material for its portion  122  within the actuator housing  102 . 
     The housing  102  includes a solenoid coil  130  which surrounds the magnetic portion of the length of the drive post within the actuator housing and a pair of wires that extend out through the housing to a  115  volt AC voltage supply source  132  for actuating the coil  130  upon closure of a switch  134 . 
     Closure of the switch  134  by an operator causes the drive post  116  to drive the movable contact  114  into engagement with the stationary contacts  110  and  112  to complete a short circuit with the fuse  62  across the anode to cathode circuit of the thyratron  50 . 
     The shortcomings of the prior art crowbar verification circuit of FIG. 2 include the required external power source  132  and electrical switch  134  in order to operate the vacuum switch. Also, the switch has problems when installed inside an HV compartment because the required transmitter interlocks are defeated. Also there is no positive provision for connecting the fuse  62 . 
     The present invention is directed toward improvements over that of FIG.  2 . The crowbar test fixture as shown in FIGS. 3,  4  and  5  herein is constructed primarily of non-conductive materials and the operation of the switch is pneumatic and therefore the fixture requires no external power source or conductor wiring outside of the transmitter cabinet. As will be noted, a pulse of air is supplied by manually actuating a foot-pedal type air pump. This pulse of air travels through non-conductive hose and tubing to a cylinder-piston assembly. The piston has electrically conductive contact surface that is connected by way of a high voltage wire to earth ground. The air pulse causes the piston to quickly rise and make engagement with a stationary contact on the fixture. The stationary contact is connected to one end of the fuse wire. The other end of the fuse wire is connected to a lower fuse wire terminal and, in turn, is connected to the cathode of the thyratron  50 . This fixture is constructed such that it can be placed inside a high voltage compartment. Voltage standoffs are such that the fixture can safely be set onto a grounded or a high voltage surface. The tubing is of sufficient length and dielectric resistance to allow the foot pedal to be placed safely outside the high voltage compartment. The diameter of the tubing is relatively small and this makes it possible to close safety panels and take full advantage of all safety interlocks during the test. 
     Reference is now specifically made to FIGS. 3,  4  and  5  which illustrate the preferred embodiment of the crowbar test fixture in accordance with the present invention. This crowbar test fixture  60  includes a pneumatically operated switch and a fuse in a series circuit that schematically takes the form as illustrated with reference to the fixture  60  in FIG.  1 . However, fixture  601  differs substantially from fixture  60  in FIG.  2 . 
     The crowbar test fixture construction in accordance with the preferred embodiment of the present invention includes a horizontal support  200  and a vertically extending mounting panel  202  suitably secured to the support  200 . Support  200  and panel  202  are preferably constructed of non-conductive materials, such as plastic or glass. A hollow plastic tube  210  is mounted to the mounting panel  202  by means of a pair of suitable nut and bolt assemblies  212  and  214 . The upper end  216  of tube  210  is open and the lower end is connected to a suitable fitting  218  which, in turn, is connected by way of a hose  202  to a foot operated air pump  204 . 
     A second hollow plastic tube, hereinafter referred to as piston  230 , coaxially surrounds tube  210  and has an inner diameter which is somewhat greater than that of the outer diameter of tube  210 . The upper end of piston  230  carries a cap  232  which is made of copper or another suitable electrically conductive material. The cap  232  serves as a movable electrical contact during operation. Spaced upwardly from cap  232  there is provided a stationary electrical contact  240  which may take the form of a suitable bolt  242  or the like which is mounted to and extends through the panel  202  and held in place with suitable nuts on both sides of the panel. The bolt  242  defining the upper contact  240  extends through the panel and the opposite end thereof (the left end in FIG. 3) is provided with a nut  244  and another nut  246 . 
     At the lower end of mounting panel  202  a bolt  250  extends through the panel and one end of an electrical cable  252  is electrically and mechanically connected to the bolt  250  by suitable nuts. The distal end of this bolt (as viewed in FIG. 3) carries a nut  254  and another nut  256 . The fuse wire  621  is mounted to the bolts  242  and  250  by backing away the nuts  246  and  256  from the nuts  244  and  254 . At least one turn of the wire is wrapped around each of the bolts  242  and  250  and then the nuts are tightened to secure the fuse wire in place. 
     The unfastened end of the electrical cable  252  is provided with a suitable electrical clip  270  which is used to connect the end of the cable to the cathode side of the crowbar circuit of FIG.  1 . 
     The copper cap  232  is electrically connected to a terminal post  300  by means of a flexible length of cable  302  of sufficient length to accommodate the movement of the cap  232  between its lowermost position, as shown in the drawings, and its uppermost position when it engages stationary contact  240 . An electrical cable  304  is connected from the terminal  300  to ground, as with the use of a suitable electrical clip  306 . 
     The operative parts of the test fixture may be enclosed in a transparent plastic housing. During assembly the fuse wire may take the form of a 36 gauge wire having a length on the order of 11.8 inches. 
     During the verification test of the crowbar circuit, it may be desirable to disconnect the IOT  20 . It is suggested that the crowbar circuit be tested whenever a new IOT tube is to be installed and thereafter on an annual basis or when in doubt of proper crowbar circuit operation. 
     Although the invention has been described in conjunction with a preferred embodiment, it is to be appreciated that various modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.