Abstract:
A ground fault interrupter to be used by utility company while effecting repairs to the electrical service for a building is positioned to interrupt the power supply to the building in case of a detected ground fault and utilizes a sensor for detecting the fault current at the service entrance to a building; a contact switch, selectively movable between open and closed positions, mounted for temporary use in series with said power supply to the building; and a microprocessor based circuit for measuring and evaluating fault current detected by the sensor and controlling the selective movement of the contact switch between its open and closed positions.

Description:
RELATED APPLICATION 
     This application claims priority to U.S. provisional patent application no. 61/051,170, filed May 7, 2008, which is incorporated herewith by reference. 
    
    
     BACKGROUND 
     The present invention relates to electrical service to a building and more particularly to monitoring and controlling the electrical service to a building during periods of repair to the electrical service. More particularly, the presentinvention is a ground fault interrupter device to be installed by an electric utility company while temporary equipment of the company is in place to restore a subscriber&#39;s power after a wiring failure in the underground power line feeding the building. In even greater particularity, the ground fault interrupter of the invention is to be installed at the subscriber&#39;s house or other building between the power meter and the power meter socket. 
     OBJECT OF THE INVENTION 
     It is an object of the present invention to reduce the possibility of ground fault current flowing on the cable TV, gas, water, or telephone lines instead of over the building neutral connection in situations when that neutral connection fails. 
     It is another object of the invention to provide a temporary monitoring device to detect and prevent ground fault current over the building service during service repair operations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The interrupter is depicted in the appended drawings which form a portion of this disclosure and wherein: 
         FIG. 1  is a side elevation view of the collar housing the interrupter; 
         FIG. 2  is a pictorial side elevation view of the interrupter on the side adjacent the building meter socket; 
         FIG. 3  is a pictorial side elevation view of the interrupter facing the power meter; 
         FIG. 4  is a plan view of the fault current sensor; 
         FIG. 5  is a schematic diagram of the electrical circuit of the interrupter; 
         FIG. 6   a  to  6   h  are flow charts of the main loop of the interrupter control process and of the subroutines of the interrupter control process. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings for a clearer understanding of the apparatus, it will be understood from  FIG. 1 to 3  that an electric utility company utilizes the present apparatus with a residence or other building to monitor and control the service to the building at the power meter location. To utilize the apparatus, an existing power meter is removed from its complementary existing meter socket on the building. An appropriately sized collar  11 , typically made of fiberglass-reinforced polycarbonate, housing the interrupter circuit  12  is installed in the meter socket making electrical contact with the building wiring through contacts  101   s  to  104   s , and then the existing meter is re-installed into collar  11 , making electrical connection with the contacts  101   m  to  104   m . A mechanically-held 200-amp two-pole contactor  18  , such as a BLP 200 Amp rated model shown in  FIG. 3 , which has two coils- one for OPEN and the other for CLOSE, is mounted in collar  11  and is used to interrupt service to the building if a ground fault is detected. Contactor  18  is connected to the control circuit via connector J 1 . 
     Fault current sensing is accomplished by means of a split-core current transformer  13 , shown in  FIG. 4 , that is installed around the conduit providing the electrical service feeding the building. This current sensor may be a flexible Rogowski coil adapted for engagement about said service entrance. Transformer  13  can be used with both PVC and metallic conduit. As shown in  FIG. 4  transformer  13  is formed with two halves  13   a  and  13   b  which are held together using neodymium magnets  14  rather than a more complex latching assembly. The sensing transformer is connected to the interrupter circuit  12  by line  13   c  and plug  13   d.    
     Referring to  FIGS. 2 &amp; 5  , the signal from current sense transformer  13  is fed via plug  13   w  to a shunt resistor R 11 , then to a low-pass filter, and then to an operational amplifier U 3 , such as a MCP602 available from Microchip Technology. The output of the operational amplifier is AC coupled into a peak detector. The signal from the peak detector is fed into an analog to digital converter at A 4  in a microprocessor U 1 , such as a PIC 16F88. Microprocessor U 1  controls contactor  18 . The PIC microprocessor is programmed via connector J 2 . 
     The circuit includes a wide-input 15 volt DC supply U 2 . The wide input is necessary so it can run on either 120 or 240 volts. The input to the supply is connected across both 120 volt legs, but it must be able to operate if one leg is faulty. The voltage from the power supply is reduced to 5 volts at U 4 , which may be a 78LO5UA voltage regulator, to supply microprocessor U 1  and operational amplifier U 3 . A large capacitor C 2 /C 3  is connected across the supply to microprocessor U 1  to enable the microprocessor&#39;s internal timer to function in the absence of input power. 
     A bank of storage capacitors C 4  and C 5  connected to the 15 volt output of power supply U 2  through an appropriate current limiting circuit. The energy stored in these capacitors is used to operate mechanical two-pole main contactor  18  in the absence of input power. Two MOSFETS Q 1  &amp; Q 2  driven by the microprocessor at A 1  and A 0  dump the charge from energy storage capacitors C 4  and C 5  into contactor  18  via J 1  to cause it to operate in the absence of actuating power from power supply U 2 . 
     A pushbutton P 3 , mounted on collar  11 , is connected to microprocessor U 1  at B 3 , and two status-indicator LED&#39;s—one red D 6  nd one green D 5  also mounted on collar  11 , are connected to the microprocessor U 1  at B 0  and B 1 . An optically-isolated circuit D 1 /D 7 /U 5 , using a device such as a 4N38 phototransistor-type optically coupled opto-isolator, monitors line voltage and presents a pulse train to the microprocessor any time the input voltage is above a specified minimum value. 
     The operational features of the circuit described above are described and depicted in the flow charts of  FIGS. 6   a  to  6   h.    
     The contactor  18  is always in the OPEN position when power is initially applied, thus in  FIG. 6   a  at step  201  the microprocessor U 1  will first attempt a self test. Specifically, the microprocessor checks at  201  as to the state of the contactor  18 , at  202  as to the presence of the current sensing transformer  13 , at  203  as to AC voltage ensure that it is above a preset minimum value. Assuming the AC is OK, then microprocessor U 1  at step  204  checks its nonvolatile memory to determine whether the main contactor was open or closed the last time the interrupter was used; i.e. was the power to the subscriber off or on. If it was closed, power ON, the sub routine InitClose is initiated. If it was open, power off, the last time the interrupter was used, subroutine InitOpen is initiated at step  205 . If the AC power is not OK, then the red LED D 6  is illuminated and green LED D 5  is turned off. 
     Once the startup conditions are satisfied, the contactor is closed, current imbalance monitoring begins as shown in  FIG. 6   b . During startup and while the Interrupter is in monitoring mode microprocessor U 1  is constantly checking at step  501  to ensure that the current sensor  13  is still attached. If current sensor  13  is removed, the contactor  18  immediately is signaled to open and the red and green LEDs alternate rapidly as shown in  FIG. 6   c.    
     In the ground fault monitoring process, after checking the AC input at  502 , the microprocessor U 1  converts the voltage from the analog processing circuitry of sensor  13  and U 3  described above to a digital value at  505 . This digital value is then averaged over several samples at  506  and then compared to a preset threshold at  507 . If the threshold is exceeded, the trip sequence is started at  508  whereupon a signal is sent to open the contactor, as shown in the trip sequence in  FIG. 6   d . At the same time, the microprocessor writes the new “tripped” state to its nonvolatile memory and blinks the red LED D 6  rapidly. An internal trip counter is also incremented. The user is allowed to re-close the contactor by pressing and holding the pushbutton. This process may be repeated a limited number of times, for example 3 times depending on the selected programming, and then it locks out the contactor until input power is removed. 
     In addition to monitoring for current imbalance using the input from current sensing transformer  13 , if the input voltage drops below a preset minimum, usually 180 volts, as indicated by optically-isolated circuit D 1 /D 7 /U 5 , for longer than a preset time, usually two seconds, the microprocessor will send the signal to open the contactor, as shown in  FIG. 6   e . Once the input voltage returns to a normal value the contactor will re-close. This feature allows power to the building to automatically restore itself after brief outages or brownouts. 
     If the check of the AC input at  502  indicates that AC power has been lost while the service was running properly, the lost AC subroutine shown in  FIG. 6   f  is initiated. 
     As shown in  FIG. 6   g , if power is ok and the state was ON when the interrupter was last used, at step  301  microprocessor causes the green LED to blink rapidly to indicate that the contactor is about to reclose. After a predetermined time (usually 5 seconds) has elapsed during which time the power is checked again at step  302 , microprocessor U 1  turns on the close coil command to Q 1  or Q 2  at step  303 , clears the averaged current value in memory at  304 , closes the main contactor and checks to see if power was lost when the coil close command is turned off at  308 . If power is lost the open coil command to Q 1  or Q 2  is turned on at  310 , the new contactor open state is written to memory at  311 , average current values are cleared, the green LED D 5  is turned off at  313 , the low voltage trip count is incremented at  314 , and the open coil command to Q 1  or Q 2  is turned off. The routine then returns to start. If power is not lost, the green LED D 5  is illuminated steadily and the microprocessor iterates to the monitor subroutine. As indicated in  FIG. 6   h , if power is ok and step  205  indicated that power was off when the interrupter was last used, then at step  401  microprocessor U 1  causes the green LED D 5  to blink slowly to indicate that it is permissible to close the contactor by pressing the push-button. If it is desirable to close the contactor  18 , the user will press and hold the pushbutton P 3  for a predetermined time (usually 5 seconds), during which time the AC is checked again at step  403 . During the time the pushbutton is held the green LED D 5  blinks rapidly to indicate that the contactor is about to close. At the end of this delay the microprocessor U 1  completes the routine and the contactor closes. LED D 5  is illuminated steadily. As with  FIG. 6G , if power is lost the microprocessor goes through a routine to open the contactor and returns to START. 
     In the event that one or more of the power legs has a resistive fault that allows the voltage to drop when the contactor closes, the contactor could continue to cycle on and off. This is obviously undesirable, so software has been included in the microprocessor to detect this situation and lock out the contactor after three tries as shown in  FIGS. 6   g  and  6   h . In the event of this type of lockout, the green LED extinguishes and the red LED gives a repeating double blink per  FIG. 6   f.    
     Additionally, we have provided a means of manually turning off power to the building while everything is operating normally by pressing the pushbutton while the green LED is lit as illustrated in  FIG. 6   b . This causes the contactor to open and the green LED will blink slowly, indicating that it is permissible to reclose when desired by pressing and holding the pushbutton. 
     The foregoing features and embodiments are presented by way of illustration rather than limitation therefore the appended claims should be considered as defining the proper scope of the invention.