Abstract:
An apparatus and a method of controlling a load in response to a ground fault condition includes measuring a ground fault alternating current flowing from the load. A real part and an imaginary part of the ground fault alternating current is ascertained. Electrical power is removed from the load and/or the ground fault condition is indicated to a user if a magnitude of the real part exceeds a first predetermined threshold and/or a magnitude of the imaginary part exceeds a second predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ground fault protection apparatus and method, and, more particularly, to a ground fault protection apparatus and method for preventing shock and/or equipment damage. 
     2. Description of the Related Art 
     The U.S. National Electrical Code (NEC) requires ground fault protection for both shock and equipment protection. Although shock protection requires a 6-milliampere limit, there is no NEC current limit for equipment protection. In the U.S., a figure of 30 milliamperes is commonly used and 100 milliamperes in Canada. 
     The are two types of ground fault protection apparatus. A ground fault circuit interrupter (GFCI) opens its branch circuit upon detecting a ground fault current exceeding a maximum limit. Current cannot be restored to the branch circuit until the GFCI is manually reset. GFCI applications include residential kitchens, outdoor applications, and bathroom branch circuits, including those for floor warming and heating. In residential applications, the GFCI limit is 6 milliamperes for personnel protection and 30 milliamperes for heating apparatus and equipment protection. 
     The second type of ground fault protection apparatus warns of a ground fault hazard but does not interrupt current flowing in a branch circuit. Warning is used only in fire protection applications where the ground current hazard is considered less dangerous than interrupting the current to pipe trace heaters that keep wet sprinkler systems from freezing. 
     The ground fault current is the vector sum of the currents flowing in a branch circuit. If there is no ground fault current flowing, the branch currents sum to zero. In the event of a ground fault current, the branch currents do not sum to zero. Their difference is the ground fault current. 
     FIG. 1 shows a balanced electrical branch circuit  10  with no ground fault current flowing. Circuit  10  includes input wiring  12  and a two-pole circuit breaker  14  providing over-current protection and circuit interruption. Two-pole circuit breaker  14  is required in electrical systems without a grounded neutral including low voltage (i.e., less than or equal to 600 volts AC) branch circuits using U.S. common distribution voltages. These include 240 volts single and three-phase along with 208 and 480 volts three-phase. U.S. distribution voltages with a grounded neutral include 120 volts single phase along with 277 volts three-phase. 
     A current i 1  flows to a load  16  through a ground fault protector  18 . Similarly, a current i 2  flows from the load  16  through ground fault protector  18 . In FIG. 1, no ground current flows. Thus, the sum of the currents i 1  and i 2  is zero. 
     FIG. 2 shows the case with a ground fault current i 3  flowing from a load  20 . FIG. 2 is identical to FIG. 1 except that the ground current i 3  flows to equipment ground. A branch circuit  22  must supply the ground fault current i 3 . Thus, the ground fault current i 3  equals the difference between i 1  and i 2 . 
     The ground fault current i 3 , expressed as a vector, has both magnitude and phase. This is caused by the fact that there is capacitance between the current-carrying branch circuit  22  and ground. The reactive, imaginary current component  24  (FIG. 3) flowing through the capacitance is at a right angle to the in-phase, resistive, real component  26 . Since capacitance is purely reactive, current flowing through it does not cause heating. Further, such capacitance is not indicative of a shock hazard. Thus, capacitance does not indicate a threat to either personnel or equipment. The resistive component, in contrast, does cause heat and is indicative of a threat to both personnel and equipment. So far as fire safety is concerned, only the real current causes heating. The imaginary component does not. 
     In a typical cable configuration heater  28  as is shown in FIG. 4, a heater wire  30  is surrounded by insulating material  32 . Failure of the heater&#39;s insulation  32  causes a substantial in-phase ground fault current to flow. A shield  34  provides fire safety by diverting current resulting from insulation or mechanical failure to the shield  34  which is connected to the safety ground (i.e., earth ground). Shield  34  conducts this current to safety ground, thus providing protection until the GFCI or ground fault protector  18  detects a ground fault current above a threshold value and interrupts current flow in the branch circuit  22 . Thus, the fire hazard is eliminated. 
     Heating cable  28  can be used for pipe trace heating, floor warming and heating, ceiling and wall heating along with many industrial applications for process heating. Although cable heaters employ a wide variety of construction schemes and insulating material, they all employ a grounded outer braided shield  34  or stainless steel or copper jacket as required by the NEC. This construction eliminates the fire hazard that would otherwise occur if insulation  32  failed for any of a variety of reasons. 
     FIG. 5 shows the equivalent lumped circuit of the heater and the elements causing the flow of the ground fault current i 3 . A substantial capacitance  36  between the heating element  30  and equipment ground (i.e., safety ground) exists that is proportional to the heater length. The application of supply voltage to the heating element  30  causes a substantial current to flow through this capacitance  36  to equipment ground. This represents a ground fault current i 3 . 
     A leakage resistance  38  and heater-to-shield capacitance  36  are shown as acting at the center of the cable heater  28 . This simplification is reasonable since the leakage resistance  38  and leakage reactance  36  are much greater than the heater resistances  40  and  42 . The leakage currents i 4  and i 5  flow into the equipment ground  44  (i.e., safety ground). 
     The vector sum of the currents i 4  and i 5  equal i 3  which is the ground fault current. From FIG. 5, it is shown that the ground fault current i 3  has two components: i 4  which is real and i 5  which is imaginary. The real component i 4  is in phase with the branch distribution voltage across input wiring  12 . The imaginary component i 5  leads the real component i 4  by ninety degrees. FIG. 3 shows the vector relationship between these currents when expressed as phasors. 
     The commonly used 30-milliampere GFCI setting for equipment protection does not eliminate the shock hazard. In heating applications, the 30-milliampere limit creates both economic and safety problems. The 30-milliampere GFCI setting limits the length of heater cable that can be powered by a single branch circuit—particularly at the higher distribution voltages of 277 and 480 volts (600 volts in Canada). The capacitance  36  between the shield  34  and the heater wire  30  is proportional to length, as is the ground fault current. The 30-milliampere setting is too high to provide shock protection. 
     What is needed in the art is a method of identifying the real and imaginary parts of a ground fault current. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for providing both shock and equipment protection in a single GFCI or ground fault protection device by rejecting or ignoring all or most of the ground fault current that is due to capacitance between the heaters and distribution bus wiring and ground. 
     The invention comprises, in one form thereof, a method of controlling a load in response to a ground fault condition. The method includes measuring a ground fault alternating current flowing from the load. A real part and an imaginary part of the ground fault alternating current is ascertained. Electrical power is removed from the load and/or the ground fault condition is indicated to a user if a magnitude of the real part exceeds a first predetermined threshold and/or a magnitude of the imaginary part exceeds a second predetermined threshold. 
     An advantage of the present invention is that it is possible to consider only the real part of a ground fault current when determining whether the ground fault current requires a response. 
     Another advantage is that it is practical to simultaneously provide ground fault protection to both personnel and equipment. 
    
    
     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 diagram of a known branch circuit with no ground fault current; 
     FIG. 2 is a schematic diagram of a known branch circuit with a ground fault current; 
     FIG. 3 is a phasor diagram of the ground fault current of FIG. 2; 
     FIG. 4 is a schematic, cross sectional view of a known heating cable; 
     FIG. 5 is a schematic diagram of a simplified equivalent circuit of the heating cable of FIG. 4; 
     FIG. 6 a  is a plot of the ground fault current of FIG. 2 versus time; 
     FIG. 6 b  is a plot of the voltage across the input wiring of the branch circuit of FIG. 2 versus time; 
     FIG. 6 c  is a pulse waveform indicative of the time difference between the ground fault current of FIG. 6 a  and the branch voltage of FIG. 6 b;    
     FIG. 7 is a block diagram of one embodiment of a ground fault protector of the present invention; 
     FIG. 8 is a schematic diagram of the microcontroller subsystem of FIG. 7; and 
     FIG. 9 is a schematic diagram of the power control and sensing system of FIG.  7 . 
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention accurately measures both the real and imaginary parts of the ground fault current. Although this measurement can be performed using analog or digital techniques, a digital method, as described herein, is simpler and lower cost. 
     Ideally, rejecting or ignoring the imaginary component i 5  of the ground fault current i 3  provides superior protection since the real component i 4  which causes heating is detected. Furthermore, sensitivity to the real component i 4  is such that the 6 milliampere personnel safety current limit can be maintained while providing a 30 milliampere, or higher limit for the reactive component. Thus, simultaneous equipment and personnel protection is both practical and possible. 
     By assuming sinusoidal steady state conditions, the real component i 4  and imaginary component i 5  of i 3  can be trigonometrically calculated from the magnitude of i 3  if the phase angle θ is known. The equations follow: 
     
       
         Re{i 3 }=i 3  cos θ 
       
     
     
       
         Im{i 3 }=i 3  sin θ 
       
     
     The phase angle θ is determined by measuring the phase shift between the ground fault current i 3  (FIG. 6 a ) and the branch voltage (FIG. 6 b ) across input wiring  12 . This can be accomplished by measuring the time difference between the positive going zero crossing of the ground fault current waveform (FIG. 6 a ) and the next positive going zero crossing of the branch voltage waveform (FIG. 6 b ). The phase angle θ is calculated using the following equation: 
     
       
         θ=360° (time difference) frequency 
       
     
     wherein 
     ‘θ’ is the phase shift in degrees; 
     ‘f’ is the power line frequency in Hertz; and 
     ‘time difference’ is the time between zero crossings in seconds. 
     The time difference is indicated by a width of each individual pulse in a pulse waveform (FIG. 6 c ). 
     Using the above procedure eliminates the need for high speed real-time calculation. Only the time difference needs to be measured. This is accomplished with a simple time period measurement that is a built-in function of most microcontrollers. After determining the time difference, the phase shift angle θ can be easily calculated from the expression show above. The values of sin θ and cos θ can be determined from a look-up table. Simple multiplication yields the values of Re{i 3 } and Im{i 3 }. 
     The above-described procedure for calculating the values of Re{i 3 } and Im{i 3 } is a simple and inexpensive method for obtaining the desired result without the need for high speed arithmetic. For example, these calculations can also be performed real-time using a digital signal processor. 
     One embodiment of a ground fault protector  44  (FIG. 7) of the present invention is shown attached to a load  20 . A power control and sensing subsystem  46  performs the higher level functions including power control and converting the ground fault current and branch circuit voltage into signal levels required by a microcontroller  48  (FIG. 8) of microprocessor subsystem  50 . Microprocessor subsystem  50  performs computational, timing, display and operator interface tasks. 
     Wires  52 ,  54 ,  56  and  58  conduct signals between power control and sensing system  46  and microcontroller subsystem  50 . Operating voltage and ground connections have been omitted for clarity. 
     The branch circuit connections are made through input wiring  12 . Load  20  is connected to system  46  through wires  60  and  62 . 
     Power control and sensing subsystem  46  is capable of sensing the ground fault current, functionally checking the operation of ground fault protector  44  and interrupting load current upon command. 
     A control transformer  64  (FIG. 9) reduces the branch voltage to a convenient value without introducing phase error. Its secondary voltage provides current for self-testing the ground fault protection function and the phase reference used to determine the real and imaginary components of ground fault current i 3 . 
     A double pole contactor  66  interrupts current to load  20  during self-test of ground fault protector  44  or in the event the ground fault current i 3  exceeds a preset value. Both sides of the branch circuit are broken. This is necessary to interrupt the ground fault current in power distribution systems without a grounded neutral, e.g., 240 volts 3-wire, 208 volts 3-phase and 480 volts 3-phase in the U.S. 
     Microcontroller  48  placing a logic voltage on input lead  56  to the gate of N-channel metal oxide field effect transistor NMOSFET  68  causes current to flow through the solenoid coil of double pole contactor  66 . This pulls-in or closes the contacts  70  of contactor  66 , thus applying the branch circuit voltage to load  20 . The contactor coil operates from the DC supply voltage V++. A diode  72  protects NMOSFET  68  from a destructive inductive voltage transient when interrupting current to the contactor coil. A pull-down resistor  74  prevents spurious contactor operation if the NMOSFET gate wire  56  is open-circuited, as is often the case during power-on initialization of associated microcontroller  48 . 
     A four-winding current transformer  76  performs a variety of functions. One function is summing the branch circuit currents flowing through single-turn winding number one  78  and single-turn winding number two  80 . Winding number three  82  is connected to current shunt resistor  84 . This causes a voltage to appear across shunt resistor  84  that is proportional to its value divided by the number of turns of wire forming winding number three  82 . Winding number four  86  is an auxiliary winding used for self-test purposes. 
     Microcontroller  48  placing a logic level voltage on input lead  58  to the gate of NMOSFET  88  causes current to flow through a coil of a relay  90 , which causes the relay&#39;s contact to close. This causes current supplied by control transformer  64  to flow through a limiting resistor  92  and thence through the self-test winding  86  of the four-winding current transformer  76 . This simulates a ground fault current above the threshold value. 
     The coil of relay  90  is supplied from the V++ voltage. A diode  94  protects NMOSFET  88  from a destructive inductive voltage transient when interrupting current to the relay coil. A pull-down resistor  96  prevents spurious relay operation if the NMOSFET gate wire  58  is open-circuited, as is often the case during power-on initialization of associated microcontroller  48 . 
     An operational amplifier  98  is configured as a non-inverting amplifier. Its voltage gain is determined by the ratio of its feedback resistors  100  and  102 . This assumes that the value of resistor  100  is much greater than the value of shunt resistor  84 . Thus, the output voltage of the operational amplifier  98  appearing at wire  54  is linearly proportional to the magnitude of the ground fault current. 
     An operational amplifier  104  is configured as a non-inverting amplifier. Its voltage gain is determined by the ratio of its feedback resistors  106  and  108 . The output voltage of control transformer  64  is buffered and reduced in amplitude by the voltage amplifier employing operational amplifier  104 . The buffered output appears at wire  52 . 
     FIG. 8 shows the microcontroller  48 , along with support and interface elements. Support elements include a crystal resonator  110  whose function is to provide a stable accurate clock frequency for microcontroller  48 . This insures the accurate timing functions required by the invention. 
     A supervisor support element  112  insures predictable start-up of microcontroller  48  upon the application of power. Supervisor  112  also prevents electrical transients from upsetting the operation of microcontroller  48 . Supervisor  112  asserts microcontroller restart by holding the microcontroller&#39;s RST input  114  high unless the power supply voltage is stable. Supervisor  112  asserts restart input high unless its ‘watch dog’ input  116  is toggled every 100 milliseconds or so. This prevents microcontroller  48  from latching and thus failing to perform its required functions. When microcontroller  48  periodically emits the ‘watch dog’ toggle, it insures that it is properly executing its program. 
     A switch  118  selects the response to a ground fault condition. The default condition is selected with the switch  118  open as is shown. The second response occurs if the switch  118  is closed. Resistor  120  provides a pull-up to V+. This provides a logic level change that is inputted to microcontroller port  122 . 
     A pushbutton switch  124  toggles the TEST/RESET of the ground fault protection function. A resistor  126  performs a pull-up function. Pushing switch  124  provides a logic level change at the microcontroller input port  128 . 
     A light emitting diode (LED) ground fault indicator  130  operates while a microcontroller port  132  is logically high. A resistor  134  sets ground fault indicator  130  to its design value. 
     A potentiometer  136  creates an adjustable bias voltage at an analog-to-digital converter (ADC) input port  138 . This bias sets the ground fault trip current value for the real part of the ground fault current i 3 . 
     Another potentiometer  140  creates an adjustable bias voltage at an ADC input port  142 . This bias sets the ground fault trip current value for the imaginary (i.e., capacitive) part of the ground fault current i 3 . 
     A microcontroller output port  144  provides an output signal for operating two-pole contactor  66 . Similarly, a microcontroller output port  146  provides an output signal for operating relay  90 . 
     Comparators  148  and  150  along with the NAND gate  152  generate a pulse proportional to the phase difference between the branch circuit voltage and the ground fault current i 3 . The pulse (FIG. 6 c ) is inputted to a microcontroller timer port  154 . 
     Microcontrollers commonly provide a facility for measuring the period of an external waveform. This is accomplished by gating a train of internally generated pulses, derived from the microcontroller&#39;s crystal-controlled clock, into an internal register. The external signal controls the gate. For example, the gate could open on the leading edge of the external waveform and close on the trailing edge. The contents of the register, which is proportional to the period of the external waveform, can be transferred to the microcontroller&#39;s program counter, or equivalent register. 
     Either of two ground fault protection responses are provided to a ground fault condition. Switch  118  selects the response. The default response is to maintain power interruption to the load  20  until the resetting of the ground fault condition, which is accomplished by pressing switch  124 . Leaving switch  118  in its normally open position selects this response. When selecting this mode, microcontroller  48  must store the information that a ground fault occurred in EERAM  156  or its equivalent (i.e., flash RAM). The U.S. and Canadian NEC require power interruption to the load after operating power is restored to ground fault protector  44  in the event of an interruption. 
     The second response is to indicate the ground fault condition while the ground fault condition exists. Power to the load  20  is not interrupted. No reset action is required if the condition clears. This response in enabled by closing switch  118  only in certain fire protection applications where the ground fault condition is a secondary consideration to maintaining load power. This is the case for heater controls in wet sprinkler systems. 
     If no ground fault condition exists, pressing switch  124  automatically verifies proper ground fault protection operation. Verification consists of a sequence of steps. Immediately after switch  124  has been pressed, two-pole contactor  66  is de-energized, if it is energized, thus removing power to load  20 . This removes external ground fault current to insure verification accuracy. 
     Next, relay  90  is energized, thus applying the test current to the current transformer winding four  86 . This current simulates a ground fault current above the real threshold value. The ground fault indicator  130  will operate for approximately two seconds. If the ground fault test fails, ground fault indicator  130  will flash continuously and two-pole contactor  66  will remain de-energized as is the case with the default response (i.e., when switch  118  is open). 
     If switch  118  is closed, thus selecting the warning mode, a verification failure is identified by ground fault indicator  130  continuing to flash. However, normal operation of two-pole contactor  66  will resume. 
     If the test is successful, relay  90  is de-energized along with ground fault indicator  130 . Next, two-pole contactor  66  resumes the state that it was in before switch  124  was pressed. A new test sequence cannot be initiated unless switch  124  has been released and is not pressed for two contiguous seconds. 
     Potentiometers  136  and  140  set the real and imaginary (i.e., capacitive) ground fault trip current levels, respectively. Currents exceeding these levels cause ground fault protector  44  to operate to remove power from load  20 . It is possible to make these calibrated adjustments accessible to maintenance personnel. Normally, potentiometers  136 ,  140  are used to calibrate ground fault protector  44  during manufacture. 
     The voltage between potentiometers  136  and  140  and ground is linearly proportional to the wiper position. This makes it possible to calibrate these adjustments. The hardware embodiment herein described operates from a single positive power supply. The AC voltages inputted to the microcontroller A-D input ports  138 ,  142  are half-wave. This reduces analog signal processing circuit complexity and costs (e.g., elimination of the need for a second negative power supply along with DC level shifting components). 
     The wiper voltages are encoded by the microcontroller&#39;s A-D converter and thereafter stored in specific random access memory (RAM) locations. The microcontroller inputs  138  and  142  are serviced by the internal A-D converter. 
     Determining the real and imaginary ground fault current values involves executing a sequence of processes. Conceptually, a process can viewed as being similar to a subroutine or subprogram. However, unlike a subroutine, a process can describe a sequence of steps that can execute as a program. The words “subroutine” and “process” are used interchangeably herein. 
     The first process includes the steps required to determine the magnitude of the ground fault current. The positive peak value of the ground fault current waveform (FIG. 6 a ) is measured since it is linearly proportional to the magnitude. 
     As is shown in FIG. 6 a , the ground fault current waveform is sinusoidal. Thus, its peak value occurs 90 degrees after zero crossing. This occurs at a point in time that is one-quarter of the period of the sinusoid after the zero crossing. With a 60 Hz power line frequency, the peak occurs approximately {fraction (1/240)} second (0.0041667 second) after the zero crossing. 
     Note that the derivative of the ground fault current waveform (FIG. 6 a ) with respect to time is zero at the 90 degree point. Thus, the amplitude of the ground fault waveform does not change rapidly with respect to time at the 90 degree point. A plus or minus one degree change at 90 degrees results in less than a minus 0.016% change in the peak value. Further note that one degree of phase shift at 60 Hz is 46.3 microseconds. 
     Measurement of the peak value of the ground fault current waveform connected to an input  158  is accomplished by triggering the microcontroller&#39;s A-D 0.0041667 second after the zero crossing. Microprocessor  48  provides timing capability for this purpose. Depending upon the resonator frequency selected, the delay time can be set with an uncertainty that is less than 40 microseconds. The A-D encodes the value at its input  158  when triggered. The encoding time can be up to 100 microseconds depending upon the resonator frequency. The encoded value is added to the contents of a specific RAM location. 
     Noise (i.e., uncertainty) in the ground fault current magnitude value could cause spurious ground fault protection operation. Filtering minimizes uncertainty. The ground fault current magnitude is filtered by adding the four most recent ground fault current magnitudes to the specific RAM location cited in the previous paragraph. After each fourth sample, the contents of this RAM location is shifted left twice. In effect, this divides the contents of the ground current magnitude location by four. The resulting value is taken as the ground fault current magnitude for another process. 
     FIG. 6 c  shows the pulse the duration of which is proportional to the phase difference between the branch voltage waveform (FIG. 6 b ) and the ground fault current waveform (FIG. 6 a ). At a branch supply of 60 Hz, the pulse width is 43.6 microseconds per degree of phase shift. The pulse is applied to the input  154  of microcontroller  48 . 
     Microcontroller  48  provides a facility for measuring the period of a pulse connected to the input port  154 . Microcontroller  48  does this by applying a gated periodic pulse train derived from its resonator controlled clock into an internal register that is configured as a counter. The period of the internal pulse train is less than the time interval of one degree of phase shift at the branch circuit frequency. For example, a period of less than 40 microseconds is adequate for 60 Hz since this provides a resolution that is better than one degree. Thus, the pulse train frequency should exceed 25 KHz. 
     The external pulse (FIG. 6 c ) connected to the input port  154  gates the pulse train supplied to the counter. Counting begins with the positive leading edge of the pulse and stops upon the trailing edge of the pulse. The resulting number stored in the counting register is linearly proportional to the duration of the external pulse. This number is transferred to a unique RAM location for storage until needed. 
     The scaled values for the both the sine and cosine functions are stored in a single, common lookup table or array of ninety contiguous locations in program memory  156 . Scaling of these values simplifies future calculation. The array index, that is, phase angle, determines the sine or cosine value. The symmetry of these function eliminates the need to store separate values for the sine and cosine functions. That is, the single array of ninety contiguous values is used to determine both sine and cosine values. 
     Next, the array index is calculated from the counter value stored in a unique RAM location, as was described in the previous paragraph. This requires integer offsets and rotations of the counter value. Individual array indexes are required to select the scaled sine and cosine values which are stored in unique RAM locations. 
     The scaled imaginary value is determined by multiplying the stored sine and stored ground fault current magnitude values together and the result is stored in a unique RAM location. The scaled real value is determined by multiplying the stored cosine and stored ground fault current magnitude values together and the result is stored in a unique RAM location. 
     The output of the calibrated real ground fault setting potentiometer  136  is connected to the microcontroller A-D input  138 . Microcontroller  48  encodes its value and stores it in a unique RAM location. 
     The output of the calibrated imaginary ground fault setting potentiometer  140  is connected to the microcontroller A-D input  142 . Microcontroller  48  encodes its value and stores it in a unique RAM location. 
     The scaling described above in determining the sine and cosine of the phase angle assures that the internally stored real and imaginary ground fault current values match the encoded internally stored real setting and imaginary ground fault current calibrations. That is, the settings are accurately calibrated in engineering units of milliamperes. 
     Either or both of two conditions command a ground fault trip. The first condition is the stored real ground current value equaling or exceeding the stored real setting. The second condition is the stored imaginary ground current value equaling or exceeding the stored imaginary setting value. In the event that either or both these conditions occur, a ground fault condition exists and a trip is declared. 
     As discussed above, ground fault protector  44  has a choice of two responses to a ground fault condition. Switch  118  selects the response. 
     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.