Method and apparatus for limiting fault current

A method and apparatus for limiting the fault current in a portable welding gun comprising a power circuit, the power circuit comprising a circuit breaker and an isolation contactor, the isolation contactor in series with the circuit breaker, the fault current limited by providing a safety circuit, the safety circuit comprising a safety circuit comprising two resistor/relay pairs, the resistor and the relay of each pair in series with each other and each pair in parallel with a contact of said contactor, the resistors sized to limit the fault current in the power circuit to less than 50 ma.

BACKGROUND OF THE INVENTION 
The present invention relates to portable resistance spot welding guns, 
called "transguns," and, more particularly, to a safety circuit for such 
transguns. Historically transguns have been used with robots, but recently 
a number of applications throughout Europe and North America have made use 
of a human operator handling the transgun and making welds by initiation 
of the welding sequence after placing the transgun in a welding position. 
Because transguns supply high voltages during the welding cycle in the 
vicinity of the operator, safety circuitry is required in the transgun to 
protect the operator from electric shock. 
A typical transgun circuit 10 is shown in FIG. 1. 480 or 575 volt AC 
(Canada) power 12 is supplied to a welding control enclosure 14. Circuit 
breaker 15, isolation contactor 17 having two contacts and silicon control 
rectifiers (SCRs) 19 are all contained within welding control enclosure 
14. Welding control enclosure 14 is connected to the transgun 16 via 
welding cable 18. Transgun 16 comprises transformer 25, electrodes 22, and 
material to be joined 20. The transgun is manipulated into position over 
the sheets to be welded 20, and depressing a button on the transgun 
initiates the welding action. 
FIG. 2 shows the shock hazard that may be caused by wear on the welding 
cable of the transgun. All of the following conditions must exist for such 
a hazard to occur: 
1. The power supply system must have a ground fault. This is shown in FIG. 
2 as one leg of the three phase delta connected system being grounded 
(26). 
2. The weld control must be powered up to close the circuit breaker 
(circuit breaker 15 closed). 
3. The weld control must be initiated to close the isolation contactor 17. 
4. The operator 28 must contact one of the welding cable's "live" 
conductors via worn spot 31. 
5. The operator must be grounded (30). 
If all of the above conditions are met, fault current 32 will flow (as 
shown in FIG. 2) from one leg of the power supply through the closed 
circuit breaker, the closed isolation contractor, the welding cable up to 
the point of the fault, through the fault to the operator, through the 
operator to ground, and finally back to the ground at the delta power 
system. It should be noted that in grounded "Y" power systems there is no 
need for a fault condition to ground one leg of the distribution system, 
since the grounded "Y" is intentionally grounded. Therefore, for grounded 
"Y" systems, only the last four conditions must be met to provide the 
possibility of shock to the operator. 
FIG. 3 shows a timing diagram for the fault current shown in FIG. 2. The 
timing diagram shows that after operator initiation of the welding cycle, 
the welder continues through the entire welding sequence, even though 
fault current is flowing. The worst case fault current is calculated by 
assuming that an operator's resistance to current flow is approximately 
1000 ohms, so that with a supply voltage of 575 volts, approximately 575 
milliamperes (ma) of current will flow for a time that is only limited by 
the weld time adjusted in the weld controller. This time interval could be 
as long as several seconds. 
To prevent the situation described by FIG. 2 and FIG. 3, systems that 
incorporate ground fault current detection have been designed. A typical 
such system is shown in FIG. 4. These devices operate on the principle of 
current imbalance, and current imbalance detector 34 checks to insure that 
the current that is supplied to the transgun via one of the supply wires 
matches the current returning via the other wire. Any difference between 
the two currents measured is assumed to be due to a fault in the system, 
and if the level is high enough, circuitry operates the shunt trip coil of 
the circuit breaker, thus opening the circuit breaker and disconnecting 
the welding control, welding cable, and transgun from the supply voltage. 
The timing diagram of FIG. 5 shows that although this approach does not 
limit the current that the operator is exposed to, it does limit the time 
before the high voltage is removed. FIG. 5 shows that the same 575 ma 
current will flow, but because the ground fault detector 34 ultimately 
causes the circuit breaker to open within 90 milliseconds (ms), the 
operator's exposure to the high voltage is reduced. 
FIG. 6 shows curves defining "safe" operating conditions. Because there is 
not universal agreement on shock hazard, various organizations have 
adopted guidelines. The UL Class A curve is the most difficult to meet, 
because the current levels allowable are lower. Note that the worst case 
condition, 575 ma at 90 ms (36), is to the right of all the curves, while 
the preferred operating side is the left side of the curves. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a portable 
welding gun safety circuit that lowers the fault current. It is another 
object of the present invention to provide a portable safety circuit that 
operates within UL Class A Curve. It is yet another object of the present 
invention to provide a portable safety circuit that exhibits no loss of 
cycle time due to circuit operation. It is yet another object of the 
present invention to provide a portable safety circuit that provides a 
simple modification to conventional safety circuitry. 
For that reason, disclosed and claimed herein is a safety circuit for a 
portable welding gun comprising a power circuit, the power circuit 
comprising a circuit breaker and an isolation contactor in series with the 
SCRs. The safety circuit may be used with other applicable power circuits, 
however. The safety circuit comprises two resistor/relay pairs, the 
resistor and the relay of each pair in series with each other and each 
pair in parallel with one of the isolation contactor contacts. In the 
preferred embodiment the resistors are sized to limit the fault current in 
the power circuit to less than 50 ma. 
Also disclosed and claimed is a modification to the safety circuit 
comprising means for verifying the operability of the safety circuit, the 
means in the preferred embodiment comprising a current imbalance detector: 
The details of the present invention, both as to its structure and 
apparatus, can best be understood by reference to the accompanying 
drawings in which like reference numbers refer to like parts and in which:

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 7 shows a transgun comprising the apparatus of the present invention, 
safety circuit 38. Safety circuit 38 comprises two resistors 42 in series 
with fault sensing relay (FSR) contacts 40, each series resistor/contact 
pair in turn in parallel with a contact of an isolation contactor 17. The 
present invention thus provides a safety circuit that is a simple 
modification to conventional safety circuitry. Indeed, the apparatus of 
the present invention may be used as desired with any appropriate power 
controller that utilizes an isolation contactor. According to the method 
of the present invention, welder initiation does not immediately close 
isolation contactors 17, but instead closes FSR contacts 40 of safety 
circuit 38. Any fault current is connected with resistors 42 rather than 
isolation contactors 17, limiting the fault current. Fault current 44 is 
present with the addition of the present invention and starts at the delta 
distribution system, proceeds through the circuit breaker, the resistor, 
the FSR contact, the welding cable up to the point of the fault, the 
operator, and finally back to the ground at the delta power system. The 
resistors of the safety circuit are sized to limit the fault current to 
approximately 50 ma, as is shown in FIG. 8. This current value is to the 
left of the UL Class A curve, as shown in FIG. 9 (50), but provides 
sufficient current to be sensed by the current imbalance detector to trip 
the circuit breakers. Such detectors are usually set to trip at 30 ma to 
avoid nuisance tripping. 
Because the present invention relies on current being supplied through the 
FRS contacts and the associated resistors rather than the isolation 
contactor, the preferred embodiment of the present invention includes 
means for verifying operability of the safety circuit which preferredly 
comprises a current imbalance detector to verify that these components are 
operational. As shown in FIG. 7, current imbalance detector 45 comprises 
current sensors 46 that sense current flow in the safety circuit itself. 
If balanced current is not detected immediately after the FSR relay is 
energized, the current imbalance detector will not allow the isolation 
contactor to be closed, nor will it re-initiate until the fault sensing 
circuitry is operational. Although the preferred means for verifying 
operation is a current imbalance detector, other appropriate means may be 
used. 
While particular embodiments of the invention have been described above, 
the invention is not so limited. Alternative embodiments and modifications 
which would still be encompassed by the invention may be made by those 
skilled in the art, particularly in light of the foregoing teachings. 
Therefore, the following claims are intended to cover any alternative 
embodiments, modifications or equivalents which may be included within the 
spirt and scope of the invention as claimed.