Ground fault defeat cable for high current static trip circuit breaker test sets

A high current field test set is electrically connected to pass test currents of overcurrent proportions through a single pole of a multipole static trip circuit breaker. The normal electrical connections between the breaker phase and neutral current sensing transformers and the static trip unit are made via a ground fault defeat cable which serves to automatically route the phase current monitoring signals produced by the test currents and fed to the appropriate trip unit phase overcurrent signal processing network through two primary windings of the ground fault sensing differential current transformer in opposite directions, thereby inhibiting a ground fault trip function.

BACKGROUND OF THE INVENTION 
Modern circuit protection increasingly calls for circuit breakers equipped 
with so-called "static" or electronic trip units in lieu of the 
traditional thermal-magnetic or dual-magnetic trip units operating to 
effect automatic opening of the breaker contacts in response to 
overcurrent conditions ranging from light overload to heavy short circuit. 
Static trip units are found to be more versatile and precise in terms of 
selectively establishing multiple overcurrent trip pick-up levels and trip 
time delays. For example, currently available static trip units have the 
capability of selectively establishing coordinated long-time delay, 
short-time delay and instantaneous current pick-up levels, as well as 
different tolerance bands of time delays. As a consequence, the trip 
settings of a static trip circuit breaker can be readily tailored to a 
particular load so as to provide proper protection and yet avoid vexatious 
nuisance tripping. 
Another reason for the current popularity of static trip circuit breakers 
is the increasing demand for ground fault protection. Since response to a 
ground fault condition is best handled electronically, it becomes quite 
practical to integrate the ground fault trip function into an overcurrent 
responsive electronic trip unit in contrast providing an electronic ground 
fault trip unit plus a traditional electromechanical trip unit. 
Once a static trip circuit breaker goes into the field, it is desirable to 
periodically verify its continuing capability to provide the full measure 
of circuit protection intended for a particular application. To this end, 
field test sets of two types have been made available. In one type, low 
current, fault simulating signals are injected into the secondary circuits 
of the breaker phase current sensing transformers whose secondary are 
connected as separate inputs to the static trip unit. If the trip unit is 
functioning properly, it will process these fault simulating signals 
pursuant to initiating a trip function as though corresponding high 
currents of overload proportions actually flow through the breaker poles, 
i.e., the primaries of the breaker current transformers. Applicant's 
co-pending application Ser. No. 815,628, filed July 14, 1977 discloses a 
static trip circuit breaker field test set of this type. 
With the other type of static trip circuit breaker field test set, a high 
current of overcurrent proportions is passed through the breaker poles to 
verify that the breaker will trip with the appropriate delay. When a 
static trip circuit breaker is equipped with integral ground fault 
protection, the high current from the test set must be passed in opposite 
directions through two of the breaker poles in series, otherwise the 
ground fault sensing differential current transformer within the trip unit 
would be unbalanced, precipitating the initiation of a ground fault trip 
function. If the test current is passed through but one breaker pole, the 
consequent tripping of the circuit breaker cannot be said to verify the 
operability of the overcurrent tripping capability of the trip unit, since 
it is quite likely the breaker tripped in response to the differential 
current transformer unbalance. Thus verification of the overcurrent trip 
settings and trip time delays established in the trip unit cannot be 
reliably obtained. 
As noted above, the current practice in defeating a ground fault trip 
function is to pass the high current generated by the test set in opposite 
directions through two of the breaker poles in series. Since the typical 
high current field test set is portable in nature and operates at a 
relatively low voltage, the added impedance of the second breaker pole 
severely limits the maximum level of test current the test set can 
develop. As a consequence, in many situations the test set cannot generate 
sufficient current levels to verify the operability of the static trip 
circuit breaker under simulated heavy overload and short circuit 
conditions. 
It is accordingly an object of the present invention to provide a high 
current static trip circuit breaker field test set which is equipped with 
means for defeating a ground fault trip function without having to pass 
the test current through two breaker poles in series. 
An additional object of the present invention is to provide a ground fault 
defeat cable for connecting the breaker current transformers to the static 
trip unit pursuant to inhibiting the initiation of a ground fault trip 
function despite the fact that high levels of test current is passed 
through but a single breaker pole. 
Yet another object of the present invention is to provide a ground fault 
defeat cable of the above character which is inexpensive to manufacture 
and convenient to use in the field. 
Other objects of the invention will in part be obvious and in part appear 
hereinafter. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a ground fault 
defeat cable for use in conjunction with a high current field test set in 
verifying the phase overcurrent tripping capability of a ground fault 
equipped circuit breaker static trip unit. The ground fault defeat cable 
of the present invention functions to automatically inhibit the initiation 
of a ground fault trip function while test current of overload proportions 
generated by the test set is passed through but a single pole of a 
multipole circuit breaker. In its application, the ground fault defeat 
cable is simply utilized to make the normal electrical connections between 
the various phase and neutral current transformers and the static trip 
unit. The defeat cable also makes the electrical connections between the 
circuit breaker trip solenoid and the static trip unit. When a test 
current of overcurrent proportions is passed through a single pole of the 
circuit breaker, the associated phase current transformer develops an 
overcurrent indicative monitoring signal which is routed by the defeat 
cable, not only to the appropriate phase overcurrent signal processing 
network of the trip unit, but also through two primary windings of the 
trip unit's ground fault sensing differential current transformer in 
opposite directions. As a consequence, the differential current 
transformer remains balanced and a ground fault trip function cannot then 
be initiated, despite the fact that test current is passed through only 
one breaker pole. Since ground fault tripping is inhibited, the tripping 
responses of the static trip unit to varying levels of phase overcurrent 
can be thoroughly tested. Since, by virtue of the present invention, test 
current need not be passed through two breaker poles in series in order to 
prevent the initiation of a ground fault trip function, the test set is 
capable of developing significantly higher levels of phase test current, 
thus rendering it practical to test the circuit breaker under extreme 
short circuit conditions. 
The invention accordingly comprises the features of construction, 
combination of elements, and arrangement of parts which will be 
exemplified in the construction hereinafter set forth and the scope of the 
invention will be indicated in the claims.

DETAILED DESCRIPTION 
Referring first to FIG. 1, there is shown a three pole, ground fault 
equipped static trip circuit breaker, generally indicated at 10, whose 
phase overcurrent tripping capability is to be tested using a low voltage, 
high current field test set, generally indicated at 12. As illustrated, 
the breaker includes line and load terminals 14a and 14b as terminations 
of its left pole, line and load terminals 16a and 16b as terminations for 
its center pole, and line and load terminals 18a and 18b as terminations 
for its right pole. As is well understood and as schematically illustrated 
in FIG. 2, phase current transformers are inductively coupled with each 
breaker pole for individually sensing the phase currents flowing 
therethrough. If the particular circuit in which the breaker is installed 
includes a neutral bus 20, then a neutral current transformer 22 is 
inductively coupled therewith to sense neutral current. The secondary 
winding of this neutral transformer is terminated in lead wires which 
together with lead wires terminating the secondary windings of the three 
phase current transformers make up a breaker harness cable 26 which itself 
is terminated in a multipin connector 26a. When the breaker is in service, 
connector 26a mates with a connector 24a of a harness cable 24 leading to 
a static trip unit STU physically adapted to the breaker. 
Heretofore, terminal connectors 24a and 26a were mated to directly 
interconnect circuit breaker and trip unit harness cables when the breaker 
was removed from service to verify its phase overcurrent tripping 
capability using high current test set 12. When the breaker under test was 
equipped with integral ground fault protection, it became necessary, in 
order to inhibit the initiation of a ground fault trip function, to pass 
the test current in opposite directions through two of the breaker poles 
connected in series. Thus for example, load terminals 16b and 18b were 
connected together by a jumper, indicated in phantom at 28, while one test 
set output cable 30 was connected to line terminal 18a and the other 
output cable 32 connected to line terminal 16a, as indicated in phantom at 
32a. In this manner, test current of overcurrent proportions is routed 
through the right pole in one direction and the center pole in the 
opposite direction in order to maintain the ground fault sensing current 
transformer of the static trip unit STU in balance. 
In accordance with the present invention, connections 28 and 32a are 
dispensed with, and test set output cable 32 is connected directly to load 
terminal 18b such that test current is passed solely through the right 
pole in the illustrated example of FIG. 1. In order that a ground fault 
trip function not be initiated under these circumstances, a ground fault 
defeat cable 34 is electrically interposed between the circuit breaker 
harness cable 26 and the static trip unit harness cable 24. Specifically, 
the defeat cable 34 has a multipin connector 34a at one end which mates 
with the circuit breaker harness cable connector 26a and a multipin 
connector 34b at its other end which mates with the static trip unit 
harness cable connector 24a. For an appreciation of how the ground fault 
defeat cable 34 is constructed so as to prevent the initiation of a ground 
fault trip function when test current is passed through a single pole of 
breaker 10, reference is now had to FIG. 2. 
As previously noted, circuit breaker 10 includes three separate phase 
current transformers, illustrated at 36, 38 and 40 in FIG. 2, which are 
positioned to individually sense the currents flowing through each of the 
breaker poles. The two sides of the secondary winding for phase current 
transformer 36 are brought out via breaker harness cable 26 to pins A and 
B of connector 26a. Similarly, the two sides of the secondary winding for 
phase current transformer 38 are brought out via breaker harness cable 26 
to pins C and D, while the two sides of the secondary winding for phase 
current transformer 40 are brought out to pins E and F of breaker harness 
cable connector 26a. The two sides of the secondary winding for neutral 
transformer 22 are brought out via breaker harness cable 26 to pins G and 
H of connector 26a, while the two sides of a trip solenoid coil TC are 
brought out to pins I and J of cable connector 26a. 
While the ground fault defeat cable 34 of the present invention may be 
adapted to static trip units of various designs, for purposes of 
illustration, trip unit STU is generally disclosed in FIG. 2 as having the 
construction detailed in commonly assigned U.S. Pat. No. 3,786,311. Thus, 
the illustrated static trip unit includes three auxiliary transformers 42, 
44 and 46, each having a secondary winding separately connected to supply 
phase current monitoring signals to a phase overcurrent processing section 
of overcurrent and ground fault programmer logic, generally indicated at 
48. Also included in static trip unit STU is a differential current 
transformer 50 having plural primary windings 50a, 50b, 50c and 50n, and a 
secondary winding connected to supply a current signal indicative of any 
imbalance in the vectorial sum of the primary currents to a ground fault 
signal processing section of programmer logic 48. 
A power supply 52, also included in static trip unit STU, is connected to 
derive from the phase current monitoring signals operating power for the 
programmer logic 48, as supplied over connection 52a. Thus, as seen in 
FIG. 2, a phase current monitoring signal at pin A of connector 24a is 
routed via one conductor of static trip unit harness cable 24 through the 
primary winding of auxiliary transformer 42, down to the power supply 52, 
back through primary winding 50a of differential current transformer 50 
and harness cable 24 to pin B of connector 24a. Similarly, a phase current 
monitoring signal at pin C of connector 24a is routed via harness cable 24 
through the primary winding of auxiliary transformer 44, down to the power 
supply and back through primary winding 50b of differential current 
transformer 50 to pin D of connector 24a. Finally, a phase current 
monitoring signal at pin E of connector 24a is routed through the primary 
of auxiliary transformer 46, power supply 52, differential current 
transformer primary winding 50c and back to pin F of the static trip unit 
harness cable connector 24a. A neutral current monitoring signal at pin G 
is routed through differential current transformer primary winding 50n and 
back to pin H via static trip unit harness cable 24. When the programmer 
logic 48 is to initiate a trip function, an energizing voltage for 
activating the breaker trip solenoid TC is impressed across programmer 
output leads 48a and 48b, which are brought out via harness cable 24 to 
pins I and J of connector 24. 
When the circuit breaker 10 is in service, cable connectors 24a and 26a are 
mated. Under these circumstances, it is seen that the secondary winding 
for phase current transformer 35 is connected in loop circuit with the 
primary winding of auxiliary transformer 42, power supply 52 and 
differential current transformer primary winding 50a. Similarly, the 
secondary winding for phase current transformer 38 is connected in loop 
circuit with the primary winding of auxiliary transformer 44, power supply 
52, and differential current transformer primary winding 50b, while the 
secondary winding for phase current transformer 40 is connected in loop 
circuit with the primary winding of auxiliary transformer 46, the power 
supply and differential current transformer winding 50c. In addition, the 
secondary winding for neutral current transformer 22 is connected across 
differential current transformer primary winding 50n, while the programmer 
output leads 48a and 48b are connected across the breaker trip solenoid 
coil TC. 
From this it is seen that when test current of overcurrent proportions 
developed by test set 12 of FIG. 1 is passed through but one pole of 
circuit breaker 10, the phase current monitoring signal developed in the 
associated phase current transformer secondary winding is routed through 
only one of the differential current transformer primary windings. 
Consequently, the differential current transformer is unbalanced and the 
programmer logic 48 will operate as though a ground fault exists, 
generating an output voltage across leads 48a, 48b to activate the breaker 
trip solenoid coil TC. To prevent this unwanted ground fault trip function 
from being initiated, it has been the practice heretofore to route the 
test current in opposite directions through two of the breaker poles 
connected in series, as previously noted. Under these circumstances, it is 
seen that the resulting phase current monitoring signals will be routed 
through two of the differential current transformer primary windings in 
opposite directions, thus maintaining a current balance to hold off the 
initiation of a ground fault trip function. The programmer logic is then 
free to fully process the phase current monitoring signals pursuant to 
initiating an overcurrent trip function. 
To inhibit the initiation of a ground fault trip function despite the fact 
that test current of overcurrent proportions is passed through but a 
single pole of breaker 10, ground fault defeat cable 34 of the present 
invention is electrically interposed between harness cable 26 and static 
trip unit harness cable 24. Thus, as seen in FIG. 2, the defeat cable 
includes a lead 60 running between pins A of the connectors 34a and 34b at 
each end. Similarly, a lead 62 runs between pins C of connectors 34a and 
34b, while a lead 64 runs between pins E of the connectors terminating the 
ends of the ground fault defeat cable 34. Pins B, D and F of defeat cable 
connector 34a mating with breaker cable connector 26a are connected in 
common via a lead 66 running to pin H of the defeat cable connector 34b 
mating with static trip unit cable connector 24a. On the other hand, pins 
B, D and F of the defeat cable connector 34b mating with static trip unit 
cable connector 24a are connected in common via a lead 68 running to pin G 
of defeat cable connector 34b. Pins G and H of defeat cable connector 34a 
are connected together by a jumper 70 which serves, with connectors 26a 
and 34a mated, to short together the two sides of the secondary winding 
for neutral current transformer 22. This is done as a precautionary 
measure to prevent the development of potentially hazardous high voltages 
across the neutral current transformer secondary winding on the off-chance 
that there be current flowing through neutral bus 20 while circuit breaker 
10 is under test. Finally, defeat cable 34 includes leads 72 and 74 
respectively interconnecting pins I and J of connectors 34a and 34b at 
each end. Consequently, with the defeat cable interposed between the 
breaker harness cable 26 and static trip unit harness cable 24, programmer 
output leads 48a and 48b are wired straight through to the breaker trip 
solenoid coil TC. 
To appreciate the operation of ground fault defeat cable 34 in inhibiting 
the initiation of a ground fault trip function while test current is being 
passed through a single pole of breaker 10, assume that the resulting 
phase current monitoring signal is induced in the secondary of phase 
current transformer 36. This monitoring signal is routed via breaker 
harness cable 26 to pin A of connector 26a, pin A of the defeat cable 
connector 34a mated therewith, defeat cable lead 60, pin A of its other 
connector 34b, pin A of mated connector 24a and static trip unit harness 
cable 24 to auxiliary transformer 42. From this auxiliary transformer, the 
phase current monitoring signal is routed downward to the power supply 52 
and then upwardly through primary winding 50a of differential current 
transformer 50 and out via the trip unit harness cable 24 to its connector 
pin B. From pin B of the defeat cable connector 34b, this phase current 
monitoring signal is routed via lead 68 of the defeat cable down to pin G 
of the same connector 34b, pin G of mated connector 24a, static trip unit 
harness cable 24, downwardly through differential current transformer 
primary winding 50n and back to pin H of the static trip unit harness 
cable connector 24a. From pin H of the defeat cable connector 34b, this 
monitoring signal is routed via lead 66 to pin B of the connector 34a at 
its other end, pin B of the breaker harness cable connector 26a, and the 
breaker harness cable 26 to the other side of the secondary winding for 
phase current transformer 36. It is seen that this phase current 
monitoring signal developed by phase current transformer 36 in breaker 10 
is routed by the defeat cable so as to flow in a circuit loop through 
differential current transformer primary winding 50a in one direction and 
differential current transformer primary winding 50n in the opposite 
direction, thus maintaining a current balance. As a consequence, 
programmer logic 48 does not initiate a ground fault trip function, 
leaving it free to respond to the phase overcurrent monitoring signal as 
applied to its overcurrent signal processing section from the secondary of 
auxiliary transformer 42. If the programmer is functioning properly, it 
will develop an activating voltage across its output leads 48a, 48b either 
instantaneously or with appropriate delay, depending upon the level of the 
test current. This activating voltage is applied via leads 72 and 74 of 
the defeat cable to the trip solenoid coil TC, whereupon the breaker 
trips. 
If the test current is routed through the breaker pole associated with 
phase current transformer 38, it is seen that the resulting phase current 
monitoring signal is routed by conductor 62 of defeat cable 34 through the 
primary winding of auxiliary transformer 44, power supply 52, upwardly 
through differential current transformer primary winding 50b, defeat cable 
conductor 68, downwardly through differential current transformer primary 
winding 50n, and defeat cable lead 66 back to the other side of the 
secondary winding for phase current transformer 38. Again, the phase 
current monitoring signal is routed through two differential windings in 
opposite directions to inhibit the initiation of a ground fault trip 
function. By the same token, a phase current monitoring signal developed 
in the secondary of phase current transformer 40 is routed via defeat 
cable lead 64 through the primary winding for auxiliary transformer 46, 
power supply 52, differential current transformer primary winding 50c in 
one direction, defeat cable lead 68, differential current transformer 
primary winding 50n in the opposite direction, and defeat cable lead 66 
back to the other side of the secondary winding for phase current 
transformer 40. 
From the foregoing description, it is seen that the ground fault defeat 
cable 34 of the present invention is effective in inhibiting the 
initiation of a ground fault trip function by a circuit breaker static 
trip unit during testing of the overcurrent tripping capability of the 
circuit breaker by passing test current of overcurrent proportions through 
but a single circuit breaker pole. This is achieved in a unique and 
efficient manner by structuring the defeat cable to automatically route 
the phase current monitoring signal developed in the phase current 
transformer associated with the breaker pole through which the test 
current is passed in opposite relative directions through two separate 
primary windings of the ground fault sensing differential current 
transformer included within the trip unit. In this manner, the 
differential current transformer remains balanced to inhibit the 
initiation of an unwanted ground fault trip function. The static trip unit 
is thus free to process the overcurrent information content of the phase 
current monitoring signal in its overcurrent signal processing section 
pursuant to initiating an overcurrent trip function. While in the 
illustrated example, the ground fault defeat cable makes differential 
current transformer primary winding 50n common to each of circuit loops 
individually including the other three primary windings, it will be 
appreciated that a defeat cable may be constructed in accordance with the 
teachings of the present invention to route phase current monitoring 
signals through any two of the differential current transformer primary 
windings pursuant to maintaining a current balance during a phase 
overcurrent trip test. That is, in some circuit applications there is no 
neutral bus, and consequently differential current transformer primary 
winding 50n may then be omitted. In this case, the defeat cable would be 
constructed to route phase current monitoring signals in opposite 
directions through two of the remaining three differential current 
transformer primary windings. 
It will thus be seen that the objects set forth above, among those made 
apparent in the preceding description, are efficiently attained and, since 
certain changes may be made in the above construction without departing 
from the scope of the invention, it is intended that all matter contained 
in the above description or shown in the accompanying drawings shall be 
interpreted as illustrative and not in a limiting sense.