TESTING OF LOW VOLTAGE POWER CIRCUIT BREAKERS
Most low voltage power circuit breakers are equipped with Overcurrent trip device which sense overloads of fault
currents and trip the breaker. These devices can be either electromechanical ,solid state or microprocessor and
will usually have two or more of the following types of elements:
1. Long time delay element- This element is designed to operate OVERLOADS BETWEEN ITS "pick up" setting
and the "pick up" of a short time or instantaneous element. The
electromechanical long time delay pick up adjustment is generally within the range of 80% to 160% of the trip
device rating. Setting higher than an electromechanical trip device amperage rating do not increase the
continuous current rating of the trip device and in no event is the rating increased beyond the breaker frame size.
The operating time of this element ranges from seconds to minutes.
2. Short time delay element - This element has time delay measured in cycles and is used to protect against
moderate fault currents and short circuits. This element usually may be adjusted to pick up within the range of
250% to 1,000% of the trip device rating.
3. Instantaneous element - This element has no intentional time delay and is used to protect against heavy fault
currents and short circuits. The pick up settings for this type of element usually range from 500% to 1,500% of the
trip device rating.
4. Ground Fault element - This type of element is available only on solid state devices and us used to protect
fault currents at levels below those which would be sensed otherwise.
THE PROPER PROCEDURE AND METHODS USED BY SOUTHERN SUBSTATION, AND THEIR
RECOMMENDATIONS FOR THE TESTING OF LOW VOLTAGE AIR TYPE BREAKERS.
The testing of electromechanical trip device or solid state devices by the primary injection method requires the use
of high current test set capable of producing sufficient current at low voltage to operate each of the elements of
the trip device. This test must have means of adjusting the amount of the time to trip the breaker at each current
setting. At least one test should be made in the range of each element of the trip device. The long time delay
element should ordinarily be tested at approximately 300% of its settings. The instantaneous element should be
tested at 90% to 110% of its setting to assure that it does not operate at too low of a current level, yet will operate
at proper level. For the test of the instantaneous element the applied current must be symmetrical without an
asymmetrical off set, or random error will be introduced. "As found and "as left" test results should always be
recorded if any need of adjustments is found.
The following is a step by step procedure used by Southern Substation, Inc., and recommended to be used by any
other company in the testing of low voltage air breakers. This procedure should be used on new equipment as
part of your acceptance test, and also on your old equipment as part of your routine maintenance testing.
A) All breakers should be and are removed from cubicles.
B) All arc chutes are removed from the breaker and all dust, dirt any foreign matter is removed.
C) All dust, dirt and foreign matter is removed from the breaker and breaker cubicle.
D) Main contacts and arcing contacts are checked for proper alignment, gap ( main and arcing), pressure and
(If the breaker has already been in service then contacts are checked for pitting, burns and breakage.)
E) All trip units are checked:
1. Oil type units are checked for oil leaks.
2. Vacuum type units are checked for bad seals and spring tension.
3. Spring type units for proper springs and spring tension.
4. Solid State type units for loose connections and proper wiring.
F) Breaker is then load tested using Multi-Amp test equipment . Breakers, if necessary are adjusted to trip
within manufacturers specifications (within manufacturers curves).
G) Breakers are megged phase to phase and phase to ground after testing is completed.
H) Breaker is replaced in cubicle to the test position. Breaker if electrically operated, is then tested using
owners board operated sequence.
I) Any minor repair mechanical lubrication and adjustment are accomplished during normal testing
procedures. If any major problems are found, this is pointed out to the owner. New equipment is returned to the
manufacturer for repair if under warranty. If the equipment is over one year old and in operation it should be
changed out with a spare. Many major problems can be repaired on the job by Southern Substation, Inc.,
J) Prior to energizing of your board or if power had already been connected it should be turned off and the
following work done.
1. All dust, dirt and foreign matter is removed from buss and buss compartment.
2. All insulators wiped, cleaned and dusted.
3. All bolted joints torqued to ASA standards.
4. All wire connections cleaned and checked for tightness.
5. All frame bolts torqued to ASA standard.
6. All busses are megged phase to phase and phase to ground.
The preceding steps should taken without exception on all of your molded case and low voltage air breakers.
Many of the breaker parts are manufactured at different locations and sent to a central point for assembly. We
have in the past, discovered many new breakers that would not operate at all, much less on the curve supplied by
Circuit Breaker Testing Page two