Electrical Test Instruments PI-2500-G2 and PI-5000-G2
Section V - Service Information and Documentation
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Copyright(C) 1994-2021 Electrical Test Instruments, LLC. All Rights Reserved
Page IV-12
LOW VOLTAGE POWER CIRCUIT BREAKERS
Nearly all low voltage power circuit breakers are multi-pole devices. The trip
units may be either magnetic with a dash pot or solid-state electronic devices.
Test procedures would be the same for either type. However, electronic trip
types may incorporate ground fault protection. If so, it is necessary to either
block or by-pass ground fault protection when tests are being conducted on
phase fault characteristics. This is because the tests are being conducted on
one pole at a time, and the ground fault protection would consider this opera-
tion to be a ground fault.
Most low voltage power circuit breakers have either Long Time Delay, and
Short Time Delay, and/or instantaneous tripping characteristics. In this case,
for test purposes, it is recommended that the instantaneous unit be set in its
maximum calibration. This is to prevent an instantaneous trip while trying to
set the current for the Short Time Delay characteristic.
Electrical tests to be conducted on low voltage power circuit breakers include
timing test on L.T.D.; timing test on S.T.D.; and instantaneous pick-up. It is rec-
ommended that the magnitude of test current for the L.T.D. timing test be 3
times coil rating or transformer tap. For timing test on S.T.D. the test current
should be about 1.5 times the S.T.D. setting. In both cases, tripping time should
be within the manufacturer's published time range. Pick-up of the instantane-
ous unit should be +/- 10% of the
setting for the electronic device and +/- 20% of the setting for the magnetic
device.
Testing the instantaneous trip on very large circuit breakers set at or near max-
imum calibration may sometimes exceed the capacity of the test set. In most
cases, the instantaneous element has several calibration marks between "LO"
and "HI". In such cases, it may be necessary to perform the test at one of the
lower calibration marks. Always be sure to record the "AS FOUND" settings and
return the adjustments to these settings after testing. To perform testing, ad-
just the screw to a lower setting that
allows the test set to verify that the unit will pick up. If the instantaneous unit
picks up at the proper current at a lower calibration point, it may be assumed
that the unit will operate properly at higher calibration points. This has been
verified by manufacturers and by means of field tests.
Specific test procedures for all electrical tests are outlined on the following
pages.
Timing Test, Long Time Delay
