Electrjcal Test lnstruments
MAC-20 Retrofit Package
Section
lll
-
Operating
lnstructions
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.
lf
so,
it is necessary
to either
block or by-pass
ground
fault
protection
when
tests are
being conducied 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 operation 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.
ln 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.
lt is recommended 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.
ln
both
cases, tripping time should
be within the manufacturer's
published
time
range. Pick-up of the
instantaneous unit should be
+l-
10o/o of the
setting
for the electronic device and
+l-
20% of the setting
for
the
magnetic device.
Testing the instantaneous
trip on
very large circuit breakers set
at
or
near maximum
calibration
may sometimes
exceed the capacity
of the test set.
ln most cases, the
instantaneous element has several calibration
marks between
"LO"
and
"H1".
ln 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,
adjust the screw to
a lower setting that
allows the
test set to verify that the unit
will
pick
up.
lf 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
1. Consult
breaker
manufacturer's
literature to determine
any necessary
precautions
and expected
test results.
A test
current of three times the
rating of the breaker
should be used
for this
test.
2.
Follow steps
1 through
18 in
general
test
procedures,
as
required, to set up the
breaker
test
set.
3. Connect
appropriate
output
tap to
one
pole
of breaker
under test.
Electrical
Test lnstruments, lnc.
Page
Ill-'10
Copyright(C)
1 994-1 998
All Rights Reserved
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