TM 5-3895-374-24-1
Insulation Test
WARNING There is a hazard of electric shock or burn
whenever working on or around electrical equipment
Turn off power supplying this equipment before working
inside motor control centers.
CAUTION Do not megger solid state devices, capacitor
units or any devices which are not designed to withstand
megger voltage Disconnect these devices before testing
the rest of the motor control center M-37 Before a motor
control center s placed into service, (after installation or
regular maintenance) resistance measurements should
be taken Use an insulation tester (megger) with a
potential of 500-1000 volts Readings should be taken
between each phase and from each phase to ground
both with the branch disconnects OFF" and "ON The
main disconnect should be ’OFF" during all rmegger
tests.
M-37 Megger readings with all the disconnects "OFF"
will typically be 5-20 megohms Lower readings
may
be
observed
during
start-up
on
new
equipment which has been stored in a damp area
If the readings are above one megohm, a few
branch units may be energized to help dry out the
motor control center If additional readings are
above one megohm, additional units can be
energized After the equipment has been in
operation for 48 hours, readings should be in the
5-20 megohm range.
When megger readings are taken with the
disconnects "ON’ (except for the main) all devices
completing circuits between phases or between
phase and neutral (e.g. control transformers) must
be disconnected Readings may be slightly lower,
but the one megohm lower limit (during start-up)
should be observed. Record all megger readings
below Any sudden change in resistance values
(even if within acceptable range) may indicate
potential insulation failure Early detection and
replacement of faulty insulating components can
help to avoid untimely failure.
‘ If megger readings are below 5 megohms (one
megohm during start-up) consult your local Square
D field office
M-38 Return motor control center to service. Energize
main first, then each branch unit one at a time.
(See Section I-17, Page 12).
(Page 3-140)
