Hazardous voltage.
Will cause death or serious
personal injury.
The main disconnect must be in the OFF
position during all megger testing of the
motor control center. Devices such as
solid-state components, capacitor units,
or any other devices which are not
designed to withstand megger voltage,
should be disconnected before testing
the rest of the motor control center.
3.20 Insulation (Megger) Test
Take resistance measurements before a motor control center is
placed into service, after installation or maintenance. When
performing resistance measurements in motor control centers
use an insulation tester (megger) with a potential of 500-
1000V.
Take readings between each phase and from each phase to
ground. This should be done with the branch disconnects OFF
and again, with the branch disconnects ON.
3.20.1 Branch Disconnects OFF
Typically readings taken with all disconnects in the OFF posi-
tion should be between 5-20 megaohms. New equipment
which was stored in a damp area may register lower upon ini-
tial startup. If readings are above one megaohm during start-
up the following procedure may be observed to help dry the
motor control center. Energize several individual control units.
If individual readings are above one megohm, energize addi-
tional units. After the equipment has been in operation for 48
hours, the readings should be in the 5-20 megaohm range. If
at any time megger readings are below 5 megaohm (one meg-
ohm during start-up) consult your local Siemens sales office.
3.20.2 Branch Disconnects ON
Before taking a reading with the branch disconnects ON, dis-
connect all devices completing circuits between phases or
between phases and neutral such as control transformers.
Readings observed may be slightly lower than the OFF read-
ings, but the start-up one megohm lower limit still applies.
Record the megger readings on the tables below. Abrupt
changes in resistive values may be an indication of potential
failure.Even sudden changes within the 5-20 megaohm range
may be an advance signal of insulation failure. The early detec-
tion of faulty insulation components can save costly repairs
and downtime.
Branch Disconnects OFF
Date
Phase to Phase
A - B
B - C
C - A
Phase to Ground
A - GND
B - GND
C - GND
Branch Disconnects ON
Date
Phase to Phase
A - B
B - C
C - A
Phase to Ground
A - GND
B - GND
C - GND
Siemens Energy & Automation, Inc.
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