Publication 2100-IN012B-EN-P - April 2005
3-6 Final Checklist Before Energizing
18. Conduct an electrical insulation resistance test to verify the MCC wiring
integrity. Conduct this test using an insulation resistance tester (megger)
with a potential of 500 - 1000 volts.
This megger test should be conducted phase-to-phase, phase-to-ground
and when applicable, phase-to-neutral on the MCC buswork. The test
should be conducted with all of the switches or circuit breakers in the
open or OFF/O positions. Typical insulation resistance values are 50
megohms or greater. Next, check the field wiring; for example motor
cables and/or incoming line cables. Insulation resistance values are
affected by temperature, humidity or dampness which may cause a
considerably lower insulation resistance reading. If the insulation
resistance values are below one megohm (i.e. affected by dampness,
temperature or humidity) or the MCC has been stored in a damp or
humid area, it is recommended the equipment be dried out. Dry out
motor cables by injecting a low voltage current or by using space
heaters.
Once the equipment is dry, repeat the insulation resistance test. The
minimum value for insulation resistance on a new installation at start-up
or energizing is 1 megohm. These readings may be recorded in Table
3.F. Table 3.F allows for additional megger readings to be recorded
during regular maintenance periods.
19. Make sure all unit latches are secure. Close and latch all doors, making
certain that no wires are pinched. All section closing plates must be in
place.
Table 3.A General Spacing Requirements for Motor Control Centers
(1)
,
(2)
,
(3)
Voltage
involved
Minimum spacing, inch (mm)
Between live parts of opposite polarity Between live parts and
grounded metal parts,
through air and over
surfaceThrough air Over surface
125 or less 1/2” (12.7mm) 3/4” (19.1mm) 1/2” (12.7mm)
126 – 250 3/4” (19.1mm) 1-1/4” (31.8mm) 1/2” (12.7mm)
251 – 600 1" (25.4mm) 2" (50.8mm)
1"
(4)
(25.4mm)
(4)
(1)
Reference Underwriters Laboratories, Inc. Standards for Motor Control Centers, UL 845, Tables 16.1 and 16.3
(2)
Spacings at all horizontal and vertical buses, at incoming terminals, at the plug-in portion of all units and at feeder units
consisting of fusible disconnect switches or circuit breakers only, having other than horsepower ratings, shall be as
specified in Table 3.A (See 17.2.2 from UL 345).
(3)
An isolated dead metal part, such as a screw head or a washer, interposed between uninsulated live parts of opposite
polarity or between an uninsulated live part and grounded dead metal is considered to reduce the spacing by an
amount equal to the dimension of the interposed part along the path of measurement.
(4)
A through-air spacing of not less than 1/2” (12.7 mm) is acceptable at a circuit breaker or fusible disconnecting means
and between grounded dead metal and the neutral of a 277/480 volt, 3-phase, 4-wire motor control center.
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