“A Stitch in Time...”

The insulation resistance of one conductor of a multiconductor cable to all
others and sheath is:
of
where:
D
R = K log 10 D / d
= diameter over insulation of equivalent single-conductor cable
= d + 2c + 2b
d = diameter of conductor (for sector cables d equals diameter
round conductor of same cross-section)
c = thickness of conductor insulation
b = thickness of jacket insulation
(all dimensions must be expressed in same units)
Transformers
Acceptable insulation resistance values for dry and compound-filled
transformers should be comparable to those for Class A rotating machinery,
although no standard minimum values are available.
Oil-filled transformers or voltage regulators present a special problem in that
the condition of the oil has a marked influence on the insulation resistance
of the windings.
In the absence of more reliable data the following formula is suggested:
R = CE
√kVA
R = minimum 1-minute 500-volt DC insulation resistance in megohms
from winding to ground, with other winding or windings guarded,
or from winding to winding with core guarded
C = a constant for 20°C measurements
E = voltage rating of winding under test
kVA = rated capacity of winding under test
For tests of winding to ground with the other winding or windings
grounded, the values will be much less than that given by the formula.
R in this formula is based on dry, acid-free, sludge-free oil, and bushings
and terminal boards that are in good condition.
A STITCH IN TIME 41