temperature measurement, insulation
resistance measurements are of value for
comparison rather than for conformance to
stated minimums. The temperature of the
cable is important and should be recorded
with the insulation resistance. This will be
difficult if the cable is partly indoors and partly
outdoors, partly underground, partly above
ground, partly exposed, and partly in
conduits. It may be necessary to estimate the
temperature of the various lengths, and a
weighted average computed. Tests should be
made between each conductor, between each
conductor and ground with other conductors
grounded, and between each conductor and
ground with other conductors connected to
the guard circuit but not grounded.
HIGH-VOLTAGE, DC TESTS - ROTATING MACHINES
22. Application - Rotating Machines.
high-voltage, dc test method is regarded as the
best test presently available for machine
insulation, through it must be recognized that the
test also has some limitations. This test may
also be carried to the point of being a high-
voltage, dc withstand test.
An understanding of the value of the test is
aided by considering its advantages and
a. Very little supply power is required to
operate the test set.
b. The test set is portable enough to be
readily useful. (Although larger than an
insulation resistance meter, it is far
smaller than an ac, high-voltage tester.)
c. The high-voltage, dc test is less destruc-
tive than other available high-potential
d. The small leakage current is not masked
by ac charging current and therefore affords
a quantitative measure of insulation quality.
e. Much less damage is done if breakdown
should occur, because the capacity of the
test set is small.
f. The insulation resistance characteristics
up through the normal operating voltage
range appear to afford more information on
winding insulation than any other practical
a. The potential stress in some cases, such
as at the generator winding end turns, is not
entirely representative of that produced in
b. Longer time required to complete the dc
test as compared to an ac test.
c. Care necessary to distinguish change of
true leakage current from change of absorp-
In order to obtain valid data on the condition of the
machine winding using high-voltage, dc tests, it is
necessary to disconnect the buswork from the
unit. Unit buswork will quite often break down at a
lower voltage than the winding.
illustrate the difference in the results that can be
obtained with the buswork connected and
23. Correlation Between DC and AC
- The dc puncture voltage may range from
1.41 times the rms ac puncture voltage for a
cracked and abraded winding, where the puncture
path is essentially an air gap, to 2.5 times the
9 (FIST 3-1 12/91)