Understanding Insulation Testing - AEMC 5100 User Manual

Megohmmeter Models 5000N/5100/5110

UNDERSTANDING INSULATION TESTING

Insulation is the material between two points of different potential which,
through high resistivity, prevents the flow of current between those
points. Insulation failure is one of the most common problems associated
with electrical equipment breakdown.
A megohmmeter is an insulation resistance tester which is essentially a
high resistance ohmmeter (MΩ or greater) providing a high DC potential
(up to 5000V). This high potential causes low amounts of current to flow
through and over the insulation which is under test.
Many factors can cause insulation to fail: mechanical damage, moisture,
heat, foreign debris, corrosion, etc. As time passes, these factors
combine to permit excessive current to flow through insulation at points
where it would normally be blocked by the insulation resistance. Usually,
the resistance on degrading insulation will drop gradually, providing
plenty of warning. Other times it will drop suddenly, as when it is
immersed.
With periodic resistance tests and good record keeping, it is possible to
get an accurate picture of the insulation condition. Insulation resistance
testing is intended to indicate not only if equipment is bad, but also
whether it is becoming bad.
Resistance of many types of insulation can vary greatly with
temperature. The resistance data obtained should be corrected to the
standard temperature for the class of equipment under test.
Please note that although we present information on test procedures,
values and recommended frequency of testing, the manufacturer of your
particular piece of equipment is the definitive source for testing
parameters and procedures. While we refer to commonly applied rules
and practices, every test will not be practical to each piece of electrical
equipment in your facility.
- 32 -