CT p28-29 Megger Dec08.qxp 20/11/2008 15:56 Page 30
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DON’T BE LED
ASTRAY BY
INSULATION
TESTING!
High voltage insulation testing is an exceptionally useful aid to plant maintenance, says Paul
Swinerd of Megger, but, as he explains, it can have pitfalls for the unwary!
A
s more and more necessary for the safe result is surface leakage – if
companies are operation of the breaker. the surface of an insulator,
discovering, high- So far, so good, but it is for example, is dirty, the
voltage insulation testing at important to bear in mind current produced by the
5 kV or 10 kV is a valuable that an unexpectedly low insulation test voltage will
diagnostic tool, and also an insulation reading doesn’t flow through dirt as well as
essential ingredient in always indicate that through the body of the
preventative maintenance equipment has failed or insulator itself. This will
programmes for electrical even that it is on the point result is a misleadingly low
plant. of failing. test result that could easily
In particular, studies have In many cases, the be interpreted as indicating
confirmed that testing reason for the poor test a faulty insulator.
medium voltage equipment
at voltages close to its
normal working voltage
provides much more
dependable information
about its condition than
testing at lower voltages.
High-voltage testing is
particularly valuable when it
is carried out at regular
intervals, and the results
compared so that trends
can be identified. For
example if, over a period of
time, the insulation
resistance of, say, a circuit
breaker is always around
2,000 megohms, and then a
subsequent test shows that
it has fallen to 1,000
megohms, this is a good
indication that further
investigation is needed.
This is true even though
the 1,000-megohm reading
may still be greater than is
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