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4. The
green
Guard
test lead is not always used.
Connection with Guard test lead. The test leads in the drawings are only for
illustration.
IMPORTANT:
Application Note 32 explains the use of
Guard
test lead in
order to minimizing the effect of stray resistances. When measurement is
carried out between parts which none of them is grounded, (like between
high-side and low-side windings of a transformer),
Guard
test lead must be
connected to ground in order to fix the apparatus potential. At any time a
measurement is performed, either the
-R
or
Guard
test lead must be
connected to ground but never both simultaneously. If none of these terminals
are connected to ground, the insulation tester can reach a high potential that
may result in an unstable non-reliable reading. If both terminals are
simultaneously connected to ground, there is a short-circuit between them
and consequently the insulation tester will measure with error.