Circutor MI-5500e Manual De Uso página 28

The green Guard  test lead is not always used.
4.
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.
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