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3. The green GUARD (G) terminal is not always used.
BLACK
TEST LEAD
RED
TEST LEAD
GUARD CABLE
(GREEN)
Technical Note 32 explains the use of GUARD (G) terminal 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 terminal must be
connected to ground in order to fix the apparatus potential. At any time a
measurement is performed, either the -R or GUARD terminals 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.
32
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