Megger ATEC 210415 Series, Manual

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M
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AVTM21-415Ja Rev. A June 2005
effective if the equivalent current is less than 50 µA. The
stabilizer can be "IN" for the complete test but the output
voltage will take longer to reach its final value than if the
stabilizer is "OUT".
USE OF GUARD (See also Section 10)
Occasionally when measuring apparatus, the leakage resistance
of interest is masked by a parallel path. Such a situation is often
encountered when measuring the leakage resistance of a cable.
In the practical situation, the value of the resistance along the
leakage path from the conductor over the surface of the
pothead may be less than that within the major length of the
cable. Under such conditions the guard feature of the Test Set
is useful in excluding the resistance of the surface path from
the measurement (See Figures 10 and 11). This is easily
accomplished by forming an intercepting electrode by
wrapping a bare wire around the outer surface of the cable
pothead insulation close to the cable shield or ground. This
wire electrode, which will intercept the surface current, must
be connected to the Test Set guard terminal. The unwanted
current then bypasses the megohm indicating meter and is not
measured; the meter will read only the insulation resistance of
the main portion of the cable.
The same guard principle can be used on other apparatus
(Figures 12 and 13) where a suitable intercepting electrode can
be added on a temporary basis. On some kinds of apparatus,
notably bushings, the guard electrode is built-in.
The Guard terminal is only a few volts above ground and does
not pose a safety hazard; however, it must not be allowed to
contact ground.