Biddle MEGGER - Page 38

in a dielectric test is immediately noticed by the load on the
hand crank. Cranking must not be forced under load.
When using the Earth-Guard connection, the ohmmeter will
read Infinity irrespective of the current which the generator
may be delivering. The reason for this should be clear from
the diagram, Figure 45. With the Earth-Guard connection,
no current flows through the ohmmeter deflecting coil A.
Use of Guard Terminal on Motor-Driven Sets
For Dielectric Strength or Break Down Tests
Motor-driven “Megger” sets as now supplied have a protec-
tive resistance in the guard circuit sufficient to limit the Earth-
Guard current in event of a short circuit across the Earth and
Guard terminals. In order to detect a short circuit during a
dielectric strength or break down test, a milliammeter should
be connected externally in series with the Guard terminal.
The following currents must not be exceeded:
2500-volt instruments య. 12 milliamperes
10007915 instruments! య. 20 milliamperes
500- Volt instruments: యనం. 30 milliamperes
These limitations are necessary to prevent overheating of in-
ternal components and driving motor and to preserve proper
commutation. The milliammeter referred to in the previous
paragraph must be watched carefully and the motor de-ener-
gized at the first sign of current values in excess of those given.
Otherwise generator commutation may be adversely affected
to the extent that pointer excursions may be caused in mak-
ing insulation resistance measurements on specimens having
appreciable capacitance.
Older motor-driven sets, other than the 2500v, 10,000 meg-
ohm types supplied prior to Sept., 1947, do not have guard
circuit protective resistances. To avoid damaging these instru-
ments from an Earth to Guard short circuit, a resistance of
40,000 ohms should be connected in series with the Earth-
Guard connection.
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