Biddle MEGGER - Page 13

3. Discharge of Capacitance
It is very important—especially in large equipment—that
capacitance be discharged, both before and after an insulation
resistance test.
Discharging capacitance after a test is a wise safety precaution.
“Megger” instruments are frequently equipped with Dis-
charge Switches (see page 25) for this purpose.
4. Current Leakage at Switches, etc.
When apparatus is shut down for the insulation resistance
test, make sure that the readings are not affected by leakage
over or through switches or fuse blocks, etc. Such leakage may
mask the true insulation resistance of the apparatus under test.
See Use of Guard Terminal, page 32.
Or, what may be more serious, current from an energized line
may leak into the apparatus and cause inconsistent readings,
particularly if the live line is d-c. However, such leakage usually
can be detected by watching the pointer of the “Megger” instru-
ment at the moment the test leads are connected to the appa-
ratus and before the instrument is operated. (Before making
these observations, be sure that all capacitance is discharged
by short-circuiting or grounding the apparatus.)
SAFETY PRECAUTIONS
Observe all rules for safety when taking equipment out of
service. Block out disconnect switches. Test for foreign or
induced voltages. Apply workmen’s grounds.
Remember that when working around high voltage equip-
ment there is always a possibility of voltages being induced in
apparatus under test or lines to which it is connected, because
of proximity to energized high voltage equipment. Therefore,
rather than removing a workmen’s ground in order to make
a test, it is more advisable to disconnect the apparatus, such as
a transformer or circuit breaker, from the exposed bus or line,
leaving the latter grounded. Use rubber gloves when connect-
ting the test leads to the apparatus, and when operating the
“Megger” instrument. i
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