what is the safe meggar values for motor, transformer HT, LT
and Cables?
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Answer Posted / keith
If you need it to keep working, such as a sewage or storm
water pump,most reputable companys would trend the readings
beginning with the date the started servicing the motor and
watch for a dramatic decrease in test readings. One has to
be aware of the capacitance effect that water has on the
motor cable of a submersible pump as well as understanding
that it is normal to see a gradual decrease over a several
year period from above 200 megaohms @ 500 VOLT d.c. ( brand
new)to eventually down to 10 to 20 mega ohms. I have found
that although motors will run down to 2 mega ohms 230 volt
3 phase and 1 mega ohm to .6 mega ohms 480 3 phase they
don't run much longer. More important is to recognize
dratic decreases between monthly to quarterly readings as
they indicate potential failure ( direct short to ground)
which could trip main distribution breakers, thereby
effecting other motors or equipment by robbing them of
power. i.e. Alarm cicuits that are not battery back up
equipped.
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