High Voltage Testing Flashcards
What can insulation problems be caused by?
- Poor workmanship
- High ESF
- Manufacture defects
- High Temps
- Chemical changes
What do meggars induce to test?
H.V. DC
How high should resistance be in proper insulation?
Mega Ohm range
What values of voltage are induced?
500 - 15kV DC
How much time should you induce the voltage?
depends on capacitance of cable or equipment
When are readings taken?
once stablized
What should be done to the equipment/cable before testing?
isolate from all sensitive loads such as metering equipment, regulators, ballasts, switches
What do you do to cable ends?
isolate from ground and corona proof
How do you connect the megger?
- positive lead to ground/return path
- negative lead to test conductor/equipment
- guard lead to dielectric
Why is reverse polarity preferred?
due to electroendosmosis
what is electroendosmosis?
moistures polar behavior to migrate to a negative charge
What is a proof test?
to test the cable before it is pulled to make sure it is good
How long is a short time test?
60 seconds
What is the polarization index test?
two readings:
1 minute duration & 10 minute duration
how do you calculate polarization index? what ratio indicates a problem?
10 minute reading/1 minute reading
ratio below 1.5 indicates a problem
