Megger testing 600V wire at 500VDC
Megger testing 600V wire at 500VDC
(OP)
I have been megging basic wires like THHN with a 5000V megger, same with motors, I have never had any problems with insulation breakdown or failure due from megging 600V Wire at this dc voltage level. Does anyone know if it is okay to do this or shall I expect insulation failure every time? We have a new wire pull that megged good at 5000V but then a few days later no change to the system it megs bad at 500V not even 5000v. Any suggestions? Electrician is saying I damaged the wire with my megger test at 5000V...






RE: Megger testing 600V wire at 500VDC
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Megger testing 600V wire at 500VDC
Keith Cress
kcress - http://www.flaminsystems.com
RE: Megger testing 600V wire at 500VDC
At higher voltage ratings cables would not be subjected to as great an over voltage with a megger test.
During a knee point test to determine the ultimate breakdown voltage the voltage may be higher, but in a knee point test, the voltage is slowly raised and the current is monitored. When a disproportionate increase in current indicates impending breakdown the test is terminated. A megger test is more of a "Turn it on and watch for smoke" test.
833% of rated voltage is a bit much for testing most normal equipment and cables.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Megger testing 600V wire at 500VDC
RE: Megger testing 600V wire at 500VDC
I've been using a megger for 20 some years and have never thought of applying 5kV to a 600V wire. What is the logic in such a high voltage?
RE: Megger testing 600V wire at 500VDC
RE: Megger testing 600V wire at 500VDC
RE: Megger testing 600V wire at 500VDC
RE: Megger testing 600V wire at 500VDC
Performing our own test now, have had 5000VDC on a 4' wire encased in a 1/2" EMT conduit 3' long for 2 hours no failures.... HMMM Makes one wonder.... One clamp one the wire and one on the conduit..
RE: Megger testing 600V wire at 500VDC
2kV AC 1min and 500V DC is enough, imho.
RE: Megger testing 600V wire at 500VDC
RE: Megger testing 600V wire at 500VDC
On the other hand, a pinhole in wire insulation will not usually cause any real problems at only a few hundred volts.
RE: Megger testing 600V wire at 500VDC
RE: Megger testing 600V wire at 500VDC
split this into 2 issues
1. a cable was tested and there is debate around the voltage used..... more later
2. A cable was tested and passed the test -
I am not keen on the voltage the test was done at BUT IT PASSED and was good when the meter was disconnected.
-I hear "the you weakened it" argument but will say it survived at 5000V so it was good it did not slowly fade away overnight .
Look for something else - if you had carried out the insulation test for certifying it would hold true ... and the cert could be quoted in a disagreement.
I would suspect something else has happened and don't think you are the source of this particular evil.
for example
insulation knick at the gland entry,
the pressure point discribed earlier,
the nail gun fired nail we found in a 66KV cable run
and any one of a myriad of possibilties
the more later bit
Going forward ALWAYS get the manufacturers cable specs before stress testing -I know of cable which if marked 415/600V then 600 is the max rating and should not be tested higher
Go and find the manufacturers maximum rated voltage for testing -hopefully it will not be too bad
Having said that have a great day and smile
Don
RE: Megger testing 600V wire at 500VDC