Megger testing 600V wire at 500VDC

[JacksonJim]

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...

 

[waross]

The old school rule of thumb was twice the working voltage plus 1000 Volts. You are a few percent over that.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill is that RoT for cable/wire? Not for motors or transformers..?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. It was mostly for transformers and motors and other equipment up to about 4160 Volts.
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

 

[JacksonJim]

I have owned a 5000 V megger for over 14 years and , we have megged motors, heat trace, basic wire and cables. we have never had a failure of wire (That I know of). I have never returned to a "megged line" and had to replace it due to this procedure. I understand what all the rules are, but I think it may be something of a wattage thing and if the wire can handle the wattage of the low mA current and high voltage relationship. Not being an engineer, I do not know how to calculate such a thing. I have to prove my case that my meg did no damage their wire or I have to replace it on my dime.

 

[DTR2011]

NETA specs list 1000V Test voltage for 600V class wire. IN theory a Megger is non destructive and the test voltage should dive if the IR values are low (high current). Some meggers have a 'burn' mode, which acts more like a Hi Pot and will (purposely) puncture poor insulation for the purpose of fault location.

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?

 

[Zogzog]

"NETA specs list 1000V Test voltage for 600V class wire" That is the MINUMUM test voltage

 

[JacksonJim]

If I could not find a leak at 550 VDC, move to 1000V, then to 2500V then to 5000V if we did not find a leak, then we call it good. Simple, yet unrefined I am learning..

 

[LionelHutz]

2500VDC seems to be a more suitable upper limit. I typically use 2-2.5 times plus 1000, but since it's a DC test I'd use the peak voltage. Puts you in the 2700-3200VDC range.

 

[JacksonJim]

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..

 

[slavag]

5000V DC to 600V wire? for what?
2kV AC 1min and 500V DC is enough, imho.

 

[JacksonJim]

To update... We have been megging wires at this high voltage to guarantee there is no leak in the customers wire. We did not know we were actually not supposed to. We have a wire pull that we megged at such voltage one day it was good, we returned 2 days later and it was bad and the electrician is telling the customer my megger messed up his wire. We have been doing our own test here in our shop and now we have had 5000VDC on a 4' wire encased in a 1/2" EMT conduit 3' long for 4 hours no failures.... HMMM Makes one wonder.... One clamp one the wire and one on the conduit.. Going to do test for customer I guess.

 

[Compositepro]

It is not so much the voltage that is important, but rather the electric field strength through the insulation. A wire lying against a sharp grounded edge will stress insulation a lot more than laying on a smooth surface.
On the other hand, a pinhole in wire insulation will not usually cause any real problems at only a few hundred volts.

 

[JacksonJim]

Composite, pinholes in deep south east Texas always a problem with water in conduits... So, are you saying is if I test w/ 500Volts or 1000Volts, and a pin hole exists in dry conduit, chances are the unit will not short to ground w/ megger test or motor running? See I am being blamed by an electrician that damaged his wire and I am just seeing what the folks in the world out here would think.. Our test here in our shop proves us right all day/.

 

[don01]

JacksonJim
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

 

[JacksonJim]

Thanks all for your input...