LV Cable HiPot

[joan271273]

I have used meggering for 1 minute as stated in NETA for field cable installations below 1kV. However Southwire in their website identifies hipot test for LV cables and one of our cleints wants to have all LV cable above 250 kcMil hipotted. Can you explain were is Southwire geeting these values and are they really valid. for example for cables between 550 and 1000 they recommend Hipot at 9.6kV DC for 5 minutes.


thread238-130438

 

[Zogzog]

Hey, why argue? If they want to pay for that, let them. I have no idea where that comes from, DC hipoting is not really accepted anymore for MV cables because it is considered destructive, so I can only assume it would be considered destructive for a LV cable.

I have been very involved in NETA for many years and cable testing has been the hot topic for the last 2 years or so, LV cables never came into any discussion.

Why dont you ask southwire where they get this spec from? If you find out, please post.

 

[joan271273]

I have posted formal requests with Southwire on this topic. I have never seen this done or identified in standards. However when a manaufacturer publishes this type of data!.

 

[dgbpeakpwr]

I don't know where they have come up with this. They do say not to exceed five minutes on the test, not five minutes. I would limit the testing to around one minute to avoid possible damage.

 

[benlanz]

The only IEEE standard that comes to mind is IEEE 690 which gives criteria for a DC-voltage insulation resistance test for cables <600 volts. The failure mechanism for LV cable <600 voltage is completely different than the failure mechanisms of MV or HV cables. IEEE 400 recommendations against the use of DC for MV and HV cable should not be extrapolated to testing LV cables.

Since LV cables are not shielded, a withstand test is next to useless because the voltage stress distribution across the cable insulation is not equal along the length of the cable. However, if there is a ground plan near a part of cable insulation, that part will be stressed. If there happens to be a crack in the insulation being stressed, some leakage may be recorded by a sensitive micro ammeter. Go ahead and megger, hipot, all you want, but understand the risk that you can have a failure literally one minute after reenergization with out warning. One minute should be more than sufficient to detect leakage current issues.


Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[joan271273]

Thanks for the IEEE reference, so basically the Southwire recommendation is useless per your response. Maybe that explains why? other vendors like Okonite do not identify HiPOt for LV cables.
It is a problem as if we have a project with Southwire LV cables a customer might refer to this type of test as " recommended by manufacturer" type test!!.