LT Power Cables - Failure

Could you give the name of your company please?

If you mean by bursting raising the voltage until an arcover occur at the weak point, I do not think so. The weak conduction by a contaminant does not need occur at the thinnest point of the insulation. Additionally, when a cable is charged up to high voltage, it contains considerable amount of energy. When an arcing occurs anyplace on it, waves propagate from it toward both end. On the open end they undergo complete reflexion. When those waves meet again, at some points (as in multiple) they add together to a voltage ideally twice of the original voltage. At those points further arcings will occur, until the peak voltage drops low enough. This kind of behavior (minus the arcings, hopefully) is well known to radio engineers, and to good radio amateurs. I am talking about well known phenomena in radio technology. Your TDR manual has to explain the various kind of reflexions too.

There is a rude way to salvage most of the cable by successive halving and measuring. Cut the cable in half and measure. Cut the bad half in half and measure. After a few steps the remainder is unsaleable, but most of it is ok.

How about running a current in the cable and check for hot spots with infrared imaging?

What is the rated voltage/insulation specifications/conductor material/no of phases/thickness of insulation/conductor specifications of the cable?. Any thing should be manufactured according to approved standards like BS/IEC/NEC/VDE etc. Are the technicians,supervisor and engineer experienced in this type of work. Was the correct test voltage applied for the recommended duration?.

Thanx for your replies...

Applying a High Voltage is risky and that is the reason we were hesitant to do it. If a substantial amount of current can heat up the fault and result in a dead short, we may be then successful with the TDR. What is your opinion on this? Any idea of the current required?

We manufacture cables in India and the reference standard is Indian Standard 7098 Part 1. All the dimensional and material properties are subjected to this standard. As per this standard the HV routine test voltage is correct. Almost all our cable drums pass in the HV tests. Failure rate is around 2-3% only.

Quality of materials as well as good workmanship are very important.

If you test with a low value of test current, the damage done by the test current may be small and difficult or impossible to find.
The problem with testing with high current is that the cause of the failure may be destroyed and then it will be impossible to determine the cause of the failure.

Dear Mr. Paul,

Please note that with use of TDR, if the Fault Resistance is equal or less than surge( charactertistic) impedance of cable, prelocation of fault may be done( putting cursor at the begining of the inverted loop for short circuit fault for example, taking into consideration of velocity propagation constant.) For High Resistance fault, you may use TDR set along with Surge generator,Magnetic Amplifier/ Audio Amplifier and use in SIM ( Secondary Impulse Mode) for prelocation and pinpoint location of fault.Also in some cases, High Resistance Cable Fault locator Bridge is very useful. Nowadays, burning of fault resistance is prohibited as during buring process, due to flow of high current, the subject cable may develop further fault or weakening of cable insulation at other points. You may consult one very good book in this regard," UNDERGROUND CABLE FAULT LOCATION" written by Barry Clegg.

Regards,

Manindra