Finding cable fault location 1

[nraju]

I have heard that there is an empirical method of finding the approximate location of a fault in a cable in the absence of a fault locator (some call it as null point method and others as 'loop method'). Would appreciate if someone who knows the procedure / empirical formula can share the same.

 

[Shortstub]

Can you determine the nature of the fault. Three-phase? Phase-to-phase? Phase-to-ground/earth?

 

[electricpete]

nraju - Sounds like you are already more familiar with available methods than me.

I have heard of two methods:
Time Domain Reflectometer. Sends out a pulse, look at the time until the reflected pulse changes characteristic, calculate distance based on time/speed.

Also something caleed a thumper that makes noise at the point of the fault?

 

[busbar]

e’-pete — Another use of the thumper is to "burn" the fault to a greater degree so it will show up more readily with the TDR. It is a nasty method, particualrly compared to {undesirable these days} microampere levels for DC in-service testing.

 

[jbartos]

Suggestion: Visit

http://www.baur.at/en_baur/_products/c_irg3000.htm

for:
quick and easy selection of different fault location methods

 

[Skogsgurra]

For those who need a very simple and very cheap "cable radar" attachment that uses an ordinary scope to do time domain reflectometry I recommend a visit to

http://www.elecdesign.com/1998/oct0198/ifd/2IFD.pdf

Use it with Tek or Fluke portable scopes. Simple, cheap, efficient.

 

[ausphil]

nraju,

Sounds like you are trying to describe a resistance location for earth faults on cables (use of the term loop method).

This basically measures the ratio of the resistance of the length of cable to the fault to the full length of the cable. It is also called a bridge location or resistance location.

It is performed with a resistance bridge, and this is basically where the term "loop" comes from, in that the different measurements you take form a loop circuit. The different measurements take into account the resistance of the leads, and resistance of the fault.

You need to have at least one good core in the cable, or a ratio of insualtion resistance of at least 10:1 of the good core to the faulty one.

There are a number of different location methods, and I would assume the link by jbartos would give most of the good info (as Baur have been in the business of making bridges for years).

 

[RavenJoe]

nraju, As mentioned previously, thumping is a high voltage pulse that is designed to create a larger more noticable fault that can be observed visualy or audibly. I have seen the results and it is a nasty but effective meathod. Usually reserved for finding small high voltage cable faults that are indicated by a meggar prior to applying high voltage to cable.

 

[torontoele]

for phase-to-ground fault,ypu can pinpoint the location by a voltage detector:injecting dc pulse in the phase line and use the portable detector on the surface of the ground.when you pass the fault point,the detector will show a different voltage--in direction.
btw,for a long cable,you can use resistance bridge to get an 'about' location.
torontoele

 

[jbartos]

Suggestion: Beside Time Domain Reflectometer (TDR), the Frequency Domain Reflectometer (FDR) or Standing-Wave Reflectometer (SWR) might be applied.
Visit

http://www.kinectrics.com/en/serviceline/MobileFieldTes...

http://www.kinectrics.com/Library/SP - MobileTestin...

http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb0...

etc. for more info
SWR has a simpler circuitry than TDR, therefore, it is less expensive.