short circuit detector

beenthere

Joined Apr 20, 2004
15,819
When faced by the assignment -
I want a circuit diagram for tracing point of short circuit in a five core cable laid 1.2 metres underground and runs about 15 metres from a building to another.
- it does not seem as if a simple short/open detector can do the job.

If the cable is inside some piping, pulling the old cable with a length of new spliced onto the end to replace it is about the only low-tech way out.
 

Ron H

Joined Apr 14, 2005
7,063
When faced by the assignment - - it does not seem as if a simple short/open detector can do the job.

If the cable is inside some piping, pulling the old cable with a length of new spliced onto the end to replace it is about the only low-tech way out.
You still have to strip the insulation from the cable and measure the distance to the short with a tape measure.:D
 

someonesdad

Joined Jul 7, 2009
1,583
Well, if they can't afford an oscilloscope, then they're not much of a school. Here's a cheap TDR circuit. If they can't afford that, then it must be a school for breaking rocks with sledge hammers... :p
 

Thread Starter

johnbosco09

Joined Feb 5, 2011
6
the truth is that our school teacher seems not to have an answer either. The 5-core cable supplies a neigbouring transformer and runs under a coal tar public road. Breaking the road open, is a big job. We want to get an accurate point and open only once. Thanks all.
 

someonesdad

Joined Jul 7, 2009
1,583
One method that wasn't mentioned was a type of inductive tracer used by electricians. Ones that work at the depth you indicated can be expensive.

Did you read about the Murray loop/Varley loop method I suggested? If this method is appropriate for your type of fault, it might be the easiest and cheapest method of solution.

The ideas you have been given are based on knowing or assuming the physical properties of the conductor per unit length. Then you use a measuring instrument to infer the quantity of that property and deduce the distance from that measured quantity. Commonly-used physical properties are capacitance, resistance, and propagation time (with the conductor acting as a transmission line).

One of my favorite tools is a 1 amp constant current source. In conjunction with a digital multimeter, this can let you measure resistances to milliohm or microohm resolutions. These two together form a simple and inexpensive troubleshooting tool for circuits involving low resistances like wiring (Adjuster mentioned it earlier and you'll hear it called a "four-wire" or "Kelvin" resistance measurement). You can build such a current source with a battery, LM317 integrated circuit, a heat sink, and a resistor.

You've not given us any idea of your access to any measuring equipment/resources, so we probably can't help you any further.
 

Ron H

Joined Apr 14, 2005
7,063
the truth is that our school teacher seems not to have an answer either. The 5-core cable supplies a neigbouring transformer and runs under a coal tar public road. Breaking the road open, is a big job. We want to get an accurate point and open only once. Thanks all.
I was right!:D
I'm wondering if this is an actual problem the school has, and they are hoping to get a cheap solution, while giving the students a learning experience.
In other words, maybe the instructor doesn't know the answer either.:eek: He only knows that the school can't afford TDR.
 
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