# short circuit detector

Discussion in 'Homework Help' started by johnbosco09, Feb 5, 2011.

1. ### johnbosco09 Thread Starter New Member

Feb 5, 2011
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I am new here. 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. Please it is a shool assignment.

2. ### beenthere Retired Moderator

Apr 20, 2004
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You might try researching the topic "time domain reflectometer".

3. ### SgtWookie Expert

Jul 17, 2007
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Have you heard of TDR? (Time Domain Reflectometry)

(eta) I have to remember to hit the Refresh button before replying...

Dec 26, 2010
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Maybe TDR is a little advanced for a school project? Depending on what stage the students have reached, they may not have covered transmission line concepts.

Perhaps they would be expected to use more elementary resistance measurements? This is less effective, particularly for other than a dead short, but may be more appropriate to an earlier educational stage.

5. ### SgtWookie Expert

Jul 17, 2007
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Point well taken; that's why I didn't mention FDR.
Resistance measurements could show which are shorted and/or open, but our OP's requirement stated that they needed to locate the point where the wiring is shorted.

Granted, they might be able to discern the distance (if the short is "perfect") by the resistance of the wire, but that would require a pretty accurate meter.

Dec 26, 2010
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It would certainly be quite challenging - but perhaps a good illustration of the use of four-wire methods for low resistance measurement. If a four-terminal meter were unavailable, the next best thing might be to run a fairly substantial current around a pair the of shorted conductors, and measure the voltage drop with a reasonably sensitive voltmeter. Depending on the wire gauge, they should be able to get some tens of millivolts per loop metre with half an amp or so. Testing from both ends of the cable (if accessible) would help.

They also mention that the cable is buried 1.2m down. Do you think they would stand a cat in H***'s chance with a search coil, and an AC signal fed along the shorted pair? My gut feel is no, but I've never tried it. Let us hope that the OP comes back with some more details.

7. ### JoeJester AAC Fanatic!

Apr 26, 2005
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The school project if probably for a very basic TDR using a 555 timer and an oscilloscope.

In which case, the OP still needs to research Time Domain Reflectomoters.

on edit:

There is a short description of a TDR in Module 21, Test Methods and Practices, found at http://www.fcctests.com/neets/neets.htm

Last edited: Feb 5, 2011

Dec 26, 2010
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Perhaps they would need to follow that 555 by something to speed up the edges a bit. If they want to resolve to say 1 metre, wouldn't they need to get $\leq$10ns displayed rise time for the echo?

(Assuming cable velocity is about 2e8m/s, round trip delay is 10ns/metre?)

9. ### Ron H AAC Fanatic!

Apr 14, 2005
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I agree with you.

10. ### JoeJester AAC Fanatic!

Apr 26, 2005
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The OP would have discovered that sharper edges are needed after they viewed the how a TDR worked. It's a short leap to looking at the cable velocities, cable characteristics, and the delays to expect.

All of that is moot since the OP hasn't rejoined the conversation.

11. ### beenthere Retired Moderator

Apr 20, 2004
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This may indicate the depth of interest.

12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Some Fluke meters for Cat5 use line resistance and capacitance for length and distance to defect (open or short).

That method might or might not be of use depending on how close the wires in the cable are.

Measuring conductance to actual earth ground can find grounded shorts in buried cables.

A TDR will essentially do all of the above if read correctly.

Jul 7, 2009
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A Murray or Varley loop might also be of interest to the OP.

A simple TDR with one chip is here. I've also done simple TDR stuff with a scope and a pulse generator, stuff that's likely to be available to the OP if he's in college.

14. ### johnbosco09 Thread Starter New Member

Feb 5, 2011
6
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thank you brothers for your concern. I studied the TDR but it is too advanced for me. Are there other ideas that would not require an oscilloscope? Since there is no power source around the track, what is required of me is a battery powered handheld detector.

15. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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All viable methods were listed in post #12.

The cost of the equipment is most likely too great which simplifies make re-laying the short cable the best option. Under thousands of meters of distance, replacing the cable is the best option.

16. ### johnbosco09 Thread Starter New Member

Feb 5, 2011
6
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thank you all for co*ncern. I have been on research about the TDR since then. TDR is too advanced. My school expects a battery powered handheld detector maybe with an alarm but not an oscilloscope. Please are there other design ideas?

17. ### beenthere Retired Moderator

Apr 20, 2004
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Is your assignment really to produce a hand held box that will locate a short in a cable, or is it to determine if one of the included conductors has gone short? There is a world of difference between those functions.

18. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Are you being asked to locate which pair is shorted or are you asking to locate the short as in a distance from either end of the cable?

One requires a simple alarm, battery, and probes. The other requires alot more.

19. ### johnbosco09 Thread Starter New Member

Feb 5, 2011
6
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I am sorry pls. I could not rejoin b'cos our school internet was tossed.
'The TDR was is advanced' they said. A much easier and simpler method is expected. Thanks.

20. ### Ron H AAC Fanatic!

Apr 14, 2005
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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. He only knows that the school can't afford TDR.