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# I need info on sensing the path of signal in supply cables

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#### SwordFish

##### Member level 4
sensing the cables...

Hi,

I have much trouble with supply cables. I saw in a magazine that one can detect the exact place of interruption. But they don't give the solution. My basic ideea is to sense (electromagnetic fields) the path of signal through supply cables. I don't know if someone already made something in this area. Thats why I need help. I need some schematics and/or web addressor others ideeas, please.

thanks.

one method

The trouble you are up against is that once the wire is broken, no current flows anywhere. One slightly destructive trick is to use a sewing needle to penetrate the insulation and measure the voltage with a VOM connected to the needle. You go from the source of the voltage in steps along the cable until it quits. The binary search method is fast. Start at the midpoint. Then go half way to the end if you measure a voltage and half way to the source if you measure no voltage. Continue this halving and you will get within a mm of the break. There are time domain methods, but you need a generator and oscilloscope with ps range rise and fall times.

Always remember to keep yourrself insulated from the needle if there are dangerous voltages.

this concept is based on time domain reflectometry (tdr)

in fact, you send a small pulse on the wire, and you look at the echo it produces. different types of echoes gives different types and distances on bad wires.

you can look at http://www.tscm.com/tdr.html for a small example, or use google with "time domain reflectometry"

I think also there are some examples on epanorama.net

good luck, you still need a memory scope with the nanosecond resolution...

Hi xdanx2000,

I have found a simple solution.

Try to measure capacitance between two wires from one side C1, and from other C2.

L_to_interrupt = C1 * L_total / (C1+C2)

Best Wishes! klug.

Re: sensing the cables...

if i want to sense existance of a cable inside a wall in my room , Can anyone suggest me acircuit or schematic ?

klug said:
Hi xdanx2000,

I have found a simple solution.

Try to measure capacitance between two wires from one side C1, and from other C2.

L_to_interrupt = C1 * L_total / (C1+C2)

Best Wishes! klug.
Very Cool Idea! What kind of accuracy have you seen?

Re: sensing the cables...

Time domain reflectometry works quite well, but implementing it is diifficult and expensive. If you have a digital storage oscilliscope, a fast pulse generator and a calculator you should be able to get a reasonable result. However, if the cable is really long, you will need to know its velocity factor.

An easier solution is to use your brain! Cables don't break for no reason. If you can, check the outside of the cable for damage or sharp kinks. If you short one end of the cable and fit a buzzer or lamp to the other end, you can then manipulate the cable along it's length and listen or watch for a response. I have found that this works in 9 out of 10 situations.

If the cable is buried, check the ground above its path for disturbance. Heavy vehicles passing over a badly buried cable can often cause damage.

The capacitance measurement idea sounds good too.

Re: sensing the cables...

I want to find a wire in the wall... does anyone have any schematics?

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