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Detect break in cable

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Scarr

Member
Hi,

What is the best way to detect a break in a long thick cable? when I say long I mean miles (approx 5) and thick I mean thick I mean UK overhead power type thickness.

Is it resistance / voltage / data stream pumped down it?

Thanks

Steve
 
Neither. It takes a lot more advanced equipment to determine where a fault might be. Resistance is no good because you have no idea how great damaged the cable is (otherwise, you would know where the fault was located).
One method is simply to send high voltage pulses into the cable and analyse the echo that any fault would generate.

Also some new hi voltage cables is special made so it's easy to determine whether they are faulty or not.
 
As Grossel stated you can send a pulse down the cable and look for the return (inverted) pulse reflected from the break with an oscilloscope. For an open line with air dielectric the pulse speed should be near the speed of light (3E8 m/s) so the round-trip time would be double the distance to the break.
 
This is an example Grossel and crutschow are talking about and is really sort of a poor mans version of a TDR (Time Domain Reflectometer). A Google of "time domain reflectometry" should bring up plenty of hits.

Ron
 
I have used Ron's poor-mans-TDR for many years. It is finnickey but does work. Once I know the aproximate distance I hook a power amp onto the wire (a 70V output is great), the common to a good ground, play some muzak and try and find the exact location with a "Tempo" 200FP filter probe ($100 on Ebay). Of course you must be able to disconnect the wire from its feed on the known end! E
 
Thanks for replies but...

Forgot to say:"cable is in a loop"

I don't need to know where the break is just that's it's broken, hence my thought of simple methods, what I was not sure about is things like loss over distance, variation in resistance due to temperature etc. unless that's what you are referring to by saying "it's not that simple"

someone has suggested applying 24v DC from a PSU capable of supplying 500ma and use the return to hold a relay open, if cable is cut relay will change and I can use this.

Thanks

Steve
 
You mentioned 5 miles of cable? That would be over 25,000 feet of cable. Can you share the cable size and is this 2 conductor cable? Do you have access to both ends?

Ron
 
Cable

Hi Reloadron

I don't have the cable size to hand but I do know its single core, it's like overhead power line cable and yes have access to both ends

Thanks
 
Well if it is single conductor you could measure the resistance end to end. As to using a relay and running a current through it much of those methods would work but that is where we get to knowing the cable conductor size, knowing that we can get the resistance per thousand feet and work from there.

Ron
 
So there is no other signal going through this cable?
 
Hi Reloadron

I don't have the cable size to hand but I do know its single core, it's like overhead power line cable and yes have access to both ends

Thanks

hi Scarr,
What is the 5mile loop of single core cable used for, whats its function.?...:)
 
Do you know what kind of damage it is? Is it like totally not connected (a complete cut of wire) or is it a fail that triggers the ground fault protection?

This is an example Grossel and crutschow are talking about and is really sort of a poor mans version of a TDR (Time Domain Reflectometer). A Google of "time domain reflectometry" should bring up plenty of hits.
Assuming this is all about a high voltage cable: Surely I cannot affort buying such kind of test equipment. The local power company owns such a device, and thay sure need a space wagon (at least) to carry it around.

Well if it is single conductor you could measure the resistance end to end. As to using a relay and running a current through it much of those methods would work but that is where we get to knowing the cable conductor size, knowing that we can get the resistance per thousand feet and work from there.

Ron
That wont work unless either of two conditions is true:
The cable is aviaiable AND can won't take damage if a test needle is pressed into it. DONT do this to a power cable as it will damage it (and your local power company will never alow you to do so anyway).
There is several avaiable points on the cable ready for measurement.

Otherwise just forget using resistance testing; unless the cable have a conductive shield AND you have a megger to measure resistance in the isolating layer AND the cable is completely cut off in the fault spot. But that not what I'm suggesting when speaking resistance-measurement, allthought it technically is resistance measurement.
 
cable fault

1. What type of cable, screened, not screened, 3core ?

( that determines type of test )

2. What is the cable used for ? power, data, voltage level HV, LV

3. First disconnect both ends of the cable and do a continuity check and insulation resistance check.

4. Trace the route of the cable.

5. Do the test, thumper or pool of potential.

A locate can be very accurately made with the right equipment.
 
My take and pure guess based on what has been posted is the cable is about 5 miles long (26.400 ft.) or 8 km long. That from the original post, then this was posted:

I don't have the cable size to hand but I do know its single core, it's like overhead power line cable and yes have access to both ends

I don't know but that leads me to believe single conductor. The original post just mentions "break" which I take as an open rather than nick or dent. What I do know is that is one long cable and he has both ends. :)

I guess there are several ways to take what has been presented, so we wait for more info I guess.

Ron
 
Sorry all, not at home at moment hence the lack of replies, managed to grab a PC and am remote controlling my PC to answer what I can and give more info.

Cable is unscreened, it is a total break not just nick / dent, it is a loop not single ended.

Thanks will check again ASAP
 
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