Detecting a break midway in a cable .

[Tubetec]

I called out to a buddy the other day ,
The bench jack cable I had made up for him six months ago was broken ,
I was 100% confident in my soldering , so after checking that I investigated further ,
turned out the signal conductor was broken .
We ended up having a gamble and cutting the cable in two unequal lenghts ,
luckily the shorter lenght ended up the side with the fault , there was no outwards signs of damage anywhere .

I got to thinking about the problem later and a way to detect a break in a cable accurately rather than taking pot luck ,
If I energise the broken conductor with an AC source I can easily locate the break with a non contact voltage detector pen .

 

[musipol]

Depending on the type of cable, you could use a TDR. The downside is they are expensive and really only work well on coaxial cable(ime).

 

[ccaudle]

really only work well on coaxial cable

You can get two channel TDR that works for twisted pair. Or use a transformer as balun.

 

[Barry Hufker]

For those of us who don't know (maybe it's only me), please tell me what a TDR is.

 

[Brian Roth]

https://en.wikipedia.org/wiki/Time-domain_reflectometer

Bri

 

[scott2000]

I think automotive applications have something for finding breaks in harnesses .. Kinda like a power probe with a sniffer....
Or something along those lines...
I'd have to look but remember watching a convincing video that made me want one....
wasn't terribly spendy....

 

[JohnRoberts]

The phone company line service people use TDR from the switching office, to look for breaks in the wires. I had one line fault that was on the pole near my house but the phone guys thought the reflection they were reading was my actual service terminus, and ASSumed that my lines were good.

JR

 

[Barry Hufker]

Thank you Brian. I always enjoy learning something.

 

[Brian Roth]

@Barry Hufker Many years ago, I first learned about TDR from a friend's wife! He owned a PA rental company, she worked for SW Bell as a tech. She was quite the well of knowledge about all things telco related.

Bri

 

[gyraf]

For shielded cable, measure and compare capacitances from each end..

 

[Horizoneer]

Put the end to your ear and run the cable through a loop in your other hand, and you'll hear when you get to the break. Kinda hard to explain in words, there's a video out there somewhere...

 

[k brown]

For shielded cable, measure and compare capacitances from each end..

How can that locate exactly where along the cable the break is?

 

[RuudNL]

If you measure 300 pF on one end and 900 pF on the other end, the interruption must be around 1/4 (or 3/4 from the other end) of the cable.

 

[Matador]

This is done commonly for HAM radio installations with long feeder lines.

If there is a physical break in the conductor, it will mean there is an impedance discontinuity right at the break. You can feed in a high frequency square wave into the line, and scope for the reflected signal. You need to make sure you terminate the other line with the characteristic impedance of your line (50 ohms) so that you don't end up with multiple reflections if the line isn't completely broken. The difference between the send time and the reflected time will be proportional to twice the distance between the send and where the break is located.

The speed of signal in a line like coax is about 2/3 the speed of light, or about 8 in/ns. So if you see a reflection at 10 ns, then the break is close to 40 in from where you are sending the test signal (you have to divide by 2, to account for round trip time).

 

[k brown]

If you measure 300 pF on one end and 900 pF on the other end, the interruption must be around 1/4 (or 3/4 from the other end) of the cable.

Ah, ratio - got it.

 

[Newmarket]

Put the end to your ear and run the cable through a loop in your other hand, and you'll hear when you get to the break. Kinda hard to explain in words, there's a video out there somewhere...

Very much depends on the type of cable / lay up.

 

[Philbo King]

I once built a TDR setup with a pulse generator set to one-shot mode and a 500MHz oscilloscope. I don't recall the details, except it worked fine. Might need a higher bandwidth scope for short cables though.

 

[clintrubber]

It's summer, but this made me think of these Design Ideas, FWIW:

www.edn.com/simple-tester-checks-christmas-tree-lights/

www.edn.com/bad-bulb-finder-fixes-christmas-lights/