How do you find a break in a cable?

[Krwkka]

I have a Marshall footswitch that had a break somewhere in the cable. The break was not near the end of the wire.

So I had this BRILLIANT idea ... break out my cool Greenlee non-contact voltage detector with adjustable sensitivity that I bought from Home Depot and scan the wire. So I hooked up one side of a 6 volt transformer secondary and ...

- it didn't work. Even though my meter showed that the wire was open, the voltage tester showed voltage all along the wire when power was applied. Oh, and there was no bleed through to the other two wires, so that wasn't the problem.

Any ideas on why this didn't work? Or suggestions on the best way to find a tough break like this without destroying the cable?

[Jazz P Bass]

Yeah finding the break can be tough.
I usually use me DVM set on ohms & bend the cable back & forth up & down the length .
Sometimes the wire eill "make" for an instant.
Really testing wires takes special testing equipment.
Cable tester - Wikipedia, the free encyclopedia

[Jazz P Bass]

Here is a nice tester that will tell you where the break is in the length.
Link:NU Network Cable Tester/Analyzer Wireless Multifunction - eBay (item 290496298359 end time Apr-03-11 08:57:17 PDT)

[Krwkka]

Well that's a good suggestion! I shoulda thunk a that. Thank you.

I'm going to read up some more on that tester.

Still like to know why my voltage detector said there was voltage through the entire cable even though I read open with my ohm meter.

[g1]

How did you have your tester connected? You said "the other two wires" so I'm assuming it's a mic cable?
I would think you would need to put the transformer across a pair of wires and some kind of "load" at the other end.

[olddawg]

You know...I've always just cut 6 -12" off of each end.... and if that doesn't do it replace the whole thing. If you have an open somewhere else than the end stress points more than likely the cable will not be reliable.

[Raybob]

I have a Marshall footswitch that had a break somewhere in the cable...

99% sure it would be at the molded plug, if it's the molded plastic type.

Raybob

[Krwkka]

>>>How did you have your tester connected?

The voltage detector doesn't connect. You hold it in the vicinity of the wire to see if there's voltage. Mainly used by electricians for checking whether a wire is live. The cable is 20 ga, 3 conductor on a foot control for a channel switch.

>>>...just cut 6 -12" off of each end ...

That's what I ended up doing,. Then I started opening up sections of the jacket. The break was in the middle, wouldn't ya know! So, yeah, new cable. But I couldn't find the nice, flexible, rubber jacketed cable like the original.

[R.G.]

N.B.
The way the network cable testers work is to act as a Time Domain Reflectometer (TDR). This puts a very fast-edged pulse on the end of the cable and looks for faint reflections of the pulse. The speed of an electric field in a copper cable is approximately 1nS/foot, and any discontinuity in the cable makes a big, fat reflection because of the sudden change in impedance at the break. So you put in a fast pulse, and start counting picoseconds. When the reflection gets back it's 1nS/foot away.

There are other ways. If you know the capacitance per foot of the cable, you can just measure the capacitance of one end of the cable and calculate. Probably works better if you measure from both ends and calculate from both ends, as a check on accuracy.

[km6xz]

Regardless of the rated capacitance per foot, the ratio between the value read from each end will tell you pretty accurately the distance from an end as a ratio of its total length. A lot of cheapie DMM's have a suitable capacitance range.
The voltage detector works on a Hall Effect transducer and if intended to check for AC mains, the detector's gain is probably set low as to not trigger on false indications. If it is a shielded wire, that will reduce its effectiveness also.

[R.G.]

You are absolutely correct.

However - and I only mention this because I've run into it before - if there are two or more discontinuities in the cable, a proportioning reading will give you wrong results. This is much rarer, but can happen. In general, a proportion from each end is OK. I was reading both ends on an installed signal cable in a building, where simply putting in a new one was not easy. I tore almost all my hair out over that one before thinking of ... two faults ... being possible.

Knowing pF/length at least lets you test your answer for reasonable-ness. Even testing 100m or 100ft of the same type cable tells you pf/length well enough to rule out multiple faults, if you have the cable available to measure.

[J M Fahey]

Fully agree.
"Break/fault-detectors" are used by telephone or industrial control electronics guys, but are expensive (for us); they definitely pay themselves in a jiffy if you find accurately *where* to dig to repair a 100 yard wire buried under a road or whatever.
I personally use my capacitance meter from both ends; the capacitance ratio gives me a reasonably accurate idea of where the fault is.
In fact, not to actually repair it most of the time but to decide whether it's close to one end, in which case I cut , say, a foot there or whether it's more towards the middle, in which case I just junk the full wire finding it unreliable.
On single conductor guitar cables, I prick them with a thin needle and check whether it shorts center to braid or not.
And "Rock" cables should not be afraid of needles.
Yea, I know, bad joke, I have more dead friends than I deserve because of those f*ckn*ng needles. !!!!

[Krwkka]

Nyuk, nyuk, nyuk!

Thanks for the ideas, guys. I'll try the capacitance trick next time.

As to the voltage detector, the one I have has a sensitivity adjustment and the cable is not shielded. On top of that, the problem is not that I didn't get a reading - but that I DID get a reading along the entire length of wire!

[kg]

as to why your detector picked up a signal throughout the entire cable, it was probably due to capacitive coupling between the conductor and shield.

edit: ahh, ok, unshielded.

second question.. did you ground one of the two conductors to earth? if not, your ACV is going to appear on BOTH conductors, triggering the sensor.

[Krwkka]

Nor sure that I understand you, KG.

There are three conductors. Are you saying that I need to ground the two conductors that I am not testing?