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A cheapskate way to find cable breaks

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A cheapskate way to find cable breaks

Postby Folderol » Sat Dec 31, 2011 5:36 pm

It's easy enough to find which wire in a cable is broken with a simple continuity test, but finding out where is a bit different. Most people opt for checking the plugs and if there is no break there, sacrifice the cable to two shorter ones by chopping it in half. However, if you have a 20m cable you can waste a lot of it if the break is only 1m from one end.

Most modern DMMs as well as continuity testing can also measure capacitance, and this is what I use to find breaks. Having found which wire is broken using a continuity test I then measure the capacitance at one of the plugs from that wire to the screen (if there is no screen I connect all the other wires together). I then do the same at the other end of the cable. The ratio of the capacitance is then the ratio of the distances from the ends to where the break is.
With a bit of fiddling around:

Assume c1 is the capacitance at one end and d1 is its unknown distance. Similarly c2 is the capacitance from the other end and d2 the associated distance. dt is the known total cable length, and ct is the total capacitance, c1 + c2.

d1 / c1 = d2 / c2

but dt / ct = d1 / c1 = d2 / c2

so:
d1 = dt x c1 / (c1 + c2)


Plonking in some figures for a 10m cable where we find we have capacitances of 100pF and 400pF

d1 = 10 x 100 / (100 + 400) = 2
So the break is 2m from one end. As a sanity check the break will be closest to the end with the lowest capacitance.

Because this relies entirely on ratios we don't need to know the cable's specific capacitance, and can work on any cable of any length provided we have a sensibly measurable capacitance.
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Re: A cheapskate way to find cable breaks

Postby Guy Johnson » Sat Dec 31, 2011 7:44 pm

Clever. But I think if a cable's gone tits-up, and it's not at the usual point near the mic-end, it means that the wires are weakened in the whole cable, and it is unreliable. It's therefor time to chuck it on a fire and watch the pretty colours!
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Re: A cheapskate way to find cable breaks

Postby shufflebeat » Sun Jan 01, 2012 12:29 am

Guy Johnson wrote:Clever. But I think if a cable's gone tits-up, and it's not at the usual point near the mic-end, it means that the wires are weakened in the whole cable, and it is unreliable. It's therefor time to chuck it on a fire and watch the pretty colours!

I don't think you're a proper cheapskate.
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Re: A cheapskate way to find cable breaks

Postby Exalted Wombat » Sun Jan 01, 2012 1:06 am

Guy Johnson wrote:Clever. But I think if a cable's gone tits-up, and it's not at the usual point near the mic-end, it means that the wires are weakened in the whole cable, and it is unreliable. It's therefor time to chuck it on a fire and watch the pretty colours!

Maybe. Because a truck ran over it at one point doesn't make it all bad!
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Re: A cheapskate way to find cable breaks

Postby Guy Johnson » Sun Jan 01, 2012 12:04 pm

Good point about the truck! but then you see where the problem was. And I AM a cheapskate, 'coz I'd keep the old Male XLR ... Females get a bit worn out and intermittent and unbalanced
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Re: A cheapskate way to find cable breaks

Postby Folderol » Sun Jan 01, 2012 12:51 pm

Hmmm not sure it's safe to talk about worn out, slack females

It's very easy to get a snagged cable with no sign of where the snag is, especially long cables out in the open.

One of our customers is a bomb disposal organisation. They run 50m 6 core cables to some of their detector kit for power and comms. Having established an area is clear they then unplug the cables and drag them out onto drums. This is nearly always when they get damaged (they usually survive being run over by trucks). Out in the desert, nipping down to RS for a new reel of cable isn't an option so they have to be repaired on-site. However, giving their comms man a cheap meter and a bit of instruction has worked rather well!

Later, they come to us for proper repairs. Depending on the condition of the cable, we'll do several patches before chopping it up and replacing it.
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Re: A cheapskate way to find cable breaks

Postby Exalted Wombat » Sun Jan 01, 2012 2:53 pm

Folderol wrote:One of our customers is a bomb disposal organisation. They run 50m 6 core cables to some of their detector kit for power and comms. Having established an area is clear they then unplug the cables and drag them out onto drums. This is nearly always when they get damaged (they usually survive being run over by trucks). Out in the desert, nipping down to RS for a new reel of cable isn't an option so they have to be repaired on-site. However, giving their comms man a cheap meter and a bit of instruction has worked rather well!

I've seen both telephone and power company engineers use a single-ended system for locating a cable break. One explained to me that it injected a pulse and timed the reflection. I don't know if there are compact, affordable versions of this technology.
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Re: A cheapskate way to find cable breaks

Postby shufflebeat » Sun Jan 01, 2012 7:22 pm

Guy Johnson wrote:Good point about the truck! but then you see where the problem was. And I AM a cheapskate, 'coz I'd keep the old Male XLR ... Females get a bit worn out and intermittent and unbalanced

That's just being a prudentskate. In fact I grew up five miles from Donegal where getting rid of anything on the grounds that it has become "a bit worn out and intermittent and unbalanced" would be considered extravagant, hence the preponderance of Clergy in the schools.
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Re: A cheapskate way to find cable breaks

Postby James Perrett » Mon Jan 02, 2012 9:39 pm

Exalted Wombat wrote:
I've seen both telephone and power company engineers use a single-ended system for locating a cable break. One explained to me that it injected a pulse and timed the reflection. I don't know if there are compact, affordable versions of this technology.

Check your computer network interface software - some chipsets include this reflection timing. I was surprised to find it on some Dell laptops.

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Re: A cheapskate way to find cable breaks

Postby Boogyman » Tue Jan 03, 2012 10:19 am

Hi Folderol,

Using your way I was finally able to find where a wire is cut, awesome!

I have a 2.5 meter cable with 3 core wires (L/N/E) of the same diameter. The N was cut, so I measured capacitance for N/E or N/L from both sides, they were 0.225 nf and 0.242 nf. But when I measured the capacitance for the two good wires it was 0.300nf. Surprisingly 0.225 + 0.242 = 0.467 which is not 0.300.

Can you explain? am I missing something?
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Re: A cheapskate way to find cable breaks

Postby Folderol » Tue Jan 03, 2012 7:13 pm

@james
Interesting
Proper portable testers were considerably north of £1000 last time I drooled ^H^H^H^H checked.

@Boogyman
Not really sure, but probably due to the fact you were leaving the unused wire floating instead of connecting L & E together. Might have also affected the accuracy in a longer lead with the fault towards one end.

With one wire floating it will have capacitance to both the other two - the precise result of which is beyond my mathematics!
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