Here's how I find the break in the cable:

1. Make sure nothing is connected to the cable and that it isn't touching an earthed part or large piece of metal casing.
2. Warn people that the cable is going to be live.
3. Connect the cable to the live mains conductor.
4. Use a DVM with a non-contact mains frequency electric field detector to scan the cable. The point in the cable where it stops beeping or becomes less sensitive is where there's a break.
5. Disconnect the cable from the mains.

You could also use a neon lamp and a high frequency high voltage source such as a cold cathode tube driver, just make sure the voltage is below 1kV or it could burn through insulation or arc across connecto contacts.

Originally Posted by ThermalRunaway

Could you not accomplish this by measuring the phase difference between an input signal at the source and the output signal at the receiving end?

It would be crude, but you should be able to use some mathematics to calculate the approximate length of the cable based on the phase difference.

You would need to terminate the end of the cable so that you don't get reflections.

Would this work? Not sure - never tried it myself. Try it on a scope with a length of cable and see what happens! There should be a phase difference, and if there is you should be able to measure it and use the difference to calculate a cable length.

Brian.

Depending on the cable length, the phase change may go through several iterations and I see no way to know which iteration your looking at.
The TDR is the best method I know of.

For the Original Problem (post #14), (knowing the remaining contents of the box) the easiest way would be to measure the DC resistance of a full (1000 ft) box. I would connect all conductors together at the end, so I can measure at the round-trip resistance without fishing back into the middle again. Be sure to make a tight connection that can't corrode. A twist connector is a good idea. You can measure between any two conductors. Should be about 51 ohms.

You should beware that at the low end of the scale, your probe resistance could introduce errors of several ohms.

Originally Posted by bigal_scorpio

Hi Eric,

Nice to hear from you again mate.

I like the cap meter idea but would the result be affected by coiled cable?

I just want something cheap and cheerful, to quote your tag "good enough", it would be used infrequently and mainly to check if there is enough cat5 cable in the box for a particular job so I would only need a rough guide, maybe 5% or even 10% would probably be an accepable error since the boxes have 305 meters to begin with.

So whats your view on the accuracy and coiled to flat question?

Your humble student..........Al

hi Al,
The spec for CAT5 is fairly tight, so as suggested a simple resistance check would be fairly accurate.

Category 5 cable - Wikipedia, the free encyclopedia

Of course if you have some decent scales you could consider weighing it.

Last time I had a box of cat5 it had the start and end number on the box.

(Slap my forehead) of course!

And I've used the same markings to 'measure' the length of existing runs where I want to add more parallel cables.