Using TDR on single wire to detect where break occurs?

[steaky1212]

Hi all,

Maybe TDR is not the correct test methodology (and this probably isn't a beginner question either), so I'll ask the proper question...

I have a solenoid on the end of about 1m of cable. The cable goes through a glass-to-metal seal, and the solenoid terminals are covered in epoxy. There are numerous places for the cable to break (inside the glass-to-metal seal, at the solenoid terminals, inside the actual solenoid), and I would like a method of non-destructively finding this fault location.

I have used TDRs on coax and twisted pairs to locate any impedance mismatches (due to misformed twists, kinks, connectors, water, break in cable etc.) and the length down the cable that this problem occurs.

However my understanding of TDRs is that they use the characteristic impedance of the cable, and a single wire does not really have a characteristic impedance in the same way. Plus with a break there is no circuit for any return current. Normally you could put the long wire on a ground plane and almost use that as your return, however that is not possible in this situation. I was thinking maybe submerge the assembly in a conductive liquid and use that as my other conductor, but now we are going to purely theoretical ideas.

Does anyone have any ideas? I would need to resolve down to 10mm, but have a length of probably 100m of wire.

Thanks in advanced,

Steaky

[danadak]

A wire on earth is not in free space, there is a dielectric separating it from another
conductor, ground. A poor transmission line ....?

Maybe set up a simple lab experiment and see if you can detect a known fault
with a TDR approach. I am thinking because of dielectric conditions and ground
being a poor conductor that signal levels might be a challenge.


Regards, Dana.

[tggzzz]

I have used TDRs on coax and twisted pairs to locate any impedance mismatches (due to misformed twists, kinks, connectors, water, break in cable etc.) and the length down the cable that this problem occurs.

However my understanding of TDRs is that they use the characteristic impedance of the cable, and a single wire does not really have a characteristic impedance in the same way. Plus with a break there is no circuit for any return current. Normally you could put the long wire on a ground plane and almost use that as your return, however that is not possible in this situation. I was thinking maybe submerge the assembly in a conductive liquid and use that as my other conductor, but now we are going to purely theoretical ideas.

Does anyone have any ideas? I would need to resolve down to 10mm, but have a length of probably 100m of wire.

TDRs measure reflected voltage as a function. You can, if you wish, interpret that as impedance variations as a function of distance. The time at which reflection occurs will depend on the velocity of propagation, which depends on the dielectric.

The resolution is determined by the TDR's system frequency response; 10mm is short. The range is determined by the power; typically higher power => reduced resolution.

To understand some limitations, have a look at the specs for commercial TDRs, e.g. Tek 1502, 1503.

If you have a fast scope (>2GHz) and an edge generator (<200ps) then why don't you simply do a quick experiment.

[Muttley Snickers]

This sounds like a tricky one and a bit of a challenge but I am still slightly confused as the initial post stated a solenoid at the end of a one meter cable but later declares that they have 100 meters of wire so I don't really know which is applicable, anyway in this case a cable toner/ tracer/ oscillator should be able to identify at which point along its length the cable is open, perhaps some additional information, clarification or even pictures may assist others here in providing alternate solutions.

[steaky1212]

Hi all,

The issue is that I don't have access to a TDR (or fast scope with edge generator) anymore, and wanted to make sure the theory was there before blowing a load of cash.

Regarding the 100m comment, this is the amount of wire that is in the coil.
I can't use a cable toner as I don't believe the resolution is fine enough.

[alanb]

Do you have access to both ends of the solenoid? If so I suspect that if you use TDR from both ends some of the variables will drop out of the equation.

[kulawy]

Hi
I have been working with TDR's for long years (since analog CRT ones) and yes , you can use one leg to the ground and other to measure wire but waveform on your display will be very dificult to read and identify breaks or loops, it looks very disturbed compared to testing between legs . Also precision you demand (10mm) is impossible to achieve (at least telecom TDR that mostly works with long distance cables and is not very precise at low range). I remember tektronix 1503B one that we used to test LAN coaxial cables, was not good at long range but very good with short cables still only 30mm resolution with lowest 2 nsec pulse width. Not sure if fact that cable is coiled may affect as I used TDR's mosty with standard twisted or coax cables.
I hope this helps and good luck
regards
tom

[jmelson]

I have a solenoid on the end of about 1m of cable. The cable goes through a glass-to-metal seal, and the solenoid terminals are covered in epoxy. There are numerous places for the cable to break (inside the glass-to-metal seal, at the solenoid terminals, inside the actual solenoid), and I would like a method of non-destructively finding this fault location.

Is there another wire in the cable that parallels the broken one?  You say you have 1 m of wire one place, and 100 m another place.  If you have 2 parallel wires, then you DO have a transmission line.  So, you could use TDR, with the appropriate interface to the terminals.  Now, not too clear you will get 10mm resolution on this, but maybe, if the wires have a constant-enough impedance and you have a fast enough pulse generator.

Jon

[jmelson]

Hi all,

The issue is that I don't have access to a TDR (or fast scope with edge generator) anymore, and wanted to make sure the theory was there before blowing a load of cash.

Regarding the 100m comment, this is the amount of wire that is in the coil.
I can't use a cable toner as I don't believe the resolution is fine enough.

get your friendly neighborhood dentist to X-ray the thing.

Or, it may be possible to excite the wire with RF or fast pulses and make a little capacitive pickoff that detects the signal.  When your pickoff crosses over the break, the signal will stop.  Should easily get down to a few mm with a fast enough edge.

Jon