mdshunk said:
I'm curious if anyone has actually ever successfully used a TDR for NM cable?
I think you'd see a reflection at every snug staple and every 90 degree turn. Plus, we have this funny tendancy to feed through receptacles and cram wired back into boxes, so that would involve taking apart every single connection on the circuit... maybe even grounds too that were done with crimp sleeves. I think the general idea has potential, but I'm not sure the present batch of TDR's are quite suited to the task.
This has my gears spinning on how a longitudional stress test from a telco type kick meter might be useful in pinning down the location of this sort of fault.
I have used them on NM and MC many times - in conduit too with very good results... A former employer had an older version of the Megger one I linked to earlier, and I have/had a
much less expensive numeric model of which was fairly relaiable on direct shorts and wide opens - it since has stopped working after <3 years use and full-time in the truck. But when it was working it did pay for itself (<$300) on a few occassions finding various shorts. I once traced a NM through 3 floors (from both ends) to find a sheet rocker had used a 2 1/2" (on 5/8" rock) and put it right through an NM cable. I got within 3" of it with the TDR mode. Toning and measuring the path. Then while toning (same tool) I noticed a loud spot. Pulled out the screw - short gone! Opened the wall, pulled the NM out of the stud to expose where the screw went through - j-boxed it and back charged the rocker. I have done this a number of times - and thats why I say it paid for itself...
As it is no longer working - I'm not sure I can faithfully recommend it.
But I have wanted to replace it since it died on me....
The graphic models (Although much more expenive - and not nearly as easy to use) are
far superior in finding things IMO. With the one my old shop had - I have found burn wirenuts - hidden j-boxes - burnt wire nuts in hidden j-boxes.... Dead shorts.... Resistive corroded neutral in underground feeder cable, a knick in the jacket allowed the neutral to rot - that one alone was worth its $5000 price tag at the time. That one was a "DIG HERE!" moment on a 1000' feeder. We were the 3rd and smallest shop in to find that one - and the only one that did. And the customer confidence after it did bring in some BIG jobs. I drove nearly an hour out of my way to a supply house that had a simular cable in stock and willing to lay out a measured 50' length of it for me to dial in the VOP settings. I nailed it on site within 2'... I was lucky the trench was 4' long, but the other guys weren't even close, and not confident enough to say "DIG"...
Put it on an old lead jacket industrial 500MCM feeder once - I looked at the run and it had 3 boxes I could visually see. On the TDR it said 2 more splice locations I didn't see under the floor.... And which one had all of the resistance that I was looking for...
The funny thing is once you have and understand how to use a tool like that - word seems to get around, and you get calls to use it more. Even if it is to save your own butt in an expensive back-charge - it's worth it IMO. They are not 100% accurate, and they do take a bit of education and understanding of the circuit and how it works, often through some trial and error. But when you do find stuff - you look like a Guru.
