Tdr

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I do fault locates on buried conductors, usually faulted to ground and have had luck finding them even while buried in pvc conduit. Occasionally I have a conductor that is open and not faulted to another cable or earth.

I know a TDR can give me the distance to the open but would like to hear some comments from those that have used one before I purchase.
 
We used TDR's with a lot of success in a refinery, especialy in locating breaks in pipeline heat-tracing cables, overhead insulated cables and underground circuits. That was some years ago. I assume modern instruments are much easier to use and would do well in your application.
 
pton, I use a TDR on a regular basis but not for AC electrical rather optical and RF applications. They do what they advertise to do very accurately. Like any accurate instrument of this kind comes a very large price tag, so unless you need to use one on a regular basis, like once a week, you might just consider renting one when needed,
 
I've used a couple of different types on network/comm cable. With practice, you can even count loopthrough jacks and connectors, but it does take practice. Definitely learn the theory of TDR, it makes using the instrument so much easier.
 
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find a good underground cable locator on EBAY

find a good underground cable locator on EBAY

I found dozens of them. I got a ditchwitch 950r (2700.00) for 225 american dollars about a year ago in new like condition you just need patience. A goldack unit for 87 american dollars and an rd400 I have used all 3 extensively Amprobe 2005 is one of my favorites and it has never been wrong yet
 
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I'd recommend that you get one with a screen, as it is not alweays the first impedence discontinuity you are searching for. The ability to see "stuff" as little bumps lets you calibrate the velocity ratio to the exact scenario you are testing.
 
If you do a lot of fault finding - make the investmant in a Megger brand one with graphic. (Numeric can lie a little... Not what you want when being paid for accuracy.) The graphic takes a little interpetation, but once used to it, it will give you much more information.

For instance, I tried my numeric on an open phase I was looking for - I got an odd number... (The unit will self-translate and spit out a number based on what it thinks it sees.) I rightfully didn't trust it, and got the shops graphic, and retested and realized all four conductors were spliced, as opposed to the straight run that I thought I was testing. Turned out to be a bad splice in a christy box that got paved over - easy money!

A shop I used to work for had an older graphic style, and I have my own numeric style, (Cheap!) they can be a time and back saver when accuracy counts. Like I said, the numeric lies a little, and I tend to double and triple check from both ends. Especially if there are un-known splices or resistrive connections.

It may also pay to lease one if you have the work for it, but don't want to own such an expensive peice of equipment. There are a few places that will over-nite FedEx you a rental, but often when these things are needed it is needed NOW! And it had better be one you are familiar with... Mis-interpet a trace on unfamiliar equipment and you are just wasting time IMO, you can look like a moron, or you can look like a guru - you choose. :wink:
 
Of Course:

Of Course:

dereckbc said:
Well I guess ones for AC wire/cable are a lot less expensive. In my biz they run 17K to 50K
Click to expand...

Sure, but as you know the telecom people are looking for subtle changes in impedance as well as opens and shorts. I would think Romex would look something like twin line, but wires in conduit would show up rather lumpy.
 
TDR for Locating Heat Tracing Cable Failures

TDR for Locating Heat Tracing Cable Failures

I got a nice bonus using TDr to find failed heat tracing on long pipelines. The heating cable was in 100' lengths of resistance heating wire with cold lead copper pigtails on each end. The 3-mile line had condulets every 100' where the cold leads in adjacent sections were spliced. The old method when the circuit tripped was to start at the middle, meggar each way at a splice, then repeat the test until the one bad 100' section was located. Removing & replacing 100' of 6" insulation was a lot more time consuming and expensive than actually repairing the cable.

When we tried the TDR, the trace showed a lot of evenly spaced blips from the cold junctions, some hash, then more blips. By counting blips we got directly to the bad section. With the trace calibrated on the adjacent good 100' section we located the burn out within 8" without opening the insualtion. Many hours and $$ saved.

(That TDR operator later went on to found NETA, but he didn't get a bonus.)
 
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