HV, MV Underground Cable Fault Location

taking the cable off the live circuit, test with a TDR, should give the location where you have a open/short.

I don't see why you need to turn off the power to do TDR. It would take some careful coupling and more power and selectivity in the TDR device but the power at several MHz reflected by the fault should not be changed much by 60 Hz no matter the power.

You might have to do TDR from both ends if there is current flowing in the line as the magnetic and electric fields from the 60Hz power may charge the propagation of the higher frequency though the line to an undeterminable amount.

A good enough TDR setup will find more than breaks. It will find any change in impedance it is sensitive enough to detect.

gc

only if you have a dead short or open ciruit other than that you can dump the tdr.max

Guys, Thanks for the comments.

Can you suggest any particular brand for TDR. Moreover, what about the Thumper. I googled it and found that there are certain instruments based on magentic and shock wave detection methods. TDR will give the location calibrated in distance. We will also need something to trace the route in the probable area and find out the fault location precisely.

TDR is nothing more than an oscillator and an oscilloscope. The oscilator needs to be coulpled to the cable so the impeadeses matchs the line pretty well. I would use a pi network or antenna turner for the job and connect the scope at the end of the tuner that goes in the wire so the I wouldn't be confused by the complex delays in the tuner or the reflection of the miss match at the insertion point. The turner is adjusted so most of the power goes in the wire. An oscilloscope will see the signal enter the wire and see the how long it takes the refections to return. You need to know the velocity factor of the line the precent less than the speed of light the signal goes down it to calculate the position of the break.

If the full length of the wire is accessible putting a noise source on it that you can pick on a an AM radio between stations on one end of the wire and possibly a 50 to 200 ohm resistor across the other end to cut down on refection form it. Then move the radio down the wire and the noise will change at the break. If you use a single frequency oscillator for this and the line is very long their will be peak and valleys normally that can give problems.

TDR or Transmission Domain Reflectometry is nothing more than the SWR or Standing Wave Ratio whit the Reflected Power viewed or timed as it returns to the source. All the rules of of Transmission Line can be used to find faults in the in some way or the other. All wires that carry AC current are transmission lines and the transmission line theory applies. At very low and high frequencies things are a little different. At high frequency on circuit boards the signal tends to fly off right angle corners if not made correctly and it take a very long set of wires to get 20 Hz to act right on a line. But from DC+.0001 HZ to day light and on up to high energy gamma ray AC obeys the same laws but act a bit differently.

You can see the transmission line defects on television and use a a piece of aluminum foil wrapped around the twin lead leading to the set to tune the transmission line and tune out ghosts from multi path signals and bars and such from SWR in the line. After a coaxial cable with a slit in the side that a an adjustable length quarter wave stub fit in that can open or shorted and can be moved along at least a half wave of the frequency the TV is tuned to the tin foil on the twin lead is the best demonstration of transmission line theory I have ever seen. As you can see SWR and other other transmission line, antenna and signal problem on the screen and don't have to visualize them in your minds eye. Something that's not hard for me to do but impossible for people who's brains aren't wired that way.

I hope that give you some ideas on how to solve your problem. Of course there the old method of cutting it in half and seeing which half is bad. I sure the cost of repair will be half as much for materials. but not good engineering. But some time the expedient solution.

Gordon

Take a look at Hipotronics (www.hipotronics.com). They make several different types.