In daylight, we look for the smoke. At night, we look for the flame. When we get close, we sniff for the ozone. But seriously, we have a gizmo with a 600V contactor and a bank of lights. We place it between our DC bus and the load side of an open bridge feeder or traction power breaker. It flashes the lights through the fault. Then we clamp on in various manholes and see the current pulses before the fault, and not after. For signal or control cables I'll give it a shot with a metallic TDR. Guessing the velocity of propagation is always fun but not really that important. Joe
Re: Cable Fault location#60793 01/12/0609:08 AM01/12/0609:08 AM
The park that I manage is filled with defence contractors and one day one of them observed me trying to track down a underground fault in my outside lighting. Seems they were testing some equipment that was designed for this purpose to track electrical faults in tanks and aircraft wiring harnesses. They hooked up this small hand held gismo and entered the wire size into the parameters and vada bing it told me which wire had the fault and how many feet it was to the fault from the point that the gizmo was attached, it worked great. I am sure we will be seeing this in civilian applications real soon.
Mshaw, The device you mentioned sounds like a TDR. I guess I should have mentioned that TDR stands for Time Domain Reflectometer. They work by sending pulses down conductors or fiber for Optical TDRs. When you get to an impedance change, some energy couples forward, while some reflects back. It works similar to SONAR or RADAR where they gate on a sensitve receiver after the pulse is sent and time the round trip. They calculate the distance to the fault by using the time, speed of light, and the velocity of propagation percentage correction to the speed of light. The guess work I mentioned is because those characteristics vary alot with power conductors and paired cables when compared to coax, and especially fiber. I usually just check a known length of the closest cable I can find and adjust the VP to jive with my sample. Joe
We did TDR with a garden variety O scope. You have to do a little math but if you start thinking 2ns a foot and use a little Kentucky windage by causing faults at known locations to get your bearings, you can usually get to the general area of the break/short.
1. cable safely isolated from supply. 2. both cable ends completely disconnected, including screen earths if used. 3. insulation test Ø to Earth. 4. insulation test Ø to Ø. 5. insulation test screen to earth if applicable. 6. continuity test.
Fault loaction for most screened cables:
7. If possible get the cable route traced with a cable locator if listening devices in conjuntion with a thumper are used. 8. A TDR or Dart can give approximate distance prior to thumping a cable.
For single core non screened cables
9. POPIE, pool of potential in earth can be used very accurately within < 5 cm to pinpoint the fault.
The product of rotation, excitation and flux produces electricty.
Re: Cable Fault location#60799 01/28/0607:07 AM01/28/0607:07 AM
How Do you use an O-Scope as a TDR? It Sounds like something that could come in handy Someday.
I didn't say anything at the time but I'm thinking our friend probably had a pulse generator from his laundry room and some kind of gated amplifier from his attic to go with his garden variety O-scope. Joe