Electronics > Repair
Finding fault location for broken power cables in walls
calzap:
We recently bought a 25-year-old house from the original owners. They were pretty backward technologically and not great on keeping records on the original construction and subsequent modifications. As typical in California, it was built on a slab with copper pipes running through the concrete. And as often happens, a few years ago water started coming out under the slab.
So, they had the house replumbed with PEX. This process involved plumbers tearing out drywall and drilling holes in walls and framing to run the PEX. Then carpenters covered things over with some extension framing and new drywall. There were no pictures, drawings or notes made.
I tested outlets in the house. One was dead because of crummy hardware, and one was mis-wired (yep, installed by the owner). However, one in the garage and one on an outside wall opposite the one in garage were dead as was one on an upstairs, outside balcony. I’m presuming the plumbers or carpenters cut the cables (type NM) to these outlets somewhere. But where?
I have experience tracing underground cables using the 60 Hz emanations or by injecting RF and tracing that. And I have equipment for injecting RF into house wiring for tracing cables. Problem is that in this house, a lot of the cables run in bundles and the RF jumps from the injected cable to others. The two garage outlets, inside and outside, are connected to each other. They should be connected to a GFCI (RCD) outlet in the garage. Neither an RF signal or back-fed power put into the orphan outlets makes it to the GFCI outlet. Through inspection of wiring in boxes and deduction, I’ve narrowed the fault location to a 4 m section of wall. But injecting RF and trying to trace it is useless because other cables in the wall pickup the signal. So, there’s no discontinuity when moving the RF receiver across the wall. It looks like a rewire job from the attic is needed unless the fault can be located precisely. Any ideas?
The upstairs outdoor outlet is less of a concern. I can create a new outlet further along the wall opposite an outlet on the inside wall and bring wiring across.
Mike
BILLPOD:
Good Morning Calzap, Several of my DMMs have a feature of sensing AC voltage when the meter, or a specific portion of the meter, gets near the AC voltage. Maybe your DMM(s) have this feature. If so, it might help you narrow down the location of where the voltage stops.
There are also small detectors that detect only, such as this Fluke FLK2AC/90-1000V Voltage Detector. Try whatever detector you have, on live conductors that aren't behind a wall, to get a feel for the distances involved. That might get you 'in the ballpark'. :popcorn:
CatalinaWOW:
Put a fast pulse into the wire and watch for the reflection from the break. Since a nanosecond is about a foot the timing will give the distance to the break. There are oscilloscopes specialized for this. With a fast scope and a fast rise time pulse generator you can do it without the specialty equipment, but it is challenging. Among the problems is that the actual propagation speed is dependent on the cable type, and the value for NM is probably going to be hard to find.
calzap:
These are good suggestions to consider. I have inductive 60 Hz emanation detectors (one handheld standalone and one in an Extech DMM). Unfortunately, they cannot pick-up a signal from wiring through 50 mm of air and 15 mm of drywall. But the much bigger detector I use for underground wiring might work.
I hadn’t thought of pulse reflectometry and should have. I have the equipment (generator with fast rise/fall times and 200 MHz scope) and have done it on coax before. Strangely, it never occurred to try it on power wiring. I wonder what velocity factor so use ... same as coax maybe.
Mike
coromonadalix:
best luck would be to find tdr cable tester equipment ... you'll find the distance ... but getting them ...
Navigation
[0] Message Index
[#] Next page
Go to full version