Finding fault location for broken power cables in walls

[forrestc]

So, I bought a tool a few years ago which I love which might help you.

At walabot.com, they sell a handheld radar imager which connects to a smartphone.  It provides a 2d view of what is on the other side.   Look at their videos to see if this might help.

If I was buying a new one, I'd probably buy the 2 instead of the 2m since the 2 M is a simplified version with fewer features and the 2 gives you more details.

[calzap]

G'day Mike,

If you find the location of the Break/Cut, I imagine you will still need to cut a hole at least 300 x 300mm to be large enough to access the cut ends.

There may not be enough slack in the cable for a simple butt joint. You may need to insert a short length between the two cuts ends.

Wago are good for that type of work.

https://www.wago.com/au/electrical-interconnections/discover-installation-terminal-blocks-and-connectors/221#durchgangsverbinder

There's usually little or no slack in NM cable in residential wiring, and it's required to be secured (usually stapled to framing) at intervals.  So to fix a break or add a branch, two boxes are required (splices must be in a box).  Within a box, there is a requirement for minimum working length, and any listed connectors (wire nuts, Wagos,  crimp-ons, etc.) of appropriate size may be used. 

Even with direct visual exposure of a break location for NM cable, it can be hard to see the actual break.  A small drill bit can cut the phase conductor and the plastic covering entry point can re-close over the penetration.  A saw blade can cut one conductor and also twist the cable, so the the cut is opposite your viewing angle.   If there is insulation in the wall or cables are grouped, it's more difficult.

Personally, I hate NM cable and prefer conduit which allows flexibility for present and future wiring.  If conduit should be accidentally cut in a hidden location later, there are electrical, mechanical, acoustical and optical means of finding the break.  In walls and attics, I like flexible, corrugated plastic conduit, called ENT in the US ... almost as easy to install as cable.  But in the U.S., NM cable rules in residential applications because it's cheapest and quickest.

Mike

[Poroit]

G'day Mike,

Is there accessible roof space above the garage ?

[calzap]

G'day Mike,

Is there accessible roof space above the garage ?

Yes.  I'll probably be able to take a look up there next week.    Mike

[Poroit]

That may help you solve your problem.

Some of them aren't the easiest place to work in but you may be able to get a new power cable across to the South wall and remodel the wall so you can drop the cable down between the studs.

[MathWizard]

4 meter is a little short for a 200MHz scope.  The fault could be hidden in the rising edge.

I have a couple techniques, depending on the situation, when the TDR route is impractical. 

My most favorite gadget is a Carlson Superprobe (diy project from youtube).  Instead of using a piece of coax as the probe, I added an SMA connector so I can change from an E-field probe (for open circuits) to an H-field probe (for short circuits).  Audio is a bit tricky getting any distance with, but if you AM modulate a LF carrier, you can easily reach through a wall.  Just don't go too high in frequency or it can couple to other conductors.

Another one of my tricks for really getting through walls, and this is primarily for following good cables, not necessarily for finding breaks, is to inject an HF carrier at the farthest outlet between the ground and neutral using a simple crystal oscillator and decoupling cap connected to an extension cord end and plugged in, and using a handheld radio scanner as a receiver with an H-field probe in place of the antenna.  In this case, since the detection is with the H-field, current flow is required.  Being the ground and neutral are connected in the box, that requirement is met.  Assuming your break is in the hot wire, this may still work in your case for following the path of the wire.

Edit: I mention the latter HF + H-field trick because knowing where the cable is in the wall greatly aids in finding the weak e-field signals on the broken wire.  As has been said, getting a tone from a few inches away is sometimes tricky, but knowing right where to look may help you pick up the signal after losing it in a corner or something.

Edit2:  and don't be afraid of trying to get a tone on the outside of the wall.  In some pre-fab or lazy builds, instead of drilling a hold in the stud, a notch is cut into the face of it and covered with a metal plate.  Often times, these are on the outside face of the wall,so getting a tone on the inside may be hard.

I built one of those Mr.C Superprobes, and have used it for checking wiring. But yeah it has limited range through walls. That was using ~1.2kHz test signal, I never tried RF. So how much better is it with the E and H field sensors? What kind were they ?

I have a string of Christmas lights that doesn't light up any bulbs, so I'm curious to see what my Superprobe would find. They might have had a fuse, or the 1st bulb is gone I guess.

[antenna]

4 meter is a little short for a 200MHz scope.  The fault could be hidden in the rising edge.

I have a couple techniques, depending on the situation, when the TDR route is impractical. 

My most favorite gadget is a Carlson Superprobe (diy project from youtube).  Instead of using a piece of coax as the probe, I added an SMA connector so I can change from an E-field probe (for open circuits) to an H-field probe (for short circuits).  Audio is a bit tricky getting any distance with, but if you AM modulate a LF carrier, you can easily reach through a wall.  Just don't go too high in frequency or it can couple to other conductors.

Another one of my tricks for really getting through walls, and this is primarily for following good cables, not necessarily for finding breaks, is to inject an HF carrier at the farthest outlet between the ground and neutral using a simple crystal oscillator and decoupling cap connected to an extension cord end and plugged in, and using a handheld radio scanner as a receiver with an H-field probe in place of the antenna.  In this case, since the detection is with the H-field, current flow is required.  Being the ground and neutral are connected in the box, that requirement is met.  Assuming your break is in the hot wire, this may still work in your case for following the path of the wire.

Edit: I mention the latter HF + H-field trick because knowing where the cable is in the wall greatly aids in finding the weak e-field signals on the broken wire.  As has been said, getting a tone from a few inches away is sometimes tricky, but knowing right where to look may help you pick up the signal after losing it in a corner or something.

Edit2:  and don't be afraid of trying to get a tone on the outside of the wall.  In some pre-fab or lazy builds, instead of drilling a hold in the stud, a notch is cut into the face of it and covered with a metal plate.  Often times, these are on the outside face of the wall,so getting a tone on the inside may be hard.

I built one of those Mr.C Superprobes, and have used it for checking wiring. But yeah it has limited range through walls. That was using ~1.2kHz test signal, I never tried RF. So how much better is it with the E and H field sensors? What kind were they ?

I have a string of Christmas lights that doesn't light up any bulbs, so I'm curious to see what my Superprobe would find. They might have had a fuse, or the 1st bulb is gone I guess.

I will set up a little demonstration and put it on youtube.  My H-field probe is just a simple homemade one.  Basically, a 1" loop from the center of the SMA to the ground side  (cant remember if I did one turn or 4-5 turns, but that hardly matters), and then a grounded shield around that with a split in the shielding so it isn't electrically continuous around the loop.  That blocks the electric field but not the magnetic field. 

Give me a few hours and I will make a video showing the difference between audio and modulated LF on the carlson superprobe, and the usefulness of an H-field probe attachment.

I should also add for the OP (since he was in a panic about RF) that coupling to other wires can be mitigated to a workable degree with careful choice of frequency.  Take underground wire tracing for instance.   Where I live, it is usually done in the kHz, not 60Hz.  The frequency choice is a tradeoff between the distance you need to go (how long the wire is), the depth of the wire (transmission range needed) and the amount of coupling to the other wires that you can tolerate.  Lower frequencies go farther if the wire is long and couple less into adjacent wires if they are present and parallel to the driven conductor, but the transmission range takes a hit because the wire is nowhere near the length of a proper radiator.  Since his situation is a short run, coupling would be insignificant under probably about 30kHz, and given the short run, there would be no problem making it to the end at that frequency.  In other words, wire tracing should be done at the highest frequency possible that is unlikely to couple into adjacent wires that can still make it to the other end and transmit through the thickness needed.  If you try a frequency and the coupling is so great that you cannot tell from the drop in signal that you are on another conductor, drop the frequency a little.  Practice practice practice, don't just assume all RF must be avoided.

[antenna]

Lost power, had to start generator.  Wet snow took down power lines.

https://youtu.be/yHNcXsUY6FY