Author Topic: Tracing underground electrical wiring - Metal Detector or Other Means?  (Read 36169 times)

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Online xrunner

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My brother got a nice new house that has a pool. Out past the pool near the little canal that runs through the properties there are four 120VAC light fixtures on brick pedestals. The fixtures are all broken and no AC voltage is present. He can't figure out where the wires go underground or where they even get their power since all his breakers are on.  ???

A. I want to trace the wires and I was wondering if a metal detector could be used to trace the wires.

B. If it can't detect the wires due to their depth, can a signal be placed on the wires that might enhance the metal detector's ability to sound off? Does a metal detector transmit a frequency, and thus if a wire has a signal with the same frequency would it help the detector to register a signal when near the wire?

C. Or, how would you trace the wire if those ideas are dumb (other than digging it all up)? We're talking about four lights that are ~20 feet from one to the next then (presumably) a conduit that goes ... somewhere.  :-//

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Offline bilko

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I recently returned from Brazil where I had a similar problem. The power cable between two properties had become damaged.

I didn't have any tools or equipment, so I devised a signal injector and tracer  ;)

The signal injector consisted of a cable wrapped around the spark lead of a motorbike (no physical connection)

The tracer was a cheap radio tuned to the AM band.

Disconnect the cables or isolate power first
Connect the signal injector cable to one of the wires
Start the motorbike
Tune the AM radio so that you can clearly hear the ignition pulses
Start following the cable

I found the broken connection at around 300 metres away from the signal source, buried 12 inches below the surface, and managed to located it within 6 inches of the break.

Good Luck
 

Offline jpb

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A simple thing to do before starting to try and trace the wires is to check if the fittings are all connected to each other ok using a multimeter.

I would guess that there is one cable to the end one and then the four are connected in parallel so you can, by shorting the end one out check there is continuity to the others.

Assuming that the fixtures are all connected to each other ok, you presumably have reduced the problem to a single underground cable with a break. Bilko's method then might be
the best. A possible alternative if you have an oscilloscope and a pulse generator of some sort is to try some time domain reflectometry though this will only give a measure of distance and not direction.
 

Offline C

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Add to jpb's
Wild ass guess.
The original installer was probably CHEAP, so look at if as to where would you put a cable to get power to those lights.
In case you have not thought of these.

Could the lights been solar controlled and just connected to nearest power?
Are there other lights turned on by a switch that these could have be connected to?
Are these lights be from before Pool creation and pool installers were to cheap to run new wire?

You may see hints of these or others if you look with this in mind.

C
 

Offline lemmegraphdat

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Before you go digging in the ground you need to call a locator service.
Start right now.
 

Online xrunner

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I didn't have any tools or equipment, so I devised a signal injector and tracer  ;)

The signal injector consisted of a cable wrapped around the spark lead of a motorbike (no physical connection)

The tracer was a cheap radio tuned to the AM band ...

Thanks for the idea.

A simple thing to do before starting to try and trace the wires is to check if the fittings are all connected to each other ok using a multimeter.
I would guess that there is one cable to the end one and then the four are connected in parallel so you can, by shorting the end one out check there is continuity to the others.

Yes I will do that at some point.

Could the lights been solar controlled and just connected to nearest power?

Doesn't look like they were solar controlled. THey were probably built along with the house becasue the bricks are exactly the same.

Quote
Are there other lights turned on by a switch that these could have be connected to?

Not that we can find.

Quote
Are these lights be from before Pool creation and pool installers were to cheap to run new wire?

Unknown. The house is new to him but not brand new.


Before you go digging in the ground you need to call a locator service.

Yes I'm aware of this need.

But, can a good metal detector trace underground wire? How deep?
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Offline IanB

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But, can a good metal detector trace underground wire? How deep?

Possibly, but I don't know. I have an idea that metal detectors can detect small metal objects like coins maybe 6-12 inches down, but you'd best do your own research on this.

Any buried wires to the lamp fittings ought to be steel armored cable or be installed in a metal conduit. Either way that gives you a good chance of detecting them. Buried items like pipes and cables should typically be about a foot down; deep enough to escape casual digging, but not so deep they are impossible to reach for repair.
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Online xrunner

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Possibly, but I don't know. I have an idea that metal detectors can detect small metal objects like coins maybe 6-12 inches down, but you'd best do your own research on this.

I had to have some irrigation valves found in my yard once. The guy from the irrigation contractor found them by tracing the wires from the controller. He had a little system that clipped a wire with a signal onto the wires going to the valves and then he was able to track it all the way to the valve with an antenna and a receiver box.  I have no idea who made it. Man, that's what I really need.
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Online NiHaoMike

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Connect an audio amplifier to the secondary of a 120V-12V transformer through an 8 ohm or so power resistor. (A heating element works if you don't have a resistor with enough power rating. Something like a hair dryer would work.) Then connect one end of the primary to ground and the other end to one of the wires to be traced after disconnecting it from the source. Play some music (preferably some that is unlikely to be on the radio) and then use a FET amplifier to pick up the signal.
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Offline Paul Moir

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Quote
Any buried wires to the lamp fittings ought to be steel armored cable or be installed in a metal conduit.

This is not true.  If it's been installed in the last 30 years it's likely direct bury cable.  If not it's likely in PVC conduit.

I don't know if armoured cable was ever approved for direct bury anywhere....

« Last Edit: May 29, 2013, 12:47:40 PM by Paul Moir »
 

Offline ptricks

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If it was done right the first time, which it appears it wasn't, they would have run flexible conduit and run the wire inside of the conduit. Code only requires type UF wire without conduit but conduit is so cheap that not using it really is just being lazy and the amount of type UF wire that has been cut, nicked, water damaged will make you wish you did.  The way to find it is with a cable locator, fox&hound locators are good for about 1-2 feet, anything more than that and you need one of the pro models that start at about $1k and those are good to several feet. You might want to call some tool rental places , here I can rent one for about $20 a day, and it is a $2400 item , so that is a good deal.
 

Offline pickle9000

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- Generally speaking inductive or capacitive metal detectors can't see anything beyond 1.5 times the diameter or the sensing coil.
- Signal injection is by far the quickest method, I like the motorcycle one very unique
- Don't overlook the obvious, a hidden or defective switch, box covered over by drywall, lamp bases corroded. You may end up tracing back to the house instead of away from it.

 

Offline IanB

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Quote
Any buried wires to the lamp fittings ought to be steel armored cable or be installed in a metal conduit.

This is not true.  If it's been installed in the last 30 years it's likely direct bury cable.  If not it's likely in PVC conduit.

I don't know if armoured cable was ever approved for direct bury anywhere....

When I say steel armored cable I was thinking this stuff, which I believe is good for direct bury in Europe:

http://zzhlcable.en.made-in-china.com/productimage/wqoxlIYBLvUy-2f0j00OMjTFstGCZpQ/China-Copper-Conductor-XLPE-insulated-Steel-Wire-Armoured-Power-Cable-YJV32-.html

However, I don't see such cable listed at places like Home Depot. What they call armored cable is something entirely different.

What kind of cable is allowed to be direct buried in North America?

I can see burying in PVC conduit being possible as the same kind of pipe is used for buried water service, being water tight and corrosion resistant; however it gets rather brittle and fragile with age, so I'm not sure I would trust it. In the old days buried pipes and conduits were made of metal and if I had a choice I would still favor buried metal conduit over buried PVC conduit.

Having seen the water supply to my house fracture and turn into a fountain due to the combination of fragile PVC pipe and ground movement my distrust of the horrible stuff is only reinforced.
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Offline G7PSK

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Quote
Any buried wires to the lamp fittings ought to be steel armored cable or be installed in a metal conduit.

This is not true.  If it's been installed in the last 30 years it's likely direct bury cable.  If not it's likely in PVC conduit.

I don't know if armoured cable was ever approved for direct bury anywhere....

armoured cable must be approved for direct burial as it is used by the utility companies for underground feeds without a conduit.
 

Offline jpb

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Possibly, but I don't know. I have an idea that metal detectors can detect small metal objects like coins maybe 6-12 inches down, but you'd best do your own research on this.

I had to have some irrigation valves found in my yard once. The guy from the irrigation contractor found them by tracing the wires from the controller. He had a little system that clipped a wire with a signal onto the wires going to the valves and then he was able to track it all the way to the valve with an antenna and a receiver box.  I have no idea who made it. Man, that's what I really need.
That is what Bilko's suggested solution was - the transmitter being the electrical noise from the motorbike and the receiver just being an am radio.
 

Offline SeanB

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Dig by each one to find the cabling, and see how they are wired. If all 4 are wired in a chain then look at the cable leading to the house and follow the line, you will generally find that electricians go point to point.

Armoured cable can be the stuff referred to above, or even older is steel tape armoured cable, which originally had a lead inner sheath, a bitumen coat and 2 wound steel tapes on the outside, all originally covered with hessian cloth impregnated with bitumen. After a century underground you find the bitumen is gone, but the cable otherwise is in good order, even in the ground near the sea. Still hundreds of kilometers of it in use here, at various voltages from 230VAC to 66kV, all direct buried. Was fun one day when a building site 300m away was digging with an excavator and picked up the 11kV feeder for us, as he pulled it up one phase dropped, then the second then the cable finally tripped out on the substation breaker. Took 3 hours to isolate the segment and get power back to the local grid.
 

Offline commongrounder

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One piece of test equipment I own that will work to a depth of 15 feet is a Pasar Amprobe AT-2000 series tracer.  It has a high power signal injector that can be clipped to the conductor of an electrically DEAD cable (there is a transmitter included that is designed for live cables). It has a unique signal that can be picked up by the receiver and heard as well as seen.  Works very well, and could be available as a rental, since the kits purchase cost is around a grand.
I know there are others, made by Ideal, etc., too.
 

Online xrunner

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I recently returned from Brazil where I had a similar problem. The power cable between two properties had become damaged.

I didn't have any tools or equipment, so I devised a signal injector and tracer  ;)

The signal injector consisted of a cable wrapped around the spark lead of a motorbike (no physical connection)

The tracer was a cheap radio tuned to the AM band.

I tried this idea today and it worked remarkably well.

I used a weed eater and a roll of wire as a test. I had a portable AM radio with earphones and followed the signal along the wire very easily. It was a good signal up to several feet away from the wire all along, but you can zero in on it with the radio amazingly well. I have no doubt I can trace the wires with this setup. Can't wait to try it on the real thing.

 :-+
« Last Edit: June 01, 2013, 07:31:24 AM by xrunner »
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Offline C

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In general, I think you will find that the cheaper the AM radio the better you will be able to detect direction, while the better AM radios will pick up the signal at a longer distance.

There is some logic as to how things are normally done, so you need to put that to use to help with the puzzle.
Use as little wire as possible. I as others expect you to find that the four are connected with the shortest amount of wire possible.
Connect to closest power source unless more convenient to operate switch location wanted.
Save switch and turn on/off with something else like pool lights, porch light.
If you were the builder or buyer when those lights were installed, you would want it to be convenient to turn them on/off.
One reason this may not be true is if the cost was to great.
 If the pool was installed later and to them the lights not a big thing then the cost of running a new wire around to pool could have been to high. Later someone could have decided that the now unused switch would work great for a back door light. In stead of running a new long wire around the pool, they may have decided to run a shorter wire to the lawn storage shed that had power. And you have we are going to remove those lights, and they never got around to it.
I would guess if lights are original, then the switch was some place you could see the lights or part of them( pool install could have removed some).

If there is powered equipment for the pool the switch could be there.

So put on your "I AM CHEAP" hat a look.
So put on your "I WANT CONVENIENT" hat a look.

Just some more ideas for when your looking.
C         
 

Offline bilko

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I recently returned from Brazil where I had a similar problem. The power cable between two properties had become damaged.

I didn't have any tools or equipment, so I devised a signal injector and tracer  ;)

The signal injector consisted of a cable wrapped around the spark lead of a motorbike (no physical connection)

The tracer was a cheap radio tuned to the AM band.

I tried this idea today and it worked remarkably well.

I used a weed eater and a roll of wire as a test. I had a portable AM radio with earphones and followed the signal along the wire very easily. It was a good signal up to several feet away from the wire all along, but you can zero in on it with the radio amazingly well. I have no doubt I can trace the wires with this setup. Can't wait to try it on the real thing.

 :-+

I'm pleased to hear that you gave it a try and that you had some success. Sometimes you don't need to get all hi tech and expensive.

I didn't have any choice, there was no other equipment available to use  :-+
 

Offline ptricks

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What kind of cable is allowed to be direct buried in North America?

It has to be type UF if you don't use conduit. UF cable has insulation that doesn't break down in wet areas and is not damaged by UV rays.
It is against code in most areas  to bury white PVC pipe and use that for wiring for two reasons, it is associated with water and can be confusing to plumbers, and white PVC is not designed for direct burial where it might be exposed to sunlight as over time it will degrade, the pipe might not get UV rays when buried but it has to exit the ground at some point .  The gray pvc is UV stable and really not any more costly than the white stuff, though it isn't suggested for  potable water usage because the UV additives have not been long term tested for potable water

What most people use for conduit now is the flexible type like this:
 

Offline eKid

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Mr. Bilko;

          I too would like to thank you for this idea!  It is both clever and brilliant - and it works very well!  I wanted to dig up some stumps and I know there is a buried 6 gauge 4 wire 230 volt cable nearby but didn't know exactly where.  This cable is about 275 metres long and I know enough about its location to know that it does NOT run in a straight line.  It does have an outlet on a post in the middle though, which made a nice connection point.  I opened the breakers at both ends and coupled the ignition system of a Shindaiwa C270 weed whacker similar to Xrunner's onto one of the normally live wires at the outlet in the middle and was able to easily trace the location to within a few centimetres.

         For people trying this I would make the following comments:

1/ You can vary the amount of signal by how many turns you wrap around the spark plug lead.  I started with 7 turns and that was too much signal.  It made the detection area too broad.  I ended up using 2 turns but your situation may be different.

2.  I used a Sony SRF-M32 AM/FM radio set to the lowest frequency on the AM band (530 KHz.). Like most radios of this sort it uses a ferrite bar antenna which has directional characteristics.  I tried the radio in different orientations relative to the cable and the best results were with the antenna in an orientation where there was a strong signal on either side of the cable and a null when the radio was centered over the cable.  Strapping the radio onto a stick can ease the bending over problem for us old folks.  A little practice with a test setup like Xrunner's helped me get the feel of how the radio reacted.

3/  I agree with Xrunner that using a set of earphones/headphones is the way to go.  I used a good pair of "over the ear" noise cancelling headphones (Bose QC3) and was able to hear the ignition noise very clearly and ignore the noises from cars / trucks / airplanes / wife while I traced the wire.

4/  This trick might work for tracing buried plastic water pipes.  If the pipe is filled with water you could dip a 10 or 20 cm. of bare wire into the end of the pipe so it makes contact with the water and couple the other end to the spark plug lead and then trace as with electrical wires. High mineral content water should work best.  I have not tried this yet, but the next time I need to trace a plastic pipe I will give it a try.

     Thanks again Mr. Bilko for a great idea!

           Vince Sullivan (eKid)
   
 

Offline theatrus

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If you filled up your irrigation pipe with salt you might have a chance. The lawn however won't.
 

Offline calzap

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Be careful with ad hoc signal injectors unless you know their combo of power and voltage won't damage the insulation.  A coil wrapped around a spark plug or spark plug wire could possibly  spike well above the what the insulation can tolerate.  You might be punching small holes in the insulation, which would, of course, make the signal easier to detect, but set the stage for future disaster.

Don't assume the original work was done to code.  If it was done to code and under permit, there should be a plan on file at the local inspector's office.  Start there.

Rent a professional detector; there's nothing better, and it shouldn't cost too much.  If the wire is in metal conduit, good luck with any signal detection through metal and earth.  If you are trying to detect 120 or 240 VAC, sensitivity can be increased by a: putting good load on the circuit, and b: if possible, run power through one underground conductor for one leg and an above-ground conductor for the other.  This avoids the self-canceling that occurs when the conductors are side by side.

In most areas of the US, underground utility detection is free only on the utility's side of the meter.  Be prepared to pay a lot if you want it done on your side of the meter.

Mike in California

 

Offline Dave Turner

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I agree with renting a 'proper' cable detector.

However be very sure that when excavating you don't mistake a tree root for the cable.

Trust me it happens! On one old site tracing underground 3 phase 11kV distribution cables for a complete revamp of the site electrics the old 'cable' was traced and very carefully excavated around and protected before excavating further for the intended new cable. Oh boy 11kV under load makes a nice bang. Needless  to say the protected cable was a tree root.

Don't blame the workers too much old lead wrapped cable can look remarkably like a tree root.
 


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