Finding break in buried cable

Son has one of these automatic mowers to deal with his lawn that goes round 2 1/2 sides of his house. The mower mulches as it goes. When it goes.

Today he was forced to get out the old Webb ride on motor mower (thanks for the help here in years gone by with advice on the repair of this). He moaned that he had to get the help of 2 men at the tip to get the grass cuttings into the skip, and that he was asked again whether this was domestic rubbish.

The automatic mower works within a very, very long wire buried a few inches deep all round the edge of the lawn. Somewhere there is a break in this. He has a tone generator and receiver and has tried with a radio as a second detector, but says that neither of these work because, he believes, where the wire is broken it is earthed.

The mower manufacturer's only suggestion is to dig at, say 20 yard intervals and divide the wire into sections until the problem section is found. He thinks this will be difficult and leave a less robust cable installation.

My suggestion was to somehow put a very high voltage into one end of the wire and look for smoke, but I have been told that this is ridiculous.

Any suggestions?

Reply to
Bill
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Bill used his keyboard to write :

I am surprised you cannot locate the break with a tone generator..

Telecom engineers have another device, which sends a 'ping' down a cable and measures the time for the signal to bounce back. The time gives the distance to the short or the break.

High voltage would just cause the insulation to break down. I would suggest a low voltage DC, where one polarity is attached to the wire end, the other via a meter, to a second probe.

Then probe around the ground to find where you get maximum current flow indicated on the meter. Where you get the maximum, is where it is leaking to earth/ground.

Depending upon the depth, you might be able to use a metal detector to trace out the route of the cable.

Reply to
Harry Bloomfield

Time Domain Reflectometer. Expensive to buy but can be hired - still not cheap e.g.:

Or look out for a telecom or network cabling technician willing to do a 'foreigner' job for a consideration :-)

Reply to
nemo

That's the beasty. I used to have one around somewhere, one of the early version with the green screen CRT's.

I suppose something could be cobbled together with a pulse generator and a storage scope?

Reply to
Harry Bloomfield

Harry Bloomfield explained on 17/05/2015 :

Lots on ebay -

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but ignore the fibre ones.

Reply to
Harry Bloomfield

I'm struggling to see how you're going to detect a vestige of the reflected pulse with this setup. A network analyser might do it but like the other bloke said, can't beat a TDR for this application.

Reply to
Cursitor Doom

I'd like a so when the phone line goes phut I know which joint post/hole an engineer has been fiddling in so know if it's walk or drive to let him know. B-)

However I have got a network cable tester that has a TDR of sorts. With a fault close to either end that is all it says, FSVO "close". But for intermediate faults it gives the distance. Seems reasonably, couple of meters when measured from each end. with one distance being about 60m the other 10. Cost about £30 on Amazon feed your favourite search engine SC8108.

It's a bit plasticy and the beep from the remote module I find annoying but may have it's use when trying to find the other end of a cable an you are in audible distance.

Donno if the TDR element would work on a single, buried, wire mind. For that I quite like the DC supply at one end then probe for maximum current or would looking for a peak in voltage be better? Current might to influenced by variations in ground conductivity. The big assumption is that the break has also resulted in a connection to the ground.

I'm not surpised a toner hasn't worked that is just a loud AF signal. It radiates from open wires well but not at all from coax. Buried will screen the signal. Now modulated RF signal of a a few hundred kHz and LF radio might work.

Reply to
Dave Liquorice

The only TDR I've used required a pair, would they work on a single cable, also would they be happy with the varying impedance of the cable in soil?

I wonder what the original tone generator setup was? I would have thought that a generator and CAT would have worked.

>
Reply to
Bill

In message , Dave Liquorice writes

Thanks, Dave, Harry and everyone.

That has reminded me that I used to have a very cheap Tandy metal detector that was useless in trying to find a mooring that had sunk. Time to go climbing in the junk in the garage and see if it still does anything.

I might try the low voltage idea. I can probably dig out some old cable long enough for the above ground return. I have an old wind up megger in the shed. I wonder if that could be any use.

The Telecom device sounds possible but very expensive if we can't find an amenable BT man. I wasn't sure whether this would produce a usable reflection in this case where there is just one wire and genuine earth.

I did see his audio tester, but can't remember what it looked like. He is off round the country on trains again for the week, but we might have a long weekend next week if the strike is on.

Reply to
Bill

Where is he Bill?

I have a TDR

Andrew

Reply to
Andrew Mawson

I'm confused, is the wire broken, or is it "earthed" or what is it that is "earthed"?

One way of approximating an open circuit is through measuring capacitance. Some meters do this, and I would measure capacitance at each end and hope the position would be proportional to this measurement.

TDR, as others have said, is another method, but given the velocity will be substantially below "c" and dependent on the cable and surrounding earth I'd be wary to believe if this method would be any more accurate?

Reply to
Fredxxx

In message , Fredxxx writes

There is a break in the cable loop. The cable is low resistance to earth measuring either way round from the little connector where whatever it is that talks to the mower plugs in. He assumes the cable is broken and so has 2 bare ends somewhere underground.

Reply to
Bill

In message , Andrew Mawson writes

We are on the Wirral - ie Chester way. From what I remember from mentions of earth moving equipment, you are somewhere in the south, but I am very grateful for the offer.

He is on a train at the moment, so in the week I'll have a look at some of the other ideas. I haven't really been involved in this up to now so a bit of quiet contemplation might come in handy.

Reply to
Bill

A break in the cable loop implies open circuit.

Perhaps I'm missing something, but then saying the cable is a low resistance to earth implies the cable is not insulated but purposely designed to conduct to the surrounding soil.

How low resistance is "low"?

Can you not measure the resistance at either end to earth to gauge relative position of the break?

Reply to
Fredxxx

Why am I not surprised Andrew!

Reply to
Bob Minchin

-- Yes sorry hundreds of miles away

Andrew

Reply to
Andrew Mawson

:) :) :)

Andrew

Reply to
Andrew Mawson

If you feed power down the cable ends you should see a rise in R to earth, and be able to use a low impedance capacitance meter. I'm sure there are higher tech options too.

NT

Reply to
tabbypurr

From your description, it would seem that the automatic mower detects the presence of the buried wire to determine the limits of its range within which to operate. Presumably, it mows strips of the lawn using the perimeter wire to determine when it should do an about face to start a new strip parallel to the previous strip.

The questions are; How does the mower detect the perimeter wire? How does your son know that there is a break in this wire?

My guess is that the wire forms a single turn current loop which carries a low frequency current which radiates a magnetic field sufficiently strong enough for a magnetic sensor on the mower to be able to detect when it gets within a short detection range (presumably of the order of 6 inches or so).

This system requires that the ends of the wire be terminated by a signal generator of some sort (possibly something that looks like a wallwart), It might even be something as simple as an ac transformer with a 1 or 2 volt secondary to drive an amp or two's worth of 50Hz current around the loop.

A more sophisticated version might well use a 6v 6VA transformer with a

5 or 6 volt secondary with a 6W filament lamp in series to act both as a current limiter and a fault indicator. Perhaps your son is concluding a broken wire from the lack of indication of the sender unit which might just be a burnt out lamp. Failing such sophistication in the sender unit, perhaps he's simply tested for continuity with a DMM.

If the wire has broken because of physical damage which has also cut the insulation, the break can be detected by sending a test voltage into both ends of the wire with respect to a ground connection (one terminal of the test voltage, AC or DC, is connected to ground and the other to the bunched ends of the wire). Once this is set up, you can probe the route of the wire with a matching detector (DC voltmeter probe using a very long trailing earth wire back to the test voltage ground connection point or else a tone detector if using a tone generator to provide the test voltage).

Unfortunately, if the break is under the insulation, such a simple fault finding technique will be doomed to failure and more sophisticated methods will be called for such as measuring the return trip time of a narrow fast rise voltage pulse from each end of the loop.

The return time for one end divided by the sum of both return times will produce a percentage figure of the total length of the wire loop to the location of the break, eliminating any guesswork on factoring in a velocity figure to complete the distance calculation.

That takes care of the fault finding aspect. I'll leave to you to figure out an appropriate repair method. I'm sure you'll receive plenty of suggestions in this news group of which a fair few will no doubt mention Self Amalgamating Tape.

Reply to
Johnny B Good

It would if it weren't for the fact that both sides of the break would seem to have a galvanic contact with earth, providing continuity of a sort.

That's extremely unlikely. The system appears to be a single turn magnetic loop which requires the wire to be insulated from contact with earth.

That's a very pertinent question. One man's 'low' is another man's 'high' when it comes to resistance measurements of earth contact faults. I suspect that 'low' in this case is more likely to be a few hundred ohms rather than a few ohms.

I doubt this is likely to help on a wire that might only have a loop resistance of an ohm or two and earth contact resistances that might be a few hundred ohms each wherein the discrepancy will simply be down to chance as to how 'good' a contact to earth each side of the exposed conductor has to earth.

In this case, assuming this more detailed fault description is reasonably accurate, the OP's son, by the OP's own account, already possesses suitable test gear (a tone sender and detector). it's just that he hasn't figured out how to employ it to best detect this particular type of fault.

The only way to connect the tone sender is to connect one side to local ground and the other to one of the ends of the loop, preferably the one showing the lowest earth resistance value. The tone detector can be put into cable detect mode and the ground along the run of the loop probed with the detector probe until the (usually) 1KHz test tone is at its loudest where it contacts the ground immediately above the break where the local ground potential has been raised by the test current being injected from the fault contact.

If necessary, a temporary earthing rod can be stuck into the ground in the middle of the area enclosed by the loop and a trailing earth lead connecting to the case or shield connection of the tone detector in order to enhance the sensitivity of the detector system.

If this fails to detect the fault location, then it's down to the use of a mark 1 eyeball to look for any potential signs of soil disturbance arising out of whatever may have penetrated the soil to damage the buried wire and exhumation of any suspected sections. Failing any such signs, then it's down to taking a best guess and literally pulling the wire out of the ground until the break is located. The usual starting point when it becomes a guessing game is to start from the middle and work towards each end in turn.

Reply to
Johnny B Good

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