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?
--
Bill

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 17/05/2015 14:49, Harry Bloomfield wrote:

Time Domain Reflectometer. Expensive to buy but can be hired - still not cheap e.g.: <http://testequipmenthire.co.uk/MeggerTDR2000.asp
Or look out for a telecom or network cabling technician willing to do a 'foreigner' job for a consideration :-)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
nemo explained :

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?
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Harry Bloomfield explained on 17/05/2015 :

Lots on ebay - http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR1.TRC0.A0.H0.Xtime+domain+reflectometer.TRS0&_nkw=time+domain+reflectometer&_sacat=0
but ignore the fibre ones.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 17 May 2015 15:23:30 +0100, Harry Bloomfield wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.

--
Bill

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 17 May 2015 15:23:30 +0100, Harry Bloomfield wrote:

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.
--
Cheers
Dave.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Bill

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Bill" wrote in message writes

Where is he Bill?
I have a TDR
Andrew
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Bill

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Bill" wrote in message writes

-- Yes sorry hundreds of miles away
Andrew
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Why am I not surprised Andrew!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Bob Minchin" wrote in message

:) :) :)
Andrew
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 17/05/2015 14:23, Bill wrote:

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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
writes

is "earthed"?
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.
--
Bill

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 17/05/2015 18:32, Bill wrote:

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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 17 May 2015 21:04:51 +0100, Fredxxx wrote:

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.
--
Johnny B Good

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 18 May 2015 02:20:26 GMT, Johnny B Good wrote:

Depends on what "tone sender and detector" is. I don't think a telcom type set will work on this. They don't work on coax for more that a few feet, no matter how you connect the sender or with the detector probe touching screen or core. At least mine doesn't but will work fine over tens of meters of CAT5 or even de-energised T&E.
I discovered this when I wanted to identify which of three coax cables, in a loom, was the one with a break. Easy I thought, bung sender on the accessable end of fualty coax, trace along. No detectable signal after a few feet. B-(
The much larger and sophisticated CAT cable tracers as (should be) used by contractors before digging holes in the street probably would find the cable but not sure of the break. They have associated senders as well which might enable that. I think you can hire these from machinery/plant hire places.
--
Cheers
Dave.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave Liquorice submitted this idea :

Did you connect to screen?
Mine worked fine around the house on the screen.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.