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
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
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.
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.
Time Domain Reflectometer. Expensive to buy but can be hired - still not
Or look out for a telecom or network cabling technician willing to do a
'foreigner' job for a consideration :-)
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
I wonder what the original tone generator setup was? I would have
thought that a generator and CAT would have worked.
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
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.
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
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.
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.
I'm confused, is the wire broken, or is it "earthed" or what is it that
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?
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.
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?
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
That's extremely unlikely. The system appears to be a single turn
magnetic loop which requires the wire to be insulated from contact with
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.
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.
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