Finding break in buried cable

On Mon, 18 May 2015 18:29:17 +0100, Harry Bloomfield wrote:

And ground/earth. that's cable tracing mode where a spare pair in an unscreened telephone cable would be chosen to carry the tracing signal.
In this case, with co-axial cable, you'd use the outer screen instead but you'd need to make sure it was isolated from any grounding contact beforehand.

Yes indeedy! It's really all about knowing *how* to use such testing kit (or else being such a clever cloggs like me that you're able to figure it out from first principles :-) ,
In this case, the 'perimeter wire' itself acts as 'the screen' (or chosen spare pair) but now we're not interested in tracing the wire so much as the fault location where the test voltage (ac or DC) injects current into the soil, thus creating a local voltage gradient above the break in the wire which can then be readily detected.
--
Johnny B Good

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On Mon, 18 May 2015 18:29:17 +0100, Harry Bloomfield wrote:

For the tone sender of my telecom tone tracer I'm pretty sure I tried every combination of the two wires from the sender to the screen/core of the coax. Couldn't detect the signal outside the cable after a matter of feet.
It may have worked if I could have got at the core but I wanted to know which cable was the faulty one along the length of the loom without breaking into the cable.
--
Cheers
Dave.
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On Sunday, 17 May 2015 14:28:14 UTC+1, Bill wrote:

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
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On Sun, 17 May 2015 14:23:41 +0100, Bill wrote:

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.
--
Johnny B Good

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Bill wrote:

Does the mower have some sort of transmitter that is connected to the "fence" wire? If so, and the break allows it to cross the wire past the broken point, can you discover what frequency the mower listens on and use a suitable radio to hear where the break is?
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Andy Burns wrote:

Several of the robomowers I could find details for use VLF between 8 and 10kHz, radio amateurs use software defined radio to monitor e.g. submarine comms, software is available
<https://sites.google.com/site/swljo30tb
I have no idea whether it's any more illegal to receive the mower's signal using an SDR, than it is for the fence to be an unlicensed transmitter in the first place .
Looks like a loop aerial plugged straight into a laptop's soundcard would be enough ...
<
https://youtu.be/L2W1x6Rb9hI

https://youtu.be/L2W1x6Rb9hI

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Well, it very much depends on how deep it actually is. I'd suggest that the problem is that he has not isolated the other end so trying to find the break is a problem. The mower itl self assumedly ca find the cable, can't it be used to find the p spot? My guess is that its near onne of the ends. Brian
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From the Sofa of Brian Gaff Reply address is active
"Bill" < snipped-for-privacy@gmail.com> wrote in message
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On Sunday, May 17, 2015 at 2:28:14 PM UTC+1, Bill wrote:

Hello, I would get a local dowser...or better still, a local dowsing group to independently dowse for the break and compare their findings. I expect they would take it as a challenge and help you without charge...maybe coffee or beer... I could do it and I am no expert....but too far away. Search "British Society of Dowsers" for their telephone number and they will put you in touch with their local group if there is one. They will be amateurs,mostly, if not all, but that should be enough to deal with the problem. Good luck, David G
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Don't dig at 20 yard intervals.
Dig half way and test. That should tell you which half has the fault. Dig in the middle of that half. Test, determine which part to halve next. etc
[g]
On Sunday, May 17, 2015 at 2:28:14 PM UTC+1, Bill wrote:

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On Sunday, May 17, 2015 at 2:28:14 PM UTC+1, Bill wrote:

It is a pair, or just a single wire?
If it's a pair and is shorted you could work out the length by measuring the resistance and comparing with the resistance of a known length of the same type of cable?
Robert
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RobertL formulated on Monday :

That would depend upon how well shorted it is. Shorted in damp soil, likely the short would be quite high resistance, when compared to the very low resistance of the copper.
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Regards,
Harry (M1BYT) (L)
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On 18/05/2015 18:35, Harry Bloomfield wrote:

That is why I suggested using capacitance measurement. As long as the "short" in earth is a much higher resistance than the cable resistance, it's length will be proportional to measured capacitance.
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Pack the mowings in several smaller bags next time? One big bag of grass cuttings is mighty heavy, if that's what he had.

Questions: What makes him think there's a break? Is it just that the mower won't do an about-turn or WHY at the appropriate point? If so, is it the mower's sensing circuitry at fault, not the cable?
Why does he believe the cable is earthed? Presumably he's measured the resistance and gets a value he didn't expect. Is the resistance to earth different to what it was before the fault developed? Had he ever measured it before the fault developed? If not, how does he know it's different? Might not one end of the cable be earthed anyway? (If you measure the resistance between mains neutral and earth, you'd get a lowish value, if you get my point).
--

Chris

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I can't answer these questions, because I don't know.
Today I was supervising fencing on his behalf and taking his chainsaw to be repaired. I seem to have retired into a full time job.
The cable is a single wire in a loop. It emerges from the edge of the lawn and terminates in a small choc-block like 2 pin plug connector. I didn't have a meter with me, but I paced out the cable - about 300 yards perimeter - and felt and looked at the ground as I did so. There is no obvious damaged area.
The mower is a Robomow RL1000. It has a docking station, but this is in the garage and not on the lawn. I assume he plugs in the driver unit for the cable to the plug when he sets the mower going. He has 3 other smaller lawns at least one of which has a loop and he and the mower passes this on the way to the main lawn so I assume he has tested that the clever bits work.
I couldn't find this unit that plugs into the wire. The pdf manual that I have found says this has a "broken wire" light, so I assume that's why he thinks the wire is broken. I'll try to remember to take a meter along tomorrow, but I expect further investigations will have to wait till the weekend when he is back.
--
Bill

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I found the following website which might be helpful. http://robomowerwiki.com/Troubleshooting_and_Repair It includes a link to another site describing how to find a break in an invisible dog fence, operating at 600kHz, and the method has been used to find the break for a Robomow RL500.
--
Dave W

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Well, I can report great success and great embarrassment.
Yesterday we pulled up a bench to the end of the loop and sat in the sun with my meter, a radio and his mower controller.
The mower controller showed a broken loop with a light and a sequence that comprised a set of audio (about 1kHz) bleeps followed by a set of LF pulses down the wire. Interestingly, the radio on long wave could be tuned into the beeps (ie they were transmitted as well as just an audio sender in the unit) where the cable was above ground for the about 1 metre to the connector, and we could follow the pulses underground with the same radio.
We bared some wire behind the connector and measured the loop. It measured a short circuit and it also measured low resistance to earth. The cable connector looks like a 2-way "chocolate block", but with a small connector on one side instead of the 2nd screw connections. He went to look for a small screwdriver and came back with that and a spare connector. Replaced the connector and it all worked.
When I asked about the earth on the loop, he said "Ah, I did dig through the wire ages ago and soldered it back together".
So, it all works. The sender is battery powered, so the one earth somewhere in the cable doesn't matter, and I should not have assumed that the break in the cable and the earthing problem were at the same place. The connector looked OK but wasn't.
Many thanks to all who helped by making me think about the problem. Shame the thinking didn't work as well as it should.
--
Bill

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On Monday, 25 May 2015 15:03:34 UTC+1, Bill wrote:

well, that's just part of electronic fault finding.
NT
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A bit late in the day, but the old-fashioned Megger with the wherewithall to do the "Varley" measurement would have suited.
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On Mon, 18 May 2015 20:25:09 +0100, Bill wrote:

It's interesting and rather gratifying that my suggested possible methods of energising the loop via a "lamp" which could provide verification of continuity or an equivalent whereby a current sensing circuit lights up the lamp in the absence of loop current.
I wonder if the 'broken wire lamp' shows a fault by *failing* to glow as per my suggested KISS method? It matters not whether 50Hz low voltage mains is used or a higher audio frequency tone is generated to excite the loop (other than to reduce the risk of stray 50Hz magnetic fields from other sources such as high voltage transmission lines running very close by[1].
From what Bill has already told us, it seems the wire, insulation and all, has completely snapped, exposing the bare ends to, presumably, damp soil. This being the case, it should be fairly easy to detect any test voltage injected into the wire with respect to a ground connection by probing the surface of the soil along the path of the wire until detection of the raised earth potential reveals where the injected current (DC or an AC tone) is leaking into the soil.
A standard telecoms tone generator may not have enough 'oomph' on its own to provide a sufficiently detectable rise in earth potential around the break to allow the probe to be passed over the ground by a foot or two of height that would allow a quick search of the ground involved. However, you could lash the probe to a suitable stick or pole and drag it over the ground above the run of the wire. If there's any grass growing, it will suffice that the probe tip brushes the blades of grass in its path.
If using the tone detection set on its own fails to produce any results, you can either use an audio amplifier to boost the voltage (and current) or else resort to using a DMM or analogue multimeter (the analogue meter may be a better option if it has a decently high enough kilo ohms per volt rating of 'goodness' - 20 to 50 thousand ohms per volt of its scale) and a car battery or two to inject a test voltage into the wire.
In this case you'd need to provide a trailing earth (a meat skewer planted in the middle of the lawn would suffice as an earth connection - also a useful enhancement when using the tone detector option) for the DMM or analogue meter and lash the meter probe to a stick or pole to probe the ground or blades of grass along the route of the wire.
You might be able to use the 200mV scale but don't be too surprised if it registers random galvanic voltages. The 2v scale might be a better choice when you're looking for a rise in local ground potential that's likely to be in the region of 3 to 6 volts maximum when using a 12 volt battery.
[1] I remember diagnosing a problem that a friend of a friend was suffering with his computer display (a CRT based colour monitor back in the days when LCD screens were an overpriced piss poor substitute for "The Real Thing"(tm)).
In the end, the only conclusion I could draw was that it was sufficiently close enough to an almost directly overhead high voltage transmission line that the radiated magnetic field strength was strong enough to disturb the electron beam scan in a high resolution CRT computer monitor (1024 by 768 NI versus a 625 line interlaced TV display with a swamping level of its own 50Hz interlace flicker to hide behind and be lost "in the far distance", relatively speaking, compared to a normally stable NI high res computer monitor designed for close up viewing).
Another factor that made the downstairs TV set seemingly "immune" to the effect was the fact that computer monitors ran at 60Hz or higher refresh rates not locked to the 50Hz mains borne (not bourne, idiots![2]) interference making the resulting 10 or 25 or 35 Hz 'beat' frequency all the more objectionable in the case of 60, 75 and 85 Hz refresh rates respectively.
[2] Apologies for that mini-rant but it just had to be said. :-)
--
Johnny B Good

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On 17/05/2015 14:23, Bill wrote:

Connect both ends of the loop to one side of a 50Hz AC source - say from a mains driven transformer secondary. You can put a lamp in series for short circuit protection if you like. Connect the other side of the AC source to one probe of a multimeter using a long insulated wire. With the multimeter on AC volts, use the other probe to find the highest voltage by sticking it in the ground near the wire.
AC is better than DC because you don't get polarising effects, and most meters will give good noise rejection at 50Hz.
All this assumes that there is some connection between the broken wire and the ground.
Dig it?
Cheers
--
Syd

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