Help finding break in buried wire

[Willis Clayton]

I have a Radio Fence from Pet Safe that does a fantastic job of
containing my well-trained and dearly-beloved mutts.

For anyone unfamiliar with the radio fence concept, the idea is that a
transmitter sends an audio-frequency signal through a 15-20 ga. wire
buried just below the ground surface forming the containment
perimeter. The animals wear receivers that detect the signal when
they approach the boundary and receive a "correction" when they get
too close.

Unfortunately, the gophers and/or moles in my yard all-too-frequently
bite through the buried wire when it happens to cross the path through
which they are tunnelling. I have seen several posts on this topic,
and none have been terribly helpful thus far.

PetSafe Corp. actually sells a device that is supposed to help locate
breaks. It looks just like their transmitteres, but instead of the
normal operating frequency, it sends two different audio frequencies
through each end of the wire. Using an AM radio tuned to the bottom
of its frequency range (530 KHz), you're supposed to be able to walk
the perimeter and listen for the change from one signal to the other.
I've seen their device used by one of their technicians, and it seemed
to me that the signal wasn't strong enough (my boundary wire is
approximately 1000 ft., for whatever that's worth).

Several weeks ago I saw a post that I've been unable to find again,
that suggested using something like a running lawnmower (spark plug)
to send an (RF?) signal through the wire and then use the same radio
to detect the clicking sound that would apparently be observed at the
break site.

I know several of you will have great ideas for me. Sorry this post
is so long, and please forgive me if I've made all this terribly
unclear.

Willis

 

[Anthony Fremont]

Willis Clayton wrote:

Unfortunately, the gophers and/or moles in my yard all-too-frequently
bite through the buried wire when it happens to cross the path through
which they are tunnelling. I have seen several posts on this topic,
and none have been terribly helpful thus far.

<snip>

You need a time domain reflectometer.

michael

 

[Anthony Fremont]

While a Time Domain Reflectometer is the preferred method, it's not
always available (I've been in the electronics racket for 50 years
and never saw, let alone used one) to the average hobbyist. Assuming

The local electronic surplus/supply/etc store sells a wire fault finder
for less than $100.00 that will measure the distance to a short/open and
display it on an LED display. I'd kinda like to have one, but can't
really justify the expense for it. I'd really like to have a "real"
reflectometer. That is a way cool piece of equipment.

 

[qrk]

While a Time Domain Reflectometer is the preferred method, it's not always
available (I've been in the electronics racket for 50 years and never saw,
let alone used one) to the average hobbyist. Assuming that the wires didn't
separate by any appreciable distance, one could probably use a common
filament transformer and pick up the arcing with an a.m. transistor radio.
The larger the separation, the higher voltage one would need to induce
arcing. An alternative would be to use an r.f. signal source (e.g. lawnmower
spark plug hash) which could also be detected with a cheap pocket radio.

If you have an oscilloscope (100 MHz or higher bandwidth) and a
squarewave signal source, you can have the basic functionality of a
TDR. Connect the o-scope to the output of the signal generator and
look for the reflected edge.

In the dog-zapper case, a TDR might not be a good tool to use since
the buried cable is probably a big loop antenna. Better to drive one
end with a healthy signal in the AM band and walk along with a AM
receiver.

Mark

 

[Jim Yanik]

Willis Clayton wrote:

<snip>

You need a time domain reflectometer.

michael

Except that his is a SINGLE strand buried wire,not a pair or cable.I don't
know how well a TDR will work on a single wire.

 

[Baphomet]

Willis Clayton wrote:

<snip>

You need a time domain reflectometer.

While a Time Domain Reflectometer is the preferred method, it's not always
available (I've been in the electronics racket for 50 years and never saw,
let alone used one) to the average hobbyist. Assuming that the wires didn't
separate by any appreciable distance, one could probably use a common
filament transformer and pick up the arcing with an a.m. transistor radio.
The larger the separation, the higher voltage one would need to induce
arcing. An alternative would be to use an r.f. signal source (e.g. lawnmower
spark plug hash) which could also be detected with a cheap pocket radio.

 

[Baphomet]

While a Time Domain Reflectometer is the preferred method, it's not always
available (I've been in the electronics racket for 50 years and never saw,
let alone used one) to the average hobbyist. Assuming that the wires didn't
separate by any appreciable distance, one could probably use a common
filament transformer and pick up the arcing with an a.m. transistor radio.
The larger the separation, the higher voltage one would need to induce
arcing. An alternative would be to use an r.f. signal source (e.g. lawnmower
spark plug hash) which could also be detected with a cheap pocket radio.

Re - reading your post, did you try going around the perimenter with an a.m.
transistor radio and see if you could pick up noise as is without doing
anything more elaborate???

 

[Stuart Robinson]

If you have an oscilloscope (100 MHz or higher bandwidth) and a

squarewave signal source, you can have the basic functionality of a
TDR.

Indeed I used one for several years to test network cables. Now I also
have a CAT 5 cable tester that does TDR, but the scope does give a better
overall picture of the impedance match along the wire.

My scope was 30Mhz, more than enough for the job.

The issue with using a TDR, it that you would need to know with a fair bit
of accuracy what the propagation velocity of the wire is, in a 1000' run
if you managed to measure to 1% accuracy, you could still be 10' out.


Stuart.

 

[Al Yeager]

Why not put the 'pet safe' on your neck and seek the spot were you don't
get corrected. That's probably where you should dig.

 

[JJ]

It's not a single wire. It's a wire buried in earth. So think of it as a
coax where an earth stake provides access to the screen. That said, I'd
expect a lot of minor impedance discontinuities due to the non-uniform
nature of the soil so finding the break may not be clear. And a TDR is
neither cheap nor readily borrowed for most people. An alternative may be to
measure capacitance to ground from each end of the broken loop. Then use
ratios to estimate where the break is. Sadly, this will likely be defeated
by an earth fault at the break. :-{

 

[JJ]

I've used the wandering receiver approach successfully for divers to track a
seabed cable where it had been silted over. I drove the cable (in my case a
multi-core but I tied all the conductors together) with respect to my local
ground using an unmodulated sine signal at a few kHz. I reasoned that the
core conductor would drive the ground at the remote end (in this case, via
the fault at the cable break) so in the vicinity of the cable the current
would not be balanced by a return current in the outer sheath, resulting in
a magnetic field leakage. To detect that I potted up about 5000 turns of
winding wire in a block of epoxy (about 200mm x 200mm) with an op-amp,
detector diode and a moving coil meter. The divers had no problems tracking
the buried cable. In the doggy loop case I'd ground the other half of the
loop to ensure the test signal doesn't partially propagate further. Leave
out the meter and stuff and stick a preamp and headphones on the coil and go
wander.

Hope that helps,
John

 

[Ian Stirling]

I have a Radio Fence from Pet Safe that does a fantastic job of
containing my well-trained and dearly-beloved mutts.

Unfortunately, the gophers and/or moles in my yard all-too-frequently
bite through the buried wire when it happens to cross the path through
which they are tunnelling. I have seen several posts on this topic,
and none have been terribly helpful thus far.

Maybe the right fix is to replace the wire with something harder to chew.
?

 

[Terry]

Willis Clayton threw some tea leaves on the floor

I have a Radio Fence from Pet Safe that does a fantastic job of
containing my well-trained and dearly-beloved mutts.

For anyone unfamiliar with the radio fence concept, the idea is that a
transmitter sends an audio-frequency signal through a 15-20 ga. wire
buried just below the ground surface forming the containment
perimeter. The animals wear receivers that detect the signal when
they approach the boundary and receive a "correction" when they get
too close.

Sorry I don't have a solution, but couldn't resist this follow up.

When you finally fix the wire, perhaps you should modify it so that
when the Gophers etc try and chew it in future, they are "corrected"
also