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Author Topic: Finding a hidden underground joint  (Read 2494 times)

aesmith

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Finding a hidden underground joint
« on: May 18, 2017, 04:29:51 PM »

Hi,

We've had two faults which turned out to be with a hidden joint on our local cable, where an old joint post had been driven over and pushed underground so effectively the line was just a bunch of crimps in the soil.   Both faults took a long time to fix, mainly because although they caused line noise the tester was still passing.  In fact in each case it took months.

I've just taken a walk along the line, and I can see that there's another buried joint somewhere, as counting from the house joint (5) has opaque grey cable about 15mm in diameter coming from it, and joint (6) has smaller cable around 8mm with a translucent outer sheath on that side (back to larger cable on the DP side).  So somewhere in between there's a joint which I'll call (5a) that joins large cable to small.

I'd like to find that joint, because if it's like the one I referred to I can report it as damaged plant in its own right.

Any clues as to how to locate a buried joint?   All this runs along a fairly narrow margin between the track and the cultivated field.  Where the cable is buried it is typically around 9" deep.

Thanks,  Tony S
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burakkucat

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Re: Finding a hidden underground joint
« Reply #1 on: May 18, 2017, 04:50:06 PM »

My first thought was "ground penetrating radar".  ::)

Thought number two was if you could map the buried cable(s), thus knowing exactly where it/they are located, close observation of the ground may then provide a hint. So what about one of those detecting devices, used by competent contractors, to locate other buried services before any excavation takes place? With the route clearly defined, you could then perform a sequence of test digs, using the classical "binary chop", to locate the hidden joint.  :-\
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JGO

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Re: Finding a hidden underground joint
« Reply #2 on: May 18, 2017, 05:12:36 PM »

Would a recreational Metal Locator do the job ?
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burakkucat

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Re: Finding a hidden underground joint
« Reply #3 on: May 18, 2017, 05:14:57 PM »

Would a recreational Metal Locator do the job ?

That's an interesting idea. It would certainly be worth a try.
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Black Sheep

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Re: Finding a hidden underground joint
« Reply #4 on: May 18, 2017, 05:37:07 PM »

There are various methods employed in finding a buried joint .................. it all depends on what you are presented with as an engineer as to which method you would use ??

If there are 'spare pairs' within the cable that are not used, the chances are they may be 'stumped' (disconnected) in the buried joint. So, we would measure these 'spares' and whilst tracking the cable using a 'Cat & Genny' (Google it  ;)), we also use a pedometer to wheel out the distance of the measured spare pairs.

The problem is, there could be a coil of cable left in the ground as sometimes happens that would throw ones measurement out quite a lot, so the key is to also listen to the CAT tools 'beep-beep' and start tweaking down the 'sweep' level. Where the spare pairs end, the 'beep-beep' will all but disappear. It needs a little bit of experience to get to grips with it.

Likewise, if there are no spare pair in the cable, ie: they are all used and jointed through, then we attach the Genny to the armoured wiring protecting the inner cable, this is a bit trickier to get to grips with as the induced signal from the Genny has a high chance of spreading to 'Earth' where the insulation has cracked over years of being underground. But, a good engineer will know a 'good signal' from a 'poor signal' and will follow the trace until the 'beep-beep' diminishes, which is where the armoured cable has been cut to fit the buried joint.

Many, many years ago, we were told to fit a metal plate to the joints so that we could use a metal detector to locate it again in the future, but that practice kinda dropped off over the following years.  ::).

Believe me when I say that when you are presented with mile after mile of rural buried cable, you become quite proficient in measuring, locating and digging down on buried joints.  :)
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aesmith

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Re: Finding a hidden underground joint
« Reply #5 on: May 18, 2017, 08:03:40 PM »

It's true that the UG engineer and the original OR guy found the previous joint pretty quick once they had a fault measurement showing up on their tester.   Since this particular joint I'm looking for now connects 5 pair to 2 pair, I guess there must be three pairs which aren't connected straight through.
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Black Sheep

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Re: Finding a hidden underground joint
« Reply #6 on: May 18, 2017, 08:46:03 PM »

Absolutely. So measuring and locating, would be easier than if it weren't so.
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tickmike

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Re: Finding a hidden underground joint
« Reply #7 on: May 18, 2017, 09:07:46 PM »


Any clues as to how to locate a buried joint?   All this runs along a fairly narrow margin between the track and the cultivated field.  Where the cable is buried it is typically around 9" deep.

You could follow/locate the cable with a medium wave radio tuned to  612kHz but not sure about joints, a good spade could be useful :D .

 
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licquorice

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Re: Finding a hidden underground joint
« Reply #8 on: May 18, 2017, 09:59:52 PM »

I think bcat's method is probably the simplest if the cable is that close to the surface, as he says, just keep halving the distance for the trial digs.
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Black Sheep

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Re: Finding a hidden underground joint
« Reply #9 on: May 18, 2017, 10:05:47 PM »

Word of caution ............. our UG cables are very similar to the electric board cables, (especially the street-lighting ones).

We have had what industry terms 'near misses', in the past. If you must start digging up our cables, be aware that electric cables have a raised embossment on their sheathing, telecoms don't.  :)
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burakkucat

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Re: Finding a hidden underground joint
« Reply #10 on: May 18, 2017, 11:51:44 PM »

Word of caution ............. our UG cables are very similar to the electric board cables, (especially the street-lighting ones).

We have had what industry terms 'near misses', in the past. If you must start digging up our cables, be aware that electric cables have a raised embossment on their sheathing, telecoms don't.  :)

Thank you for those wise words of advice.
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4candles

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Re: Finding a hidden underground joint
« Reply #11 on: May 19, 2017, 02:40:06 PM »

Perhaps try dowsing ?


If it doesn't work you've only lost a couple of coathangers.
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aesmith

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Re: Finding a hidden underground joint
« Reply #12 on: May 19, 2017, 07:59:19 PM »

I thought I was on to it yesterday.  With the soil so dry it's relatively easy to find a shallow cable once the turf is lifted, so I cut a couple of trenches across the route and I was finding the thin cable only a few feet from joint (5).  However following the thin cable it went round in two loops, then disappeared down deep and heading into the cultivated field - I think that must be a bit of obsolete cable left after some former damage.  I need to go back to the further trenches and make 100% sure there's no larger cable running across.   Or start back from joint (6) to see whether there's any thick cable visible.

There's no power there we're fed by overhead lines from the North, and the next house SE of us is fed by overhead from the South.  His water pipe and control run across the field, not along the track.  No harm in remembering the issue though.

Regarding dowsing, the missus is pretty good using brass L shaped rods.  I can't do it at all.  I'm not sure it would find a joint though, given the cable route is known and runs past the joint.
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KIAB

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Re: Finding a hidden underground joint
« Reply #13 on: June 03, 2017, 08:38:43 AM »

If you  have a spare pair, you could use CAT 3 or 4 (Cable Avoiding Tool) & genny to easily find cable, can hire them for a day, find a independent hire shop would most likely be cheaper.


https://www.hss.com/hire/p/cable-and-pipe-locator-cat-3-hire-pack-1
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burakkucat

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Re: Finding a hidden underground joint
« Reply #14 on: June 03, 2017, 08:10:43 PM »

Another good suggestion.  :)
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