Absolute proof of Sod's/Murphy's law

[usenet]

Over the weekend I put up a short length of wire netting fence to keep the rabbits and chickens out of our vegetable patch. A couple of the posts I knocked in weren't perfectly vertical so I damped the ground down (some more) and wedged a length of wood against them to push them straight with a metal stake at the other end to stop the length of wood slipping.

We have something like half an acre (or maybe even an acre) of grass around our house and there's just one MDPE pipe running across it. One of the two metal stakes I drove in was a perfect hit on the middle of the buried MDPE! I only found out when I pulled it out and got a pretty little fountain out of the lawn.

It wasn't too much of a disaster as there is a separate stop c*ck for all the garden water and I had a straight connector to fix it with. It still amazes me that I managed to hit that pipe though.

[Pete C]

Hi,

Looks like a promising career in water divining there.

cheers, Pete.

[wanderer]

Brings back memories of spending all night looking for a fault in an unarmoured 11kv cable that dog-legged across an open field. The lead sheath had been nicked when the farmer was doing some deep cultivating and then faulted after some heavy rain.

The field was originally a WWII airfield, and the cable had been nicely dimensioned in to airfield buildings that had been knocked down 30 or 40 years ago.......

[Andy Hall]

How on earth would you detect where the fault is and then repair it?

.andy

To email, substitute .nospam with .gl

[Andy Dingley]

Look for the smoking moles ?

[RichardS]

Could you inject an HF signal into it and work out the rough position from the reflections, I wonder?

Even if you know it's X metres from the diagnostic equupment, non-straight cable runs would make that a very awkward thing to find!

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[G&M]

There's some expensive HP (now Aligent) kit that does this. At each turn you get a small reflection and an expert can work out the lengths at which the turns occur. The you start sniffing around the valid arcs on the surface for the high energy pulse it puts out.

[wanderer]

First you use pulse echo equipment to give an initial location. Used to be a high voltage wheatstone bridge before the advent of pulse echo.

If the cable is, say, 500m long, then at least that way you're looking in the right area; then connect a high voltage ignitron and capacitor, and hope that the nature of the fault gives a decent if sometimes rather faint audible discharge. The ignitron discharges about every 10 secs, and you soon develop an ear for the characteristic 'thump'. Very distinctive once you get attuned to it.

If you happen to see two or three guys with their lugs pressed to one end of pick axe handles resting on the ground, or holding various types of listening aids, that's probably what they're listening for (if it's not a water leak)! One or two manufacturers have listening devices that detect the electromagnetic field associated with the discharge pulse as well as a ground microphone, so you actually get a flashing light that tells you when you *should* be hearing the 'thump'. Personally, I always preferred laying prostrate with an ear pressed to the ground.

In this particular case, luck played a part. The cable had been pulled up close to the surface and I could hear the discharge from 10 yards away, right on the corner of the non-existant guard house!

Oh, and the field mushrooms collected at 4a.m. from a near-by paddock went down a treat. :-)

[Andy Hall]

OK. This sounds akin to a time domain reflectometer (TDR) used on data networks.

I can't completely picture this. I had always understood an ignitron to be a specialised form of triggerable mercury vapour rectifier (or perhaps solid state) which can either be used as a continuous rectifier or to discharge a large capacitor. Presumably it's the second application here. So do you charge up th C with something first? What would that be? DC derived from the supply side or a special tester? Then you're discharging the C into the section of cable with the fault? Presumably though you have to disconnect all downstream equipment and then presumably the technique relies on the breakdown of insulation etc at the point of damage.

In a wet and muddy field in the middle of the night..... You must be very dedicated. :-)

OK, so now having found the cable, and I think you said that it was an old unarmoured lead one, how do you repair it? This presumably involves spicing a new section in? How is that done and with what? I guess that this type of cable is no longer used. What sort of diameter is it and how is it insulated?

Sorry for all the questions, but enquiring minds need to know. :-)

Well deserved. That was the least that the farmer could contribute

.andy

To email, substitute .nospam with .gl

[Bill]

In message , wanderer writes

I was working on a golf course some years back where one of the tent erectors found an 11KV cable, it should have been buried a couple of metres down but due to landscaping of the course it was only just below the surface. He survived with a bit of ringing in his ears but it could have been a lot worse.

[tony sayer]

In article , Bill writes

There are usually warnings on maps produced by the utilities warning that depths may have changed, owing to extra soil being added or taken away!....

[Bill]

In message , tony sayer writes

Hi Tony, unfortunately in this case the golf course had been quite heavily re developed and even if any one had checked maps it could have been difficult to relate them to what was now there. I believe there was a "bit of a to do " about it though!

[wanderer]

Back in the late 60's early 70's when the British Army of the Rhine was being brought back to this country, married quarters were being stuck up at an astonishing rate around the Garrison towns. One development had a budget of £1,200 for a two bedroom, £1,600 for a three bedroom house. Some bright spark decided that all the utilities could lay their stuff before any landscaping and levelling had been done. Cables there were (still are, I guess) at eight or nine feet deep, which puts gas and water at ten or eleven feet deep.

Oh, and then there was the end wall of a block of shops that lost a course of bricks over a 20 foot length. Or the garden wall that was quite deliberately moved over by about a foot using a JCB a few weeks after it had been built.

[wanderer]

Yes. Discharge of a capacitor, and the ignitron is solid state..

High voltage dc test set, either operated from a convenient low voltage supply, or an on-board generator. Normally up to about 10kv for the discharge, although the output from the test set could go up as high as

50-60kv.

Yup to both.

Depends on the extent of the damage, but usually, yes, two straight joints and 5 yards of cable.

Unarmoured was used predominantly in rural locations. Construction was three copper cores, paper insulation (perhaps 30-40 layers, I can't remember the detail now), the whole lot then held together with a further three or four layers of paper ('belted'), all the insulation impregnated with a mineral oil,and then sheathed in lead, with a bitumen impregnated jute overlay. Armoured cables were same construction but with steel wire armouring wrapped overall then more impreganted jute overlay. A quick description would be PILSWA. You can work it out for yourself! :-)

Cable size (at 11kv) could be anything from .025 sq.in c.s.a upto 300 sq.mm c.s.a, with overall diameter between about inch and a quarter upto

3 to 4 inches diameter, with capacities from a few hundred kva up to about 10 mva.

Modern cables tend to use cross-linked poly-ethylene for insulation.

Actually, I think the horse in the paddock had more to do with the mushrooms than did the farmer! :-)

[Hamie]

You put several thousand (10's? 100's?) volts down it in a big pulse & listen for the thump of the arcing...

Saw it done once in NZ (Palmerston North) when I worked for the MED one summer. A 33kV feeder (Underground) went bad & they had to find the fault. They showed me how it was done afterwards. Turned out the fault was only a 2m from the end so they only had to dig a small bit up & replace it.