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Topic Title: Unidentified HV cable
Topic Summary: Live HV cable tracing
Created On: 25 March 2011 03:17 PM
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 25 March 2011 03:17 PM
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vi6851

Posts: 4
Joined: 12 October 2001

I have a HV cable which comes onto and goes off the perimeter of one of our sites; it has been traced by a specialist for a considerable way off the site but neither of the ends can be found. The cable gang think it is live because it gives a strong trace on a CAT scanner. It's definitely not part of our own HV network as we have good records of cable routes and have replaced everything in the last 5 years. The local REC say its not theirs either. The problem is that the cable is crossing the site of a new building we are about to start, and it's in the way! Without knowing where it's fed from we can't isolate it; has anyone had a similar experience? I know HV cables can be 'spiked' to prove dead but how can they be spiked or traced if they are live and you don't know where the ends are? The cable must be old because it's PILC/SWA and is covered by cable tiles - when were these first and last used? The site dates from 1940 - could it be as old as this? All advice gratefully received!

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vi6851
 25 March 2011 03:30 PM
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seeker

Posts: 319
Joined: 10 March 2007

Originally posted by: vi6851

I have a HV cable which comes onto and goes off the perimeter of one of our sites; it has been traced by a specialist for a considerable way off the site but neither of the ends can be found. The cable gang think it is live because it gives a strong trace on a CAT scanner. It's definitely not part of our own HV network as we have good records of cable routes and have replaced everything in the last 5 years. The local REC say its not theirs either. The problem is that the cable is crossing the site of a new building we are about to start, and it's in the way! Without knowing where it's fed from we can't isolate it; has anyone had a similar experience? I know HV cables can be 'spiked' to prove dead but how can they be spiked or traced if they are live and you don't know where the ends are? The cable must be old because it's PILC/SWA and is covered by cable tiles - when were these first and last used? The site dates from 1940 - could it be as old as this? All advice gratefully received!


Without knowing where your site is it is difficult to comment. However I do recall from many years ago on a YEB consumers authorised persons course the tutor advising that if the cable could not be spiked or otherwise verified as dead then use a JCB and dont try to get off the machine until the cable is severed and the machine is moved away. He had a very dry wit!
As to your question how can they be spiked live - that is the whole point of spiking. If they are live they wont be for long once the spike goes in. Under no circumstances trust isolation from each end unless you can run your hand all the way along the cable without letting go even to go from one side of a wall to the other.
 25 March 2011 06:33 PM
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MickeyB

Posts: 181
Joined: 18 January 2003

I had an unidentified 'green' cable laid across a building site about 10 years ago.... it was in clay pipes and laid across the centre of the site at about 1m deep.
No records of who's it was..... not the local DNO or the local boat yard.

After much wrangling and discussion we had a ' big digger' dig it up and remove it.
Spiking was suggested, but a CAT scan found that it was dead..... so the builder stuck his digger through it.

I would first check with the owner of the land that no 'private' supplies have a 'way leave' or other agreements that may be in place....bit of due diligence etc...
If that check proves a 'negative' I would dig two pits, either side of the area you need to work in/ remove cable.
Spike the cable, leave the blade in and apply temporary earth cables whilst you cut the cable (with hydraulic cutter).
The spiking will trip the protection of the cable but I would apply some additional earth cables to make sure that the cable ends remain earthed down........ make the DNO's control room aware of the work as you'll probably get a phone call about 20minutes later from the owner of the cable.
 25 March 2011 08:25 PM
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dlane

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In my experience the spiking of an underground cable is the final procedure carried out to prove that the correct cable has been identified and isolated and is now safe to work on.

I have never used it as a procedure on a cable believed to be live with the intention of making it dead by tripping protection and I fail to see how using it in such a manner and applying extra earths can be classified as a safe working procedure for cutting and terminating the cable. The earths will not stop the fault potential rise if the cable were re-energised which could endanger people.

Are you 100% sure that this is an HV cable? How do you know it isn't an LV cable with a 50,000A fault level? Earthing out such a cable would produce quite a flashover. You could also find yourself facing a hefty fine if you disrubt a supply to another consumer etc.

I take it you are in the UK, if so I suggest you take a look at HSG47 which you can down load from here, the HSE would take a pretty dim view if you injured somebody directly or indirectly by spiking a cable that you thought was live.

You say the site is from 1940, is it an ex MOD site? They may be a source of information if it is. National Grid and Highways Agency may also have information if they have ever had any involvement over that land. What substations are near by or are there any railways close by?

AEMC make non contact HV testers that can identify if a cable is live, but its accuracy will depend upon the type of cable that it is. Cables that can't be cut into are traced by inducing a 33kHz or 8kHz signal into them depending how far you need to go.

Is it possible to build some sort of containment around the cable through the building footings and flooring etc? That would be a prefered method if the cable cannot be isolated to be redirected.

Finally, you say that a CAT scanner is telling you that the cable is live. If you are an HV site it could be that the CAT scanner is picking up a 50Hz signal from the site earth via the cable armour and what you actually have is a redundant cable but the only way you are going to prove it is to trace it.

Kind regards

Donald Lane
 25 March 2011 08:55 PM
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MickeyB

Posts: 181
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I accept that the use of a spiking 'blade' is not the preferred method of 'proving dead' a live cable. However, if the cable cannot be identified or traced to the source by hand (or other means) the use of a spiking 'blade' to prove dead is a legitimate method (with reference to a 'safe system of work').

The use of additional earth cables is to ensure that the cable ends remain tied to earth potential at all times when cutting of the cable takes place. An HV AP or SAP conducting the switching of a circuit that had 'tripped' would follow a very prescriptive procedure to confirm why the device had tripped before switching the supply back on. As I noted earlier, we used a 'big digger' to remove an unidentified cable...... it maybe not written in the text books but if you spike the cable ends first it may be your only option.....
 26 March 2011 05:42 AM
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sfchew

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A live cable can be traced and therefore eventually you can determine where it originates and ends.

In some cases surplus lengths of cables were deliberately laid for future usage.

Regards
Chris Chew
 26 March 2011 10:02 PM
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simongallagher

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Originally posted by: MickeyB


The use of additional earth cables is to ensure that the cable ends remain tied to earth potential at all times when cutting of the cable takes place. An HV AP or SAP conducting the switching of a circuit that had 'tripped' would follow a very prescriptive procedure to confirm why the device had tripped before switching the supply back on. As I noted earlier, we used a 'big digger' to remove an unidentified cable...... it maybe not written in the text books but if you spike the cable ends first it may be your only option.....


This is not always the case.

Sometimes when a fault cannot be located (faulty EFPI indications etc), do the DNO will 'split' the feeder and try it back in, either from control or an Authorised Person on site (from a CB or a fault making rated switch).

What part of the country are you in?

It's a difficult situation; let us know how you get on.

Simon
 26 March 2011 10:57 PM
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alancapon

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Some DNOs are using automatic sequences to restore supply to as many customers as quickly as possible. There are two algorithms used, one of which may result in a damaged cable section being re-energised on one or more occasions.

If the cable is LV rather than HV, it may burn for some time before fuses operate, again this will be dependant on the DNO area.

Regards,

Alan.
 28 March 2011 08:40 PM
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jointersmate

Posts: 61
Joined: 02 June 2008

I would not like to be pulling the string on the spike gun ! Good luck with this project
 30 March 2011 10:41 AM
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broadgage

Posts: 1279
Joined: 07 August 2007

Unless you are certain that this is an HV cable, I would suspect that it is LV.
Owners of HV cables normally know were they are, and private owners are not that common.
LV cables are often privatly owned, and were regularly installed without records being kept, especialy in wartime conditions.
The cable might even have been installed to feed some secret wartime facility.

Still be very careful though, LV can be as dangerous as HV due to the very high fault currents and the often crude protection.

It might be worth trying to trace the cable route from the air.
If it was laid only about 70 years ago, then traces of the trenching and backfilling might still be visable to a skilled observer.
Archaelogists have detected much older excavations than that by skilled examination of aerial photographs.
Vegetation will often grow differently above a cable route due to the disturbed soil.
 31 March 2011 12:22 PM
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pbarnfather

Posts: 33
Joined: 10 July 2008

I strongly recommend that you do not rely on spiking alone to prove the cable is dead. As others have pointed out:
- The protection may be marginal (especially if it is a long LV cable), resulting in prolonged arcing.
- You have no control over whether the cable is re-energised. If you blow some fuses, the first thing that will happen is that someone will try to fit new (possibly bigger) ones...

PILC/SWA cable could be anything over the last 10-80 years (at a guess). So not much clue there. Do the cable tiles have any identifying marks?

You need to try and trace the source if at all possible.
This is a very difficult problem to resolve safely.
 31 March 2011 09:04 PM
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alancapon

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Originally posted by: pbarnfather
I strongly recommend that you do not rely on spiking alone to prove the cable is dead. . .

I agree. I am not sure of the detailed regulations in the UK, but here the spiking or cutting of a live power cable (either low or high voltage) is a reportable event to the Health & Safety Inspectorate.

Regards,

Alan.
 04 April 2011 09:43 AM
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timothyboler

Posts: 230
Joined: 25 July 2008

Originally posted by: vi6851

The cable must be old because it's PILC/SWA and is covered by cable tiles - when were these first and last used?


Out of interest what's wrong with cable tiles. We still used them on our project. What do they say? Any markings?

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Everyone loves a fireman - but hates the fire inspector.
 04 April 2011 11:36 AM
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RobertBrown82

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Cable tiles is not something we normally lay anymore, but were ideal as it gave that added bit of armour protection. Nowadays is plastic ticker tape that says 'Danger Live Buried Electricity Cables' or something along those lines.

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 04 April 2011 09:19 PM
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jointersmate

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I wonder how this case has developed since the thread was started vi6851 about at all?
 05 April 2011 04:22 PM
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alanjhodgart

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Perhaps the spiking did not go well........lets hope not
 06 April 2011 08:25 AM
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timothyboler

Posts: 230
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Originally posted by: alanjhodgart

Perhaps the spiking did not go well........lets hope not


There's some photos of the excavation here :

http://www.snopes.com/photos/accident/gasmain.asp#photo

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Everyone loves a fireman - but hates the fire inspector.

Edited: 19 April 2011 at 06:01 AM by timothyboler
 06 April 2011 09:17 PM
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slittle

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Yep, that one was clearly LV

As Alan C often says they can burn for a while before the fuses operate !

Stu
 25 February 2013 11:27 PM
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PaulTusonInlec

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Joined: 25 February 2013

Just my 2p.

I noticed a couple of people talking about using a CAT to see whether a cable is live or dead.

The 50Hz signal that the cat picks up in power mode is just an electromagnetic signal.

Live loaded cables give off the signal however dead cables that run near a live cable can pick up the signal and live unloaded cables may not give a signal.

All in all it is completely unreliable as a method for identifying whether a cable is live or dead. It is what it says on the tin a Cable Avoiding Tool)

In the situation as described I would start off by connecting a more advanced cable route tracer to the line (probably via a ct clamp) and then track it back to its start point and then end point.

After switching off at both these points you could then do a proper ID to confirm that you where correct in order to safely spike.

I also believe in the hydraulic spiking system btw.

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Regards
Paul Tuson
Inlec Test Equipment Hire
IET » Energy » Unidentified HV cable

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