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Topic Title: Impressive bang
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Created On: 15 August 2014 04:16 PM
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 15 August 2014 04:16 PM
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broadgage

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I was called to assist a neighbour concerned about "smoke from the electrics" in a small factory.
The somewhat dilapidated installation had suffered severe water damage, with ongoing arcing from metal clad switch fuses to other services.

Did not fancy touching any of that lot and felt justified in removing the cut out fuses to make safe.
On attempting to remove the second fuse, the cut out assembly came away from the wall and went bang.
The size of the flash and bang, and force with which the debris was thrown was very startling.
It was most fortunate that I was wearing at least somewhat suitable PPE consisting of a face shield, flame retardant overall coat, and leather gloves.
The service cable appeared to be old and perhaps damaged PILC and I suspect that the slight movement of the cable end finished it off.
The DNO agreed that under the circumstances, that removal of the cut out fuses was justified and that such dramatic consequences are very rare.
 15 August 2014 04:25 PM
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perspicacious

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537.1.4?

Regards

BOD
 15 August 2014 04:35 PM
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OMS

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Originally posted by: perspicacious

537.1.4?

Regards

BOD


LoL - behave, BoD - play nice

OMS

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Failure is always an option
 15 August 2014 05:13 PM
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sparkingchip

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537.1.4 doesn't say what to do if that is the switch which is arcing to the other services.

A friend of my parents when working for the Midlands Electricity Board some forty years ago cut using a pair of side cutters in a gloved hand the overhead cables going into a bungalow that was on fire to cries from the fire brigade of "you can do that" to which he reportedly said "just watch me" I believe the weapon of choice of the fire brigade was a long handled axe.

Andy

http://shop.bsigroup.com/Produ...pid=000000000000072776
 15 August 2014 09:48 PM
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impvan

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..long handled axe..


A client of mine does street lighting, and is often called to meetings between column and car... They too prefer the fibreglass-handled axe!
 16 August 2014 11:59 AM
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alancapon

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Originally posted by: broadgage
. . . On attempting to remove the second fuse, the cut out assembly came away from the wall and went bang.

The size of the flash and bang, and force with which the debris was thrown was very startling. . .

This is the risk of removing cutout fuses. It sounds like you were lucky and were in a fairly large area. Imagine the effect of a similar flashover in the confined area known as "the cupboard under the stairs".

. . . The service cable appeared to be old and perhaps damaged PILC and I suspect that the slight movement of the cable end finished it off. . .

The paper insulation on the cores could have been water damaged by the leaks you were talking about, it could have been a fixing screw from the cutout, or other cable damage.

. . . such dramatic consequences are very rare.

They are rare, but your risk assessment always needs to incorporate a "plan B" if it all starts going wrong. I suspect you were able to write your post due to your choice of adequate PPE.


Regards,

Alan.
 16 August 2014 01:11 PM
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UKPN

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"did not fancy touching any of that lot"
(although wearing FFV, gloves and FPs) He preferred to pull fuses (3PH)
on a service which "appeared to be old and perhaps damaged PILC"

Regards
 16 August 2014 04:20 PM
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slittle

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Originally posted by: alancapon

[
They are rare, but your risk assessment always needs to incorporate a "plan B" if it all starts going wrong.

Regards,



Alan.


Mine's RUN !

Stu
 16 August 2014 05:12 PM
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John Peckham

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I would not play around with PILC cables or cast iron cut outs unless I had my PPE Brown Trousers on.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 17 August 2014 12:24 PM
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broadgage

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Originally posted by: UKPN

"did not fancy touching any of that lot"

(although wearing FFV, gloves and FPs) He preferred to pull fuses (3PH)
on a service which "appeared to be old and perhaps damaged PILC"
Regards


I felt that removal of the cut out fuses was the lesser risk. Not only was the consumers installation very dilapidated, but the numerous tails and Henley blocks, dripping wet, would have remained live and at risk of blow up or earth leakage being sufficient to start a fire.

The cut out was a modern plastic unit, but the cable looked old and in doubtful condition. Had the cable not been disturbed by the failure of the fixings holding the cut out to the wall, then it is unlikely that it would have gone bang just then.

Certainly shows the wisdom of wearing PPE just in case though.

I still find the size of the flash and bang to be a bit surprising, the installation was very close to the DNO transformer, but it is only a small transformer fed by light weight 11KV overhead.

The DNO attended very promptly and renewed the cable, perhaps surprisingly the new service is overhead, this being quicker to install.
 17 August 2014 09:41 PM
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slittle

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The latest document I have to hand from UKPN suggests a maximum fault current of 18KA for a 3 phase 100A cutout.

That's plenty to go bang me thinks.

Most important thing at the end of the day is you're telling us about it.

I'll guess all the water ruined the "meter board" if there was one which made it easy for the fixings to pull out

Stu
 18 August 2014 12:43 AM
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alancapon

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Originally posted by: slittle
The latest document I have to hand from UKPN suggests a maximum fault current of 18KA for a 3 phase 100A cutout. . .

This figure is likely to be due to the current limiting effect of the cutout fuses. On the incoming side of the cutout, the value may be higher.

Regards,

Alan.
 18 August 2014 07:32 AM
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timothyarnold

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Originally posted by: alancapon

Originally posted by: slittle

The latest document I have to hand from UKPN suggests a maximum fault current of 18KA for a 3 phase 100A cutout. . .


This figure is likely to be due to the current limiting effect of the cutout fuses. On the incoming side of the cutout, the value may be higher.



Regards,



Alan.


18kA is sizeable and would have thought it was generic based on the distance from the supply transformers and earth return path?

For a HV/LV from a dedicated transformer you would need to be directly connected to the terminals of a 1MVA transformer to get 18kA. As the op suggested a small HV transformer (pole mount) I would say 300kVA max and therefore the fault current would be in the region of 7.5kA (assuming a number of factors)

Still pretty dangerous!
 18 August 2014 02:06 PM
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broadgage

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The supply was 150 amps, 3 phase.
Transformer capacity unknown but clearly limited as is fairly small and pole mounted.
No OCPD between the transformer terminals and the cut out, as far as I know. The supply was interrupted by the blowing of the HV fuses on the primary side of the transformer.

Not much else on the transformer, one house belonging to the factory owner, a few street lights and a sewage lifting station.
Most unusually, the house has a 3 phase service, but only at 40 amps per phase, suggesting perhaps that the DNO did not want to add any significant single phase load to the transformer.

I suspect that the DNO were alerted not only by our phone call, but also by some automatic alarm re the supply failure to the sewage lifting plant.
 18 August 2014 04:03 PM
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OMS

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Originally posted by: timothyarnold

Originally posted by: alancapon


Originally posted by: slittle

The latest document I have to hand from UKPN suggests a maximum fault current of 18KA for a 3 phase 100A cutout. . .



This figure is likely to be due to the current limiting effect of the cutout fuses. On the incoming side of the cutout, the value may be higher.

Regards,

Alan.


18kA is sizeable and would have thought it was generic based on the distance from the supply transformers and earth return path?

For a HV/LV from a dedicated transformer you would need to be directly connected to the terminals of a 1MVA transformer to get 18kA. As the op suggested a small HV transformer (pole mount) I would say 300kVA max and therefore the fault current would be in the region of 7.5kA (assuming a number of factors)

Still pretty dangerous!


A bolted fault on a 1MVA unit at circa 5% impedance would get you to about 28kA, Tim


I think the DNO value of 18kA Stu quoted above is based on a typical DNO 800KVA distribution transformer attenuated by a typical 100A TP&N service cable from the distributor

Regards

OMS

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Failure is always an option
 18 August 2014 11:50 PM
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alancapon

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I agree with OMS. The figure that Stu has quoted comes from the Electricity Association Design Guides, which give maximum fault currents for installations protected by a cutout fuse of 100A or less, for single or three-phase LV installations.

The relevant standards here (published by the Electricity Networks Association) are P25/1 for single-phase LV supplies and P26 for three-phase LV supplies.

With supplies at LV and cutout fuses in excess of 100A per phase, the PSCC of the supply would need to be determined on a case by case basis, and the values may be significantly higher than those outlined in the standards I quoted.


Regards,

Alan.
 19 August 2014 12:22 AM
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alancapon

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Originally posted by: broadgage
The supply was 150 amps, 3 phase. . .

Ok, with 150s in the cutout, I would expect a minimum of 200s in the transformer LV. The smallest three-phase transformer we would put 200s in, would be a 100kVA unit. On an overhead supply, this would be protected by 10A slow-blow expulsion fuses.

In order to "go through" the LV fuses and take out the HV fuses, the fault current would have to be significant. I would need the transformer details and the HV and LV fuse sizes to carry out the calculations, but I would suggest that 15kA to 20kA would not be an unreasonable guess.

Regards,

Alan.
 19 August 2014 08:18 PM
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ebee

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Nice bit of info from our contributors.

I like the title "Impressive bang".
I think we`ve all dealt with un-Impressive bangs before (really not bangs just thwacks!) thankfully I`ve never witnessed an Impressive bang and I don`t want to.

It makes me cringe when a know all builder moves the PILC to get the job progressing. I tells `em not to touch it then move very far away `cos I know the will do no matter what I say. Pilchards.

(I`m not putting the OP in that category by the way)

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 19 August 2014 08:22 PM
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OMS

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As the old saying goes, Ebee - a good scare is worth much more than good advice - assuming the pilchard survives it, he won't be too keen to move another PILC

Regards

OMS

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Failure is always an option
 19 August 2014 08:42 PM
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ebee

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
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