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Topic Title: Proximity of alternative supply incomer to main incomer
Topic Summary: Can you help me find any guidance
Created On: 04 October 2015 07:24 PM
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 04 October 2015 07:24 PM
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Zs

Posts: 3814
Joined: 20 July 2006

Hello,

With reference to the title I'm searching for guidance relating to the proximity of an alternative supply to the main incoming supply of a building.

I'm not finding something that I expect to be there but you know how it is when you get too tired? I might be making things up in my head. Would you mind helping my sanity? Don't go to the lengths of looking anything up but if this is on the top of your head I'd appreciate help.

A building, which is a block of flats about 9 floors high, has a main incomer and an alternative supply. I'll describe it for you:

PME, labelled as such, 4 year's old.

Main incomer head, Lucy (no subsequent isolator unless you count BS 1361 fuses)
PME with a separate green and yellow going into the ground in a cable tie style.
Ryefield with the fuses.
Fuses (1361) serving various bits including flats and landlord areas, lifts, AOV's and all the stuff you'd expect in a well designed modern block of flats. I only saw one and the fuses were not labelled so I assume a touch but with a degree of confidence.

I did several this week but one of them has the alternative supply right next to it. About two feet next to it. The rest of them are at a distance. The alternative supply looks like this:

Incomer
Memshield switched fuse @ 63A
PME (assumed, not seen) but no strapped-on green and yellow into the ground.
Changeover switch with lights stating both supplies healthy.
Changeover switches in the riser cupboards for each of the landlord supplies which are normally served from the main incomer.

I don't know how much of the infrastructure would be served from the alternative supply but no question to it serving the lifts and the AOVs.

6 blocks of flats in total, all served by individual set-ups as described above but each served by the same water pump room and as such, shared bonding.

So, what is on my mind is this:

They probably come from the same transformer. I don't know that by the way but I bet you

Given that in a densely populated area, a PME is pretty much a catch-all for earthing even if at a distance, is there any more of an association between an incomer at 15 meters distance or so from an incomer at two feet?

Why am I asking? because twice in the past fortnight I have heard this mentioned as big and bad by some of you, person to person.

Where I stand? If it is connected to a PME earth or if the fault path has access to a PME earth, then it is a PME earth. If the bonding between the blocks of flats and their alternative supplies is linked then the earthing arrangement must be considered as common and all that relates to it must apply.

I am not inclined to comment other than to express having noticed the differences between five blocks and this one.

Can you guide me to anything which expresses the two mentions from other engineers in a week? Is this close proximity, given the rest of the apartment complex, so bad?

I forbid you to spend time on research because I can do that too, when I'm not gagging for some time off, but if this is your thing, then please do tell. Thank you.

Zs
 04 October 2015 07:34 PM
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sparkingchip

Posts: 9953
Joined: 18 January 2003

Something about a green and yellow earth conductor with blue identification markings on each end comes to mind.

Andy
 04 October 2015 07:39 PM
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Zs

Posts: 3814
Joined: 20 July 2006

Curious Andy, you been reading my diary? cos there was one of those on a DB, but seeing as I don't keep a diary and I didn't tell you....

Tell more please oh guru.

Zs
 04 October 2015 08:49 PM
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Zoomup

Posts: 3265
Joined: 20 February 2014

Hello Zs, parts J and K may be of interest.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/82784/GuidElectSafety_Quality.pdf

Z.
 04 October 2015 08:49 PM
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sparkingchip

Posts: 9953
Joined: 18 January 2003

Being an anorak when people post links on this forum to DNO engineering documents and the like I skim through the pdf's that end up on my phone and tablet, then odd things stick in my head.

Reading through the original post it comes to mind that linking these heads with PME earthing will allow a current to flow and if you know that a current will flow in an earth conductor under these circumstances you should identify both ends of the green / yellow conductor with blue sleeving or the like as you know it is carrying a neutral current although it is not the neutral conductor as sparks will fly if you try to disconnect it along with there being a possibly a risk of injury or death.


At least that's how I remember it!

Andy
 04 October 2015 09:32 PM
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Parsley

Posts: 1352
Joined: 04 November 2004

http://library.ukpowernetworks...n+Earthing+Design.pdf

Page 29, as per Andy's comments.

Regards
 04 October 2015 09:37 PM
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sparkingchip

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PEN conductor. A conductor combining the functions of both a protective conductor and neutral conductor.

514.4.3 PEN conductor
A PEN conductor shall, when insulated, be marked by one of the following methods:
(i) Green -and-yellow throughout its length with, in addition, blue markings at the terminations.
(ii) Blue throughout its length, in addition, green-and-yellow markings at the terminations.


Presumably if the normal and alternative supplies are both PME then a earth conductor linking their earth terminals would become a PEN as it will carry part of the neutral current.

Or am I over thinking this?

Andy
 04 October 2015 10:05 PM
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sparkingchip

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If one is PME and the other is TT then it could be more interesting as there is then possibly a breach of the electrical environment being the equipotential zone.

Andy
 04 October 2015 11:21 PM
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mapj1

Posts: 9346
Joined: 22 July 2004

At one time second supplies were made TT for exactly that reason.
Equally, it may satisfy the word of the rules, but probably not the intention, to have main and second supplies from the same TX, as then the only fault you are mitigating is damage to the cable after the split, and maybe not even that if the substation trips off

-------------------------
regards Mike
 04 October 2015 11:51 PM
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sparkingchip

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"Given that in a densely populated area, a PME is pretty much a catch-all for earthing"

Even if it appears to be TNS then it should be treated as TNCS (PME) unless it is off a dedicated transformer as it is considered that TNS does not really exist anymore.

Andy
 05 October 2015 08:23 AM
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sparkingchip

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There has been a forum discussion that I remember as going into TT back up supplies in some depth.

Andy
 05 October 2015 08:51 AM
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mapj1

Posts: 9346
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was it This one
you were thinking of ?
Where user Nad, asked about the wisdom of RCDs on emergency supplies ..

-------------------------
regards Mike
 05 October 2015 01:16 PM
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AJJewsbury

Posts: 15811
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Is the worry about PME & return currents or the closeness of the two intakes with the risk that an "event" that makes one unserviceable (e.g. fire due to loose connection etc) risks taking out the other in quick succession - so missing some of the point of having a 2nd supply in the first place.

Are the DNOs still supporting the idea of two supplies provide some resilience? (rather than stating that a HV fault will likely take out both simultaneously and suggesting you invest in a generator)

- Andy.

Edited: 05 October 2015 at 01:45 PM by AJJewsbury
 05 October 2015 02:15 PM
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burn

Posts: 239
Joined: 06 June 2003

BS8519 (2010) requires the two supplies and switchgear be separated by a 2 hour fire resistant barrier. The outgoing circuits from them have to take diverse routes (through separate fire compartments in the building, to fire fighting lifts, AOVs etc.) in addition to the earthing and bonding discussed above.

burn
 05 October 2015 05:13 PM
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Zs

Posts: 3814
Joined: 20 July 2006

Burn, you lovely man, I bet that's it. Thank you.

I too have been rambling through the current flow stuff and, as I hinted earlier, I just can't get excited about it because of the TNCSness of the installation.

I like the DNO stuff too but will have a closer look at that.

But yes, it does the fireman's lift and the AOVs. Seems like a glimpse of the blindingly obvious to me now. I have a copy of that right here so I shall digest it and make reference to it.

Thank you.

Zs
 05 October 2015 09:04 PM
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Parsley

Posts: 1352
Joined: 04 November 2004

Originally posted by: AJJewsbury

Is the worry about PME & return currents or the closeness of the two intakes with the risk that an "event" that makes one unserviceable (e.g. fire due to loose connection etc) risks taking out the other in quick succession - so missing some of the point of having a 2nd supply in the first place.



Are the DNOs still supporting the idea of two supplies provide some resilience? (rather than stating that a HV fault will likely take out both simultaneously and suggesting you invest in a generator)



- Andy.

Ukpn won't provide fire fighting supplies anymore. They quote being unable to meet the requirements of bs9999 in their supplies to multi occupancy publication.

Regards
 05 October 2015 09:34 PM
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sparkingchip

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Fire-fighting and other Standby Connections BS 9999, the code of practice for fire safety in the design, management and use of buildings, requires certain high rise buildings to have dedicated lifts and stairs for use by fire fighters. A second standby electrical supply must be provided for such facilities. This standby supply should be provided by a standby generator that is owned, operated and maintained by the owner of the building. WPD should not provide fire-fighting or standby connections to multi-occupancy buildings as it cannot guarantee that these connections will be available when required. WPD's position aligns with the guidance in ENA Engineering Recommendation G87.

Western Power

Andy
 05 October 2015 10:20 PM
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Parsley

Posts: 1352
Joined: 04 November 2004

I think flats with levels above 18m require one or more fire fighting shafts/lifts.

Regards
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