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Topic Title: Multiple incoming supplies
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Created On: 01 April 2013 02:10 PM
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 01 April 2013 02:10 PM
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smoke1811

Posts: 18
Joined: 18 December 2010

Hello,

I have searched the forum for similar subjects to this but have not managed to uncover anything. I wonder if you can help/advise ?

I am looking into wiring a new extension to a commercial retail building. There are two incoming supplies on the site, both 100a three phase. Historically the site may have been two concerns but it is now one business.

The first of the supplies (internal supply) is in the front of the main building adjacent to the main road. This supply feeds the original main building

The second supply (external supply) is in a large external cabinet. This supply feeds two small external businesses. This supply also feeds an extension to the main original building at the rear (extension no. 1)

The main building is supplied for the internal supply, extension no.1 is supplied from the external supply so both supplies are being used in the same building (there is a defined line where the change over).

The brand new extension no. 2 is to be joined onto the existing extension (no.1)

I originally intended to use the external supply to feed the new extension (no. 2), however, I noticed that the incoming cable was warm. I therefore decided to monitor both the supplies for a week (this was a peak time of year, cold, busy etc. so a good indicator). The results showed that the external supply is running at peak times between 65-75a (L1 & L3) and 85-90a (L2). The load calculations for the new extension (no. 2) have not yet been finalized but I believe them to be around 140 amps (total). It is therefore apparent that this supply does not have the spare capacity to handle the new extensions (no. 2) load.

The monitoring indicates that the internal supply had similar results to the external supply, however the aquatics department will be moving from the original building to the new extension therefore if this supply was used it would only be required to provide an additional load of around 60a which would be acceptable.

My question is this, can the two supplies be crossed over ? I feel there may be safety implications in doing this. The internal supply would supply the original building and the new extension (no. 2) and the external supply would serve extension no. 1. Extension no. 1 sits in the middle of the original building and the new extension.

Thank you
 01 April 2013 02:54 PM
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Zs

Posts: 2899
Joined: 20 July 2006

smoke, do you mean swapping part of one supply over? I don't see much benefit in swapping the supplied areas over but may have mis understood yours.

Either way, I think yes, you can.

I would be inclined to keep the areas supplied, and indeed the phases, well separated myself, but that is do-able and old fashioned. It would require something of a catasrophe not covered by big green for there to be a phase/touching/fault issue. However, the next electrician may not be very bright so we must allow for Him and his deficiencies.

Your use of the word aquatics sends a little message though. Are we talking about a livestock area etc.? Is this a pet shop? That may change the playing field somewhat.

Zs
 01 April 2013 03:23 PM
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smoke1811

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Thanks for the reply Zs. My post is a little confusing. All dis. boards are three phase and we will be installing a three phase dis. board in the new extension, it really boils down to which of the incoming supplies we will take the power from.

There are two incoming DNO supplies. one internal and one external. both supplies are currently used in the main building and there is a defined line between them (where the old building joins the existing extension). If we supply the new extension from the external supply then there will still only be one boundary between the two supplies, however, there is not enough capacity at that supply. We therefore need to supply from the internal supply, however this will create another boundary between the two supplies.

So as you walk down the building it will be as follows:
Main building - Internal supply
Extension - External supply
New extension - Internal supply

Each area has its own dis boards. I am unsure if it would be acceptable to cross over like this.. would it meet regulations, if there were a fire would it be clear for the services where to isolate the power and potentially create confusion/danger ? The premises are a garden centre (fish and a few rabbits)
 01 April 2013 03:35 PM
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Zs

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So, the way I see it is that the new bit will be a new three phase circuit coming from an existing distribution board?

I think you should wait until the real brains get back from their easter breaks (we're short of Andy Jewsbury and OMS et al today) in case they have some essential pearls of wisdom, but why not? No different from a shed down the garden. I have not looked at the special locations section, might be worth a read through though?

As for the firemens' isolation; Massive alternative supply warning labels and staff training, with as much clear switching as you can include? In truth, they already have that issue.

Sounds like a lovely job.

Zs
 01 April 2013 03:49 PM
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smoke1811

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Thanks Zs, agreed that they already have the issue of different supplies in the same building, although I want to make sure that what we do is correct. I don't think an aquarium would class as a special location, IP ratings would need to be adhered to though.
Regards
 01 April 2013 04:30 PM
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Zs

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Looks like its just you and me this afternoon smoke (one of my favourite films as it happens...Smoke). Now look what you've done, I've got GN7 off the shelf.

I think a garden ctr would fall into the special locations bracket, especially if there is risk of immersion in water. Section 13.6 of GN7 (my copy is 17th first version i.e big red and I don't think it has been updated to green yet?).

Now, as for livestock? I'm going to reserve judgement on the fish but I'll go as far as saying the rabbits definitely qualify for special treatment in accordance with Agricultural and Horticultural locations. I reckon the fish do too but they don't have legs. But then they're in water or wet.....and probably very susceptible to shock...I'm in touch voltage mode in my head. Am I rambling?

Zs
 01 April 2013 05:06 PM
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Jaymack

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Originally posted by: smoke1811
I have searched the forum for similar subjects to this but have not managed to uncover anything.

It's not in the knowing, it's knowing where to look! (A DVD with a search facility is an easy option for BS 7671).

The supplier will probably only allow 1 point of supply in this case, but get it from the horse's mouth and in writing, if to the contrary. ................... If the former, this will remove the concerns for safety and earthing/bonding .

Regards
 01 April 2013 05:36 PM
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rogersmith7671

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Quote;
Historically the site may have been two concerns but it is now one business.

This happens a lot there is usually no problem if final arrangements comply with current Reg's (iee HWAW Ect.), but as above check with supplier first.

Regards
 01 April 2013 05:52 PM
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paulskyrme

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I "KNOW" my local DNO would not be happy with 2 metered supplies feeding one building.
I've just been through this with them.
85m of 120mm sq 4C &, an IP55 250A panel board later and it's sorted!

Zs, the 400A was another job on the same site...
 01 April 2013 08:29 PM
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slittle

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We've got a few like that with two supplies and one building, in fact the last one was at the DNO's suggestion to reduce the standing charges.

I don't see a problem as long as everything is clearly labelled as any decent sparks should be able to understand what has been done (if they can't they shouldn't be doing it)

I'm with Zs on the section 7 bits as livestock would present the same risks as it would on a farm. Rats etc getting in to eat discarded feed and therefore cables. Not so sure about the fish side though as loss of supply could kill them ??

Stu
 01 April 2013 08:35 PM
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paulskyrme

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Stu,
Funny how DNO's differ!
Once again not a level playing field...
 01 April 2013 09:00 PM
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slittle

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I think even different engineers in the same DNO differ Paul.

I can think of another site we look after with three supplies in the same building. one 500A. one 200A and a 100A one. In fact they all terminate in the same room and then split out with part of the building fed from the 500A supply and the other bit from the 100A supply. The 200A supply and the other bit of the 500A supply then disappear to feed other buildings.


Stu
 02 April 2013 09:49 AM
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Parsley

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Exert below is from UKPN's latest earthing manual.
The OP doesn't mention the earthing system type.


7.8.4
Multiple Services to a Single Customer or Building
Multiple CNE services to a single customer or building are not recommended due to the problems associated with neutral current diversion (refer to Appendix F) and with the isolation of the supplies. If there is no alternative to providing an additional service or an additional service has already been provided the following requirements (shown in Figure 13) shall be satisfied for PME earthing:

The services shall be from the same secondary substation and ideally from the same main.

The services shall be positioned in the same intake room/cabinet and have the same size neutral conductors.

The earth terminals of each service shall be bonded together using a copper conductor covered in green/yellow PVC with a blue marker at each end (to indicate that it's carrying current). The size of the bonding conductor shall be not less than that of the associated service neutral conductor. The bonding conductor shall have a warning label (BS 951) fitted to each end as detailed in Table 3.

A warning label (EDS 07-0009.119) shall be installed next to each cut-out as detailed in Table 3.

Each service to the building shall be treated as a separate supply within the customer's installation and the equipotential bonding to other services (gas, water etc) installed accordingly. This ensures that equipotential bonding is maintained if one of the services is removed in the future.
If the above requirements cannot be satisfied:

Only one earth terminal shall be provided to a customer or building from the network and all other services shall use a TT earth.
The electrical installations (including equipotential bonding) supplied from each service shall be entirely isolated from one another.


Regards
 02 April 2013 12:04 PM
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perspicacious

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"Only one earth terminal shall be provided to a customer or building from the network and all other services shall use a TT earth.
The electrical installations (including equipotential bonding) supplied from each service shall be entirely isolated from one another."


That'll be interesting with RCDs "upfront" and steel common framed buildings.........

Regards

BOD
 02 April 2013 12:31 PM
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OMS

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It will BoD - but that's the "alternative solution" - the more probable one is:

Each service to the building shall be treated as a separate supply within the customer's installation and the equipotential bonding to other services (gas, water etc) installed accordingly. This ensures that equipotential bonding is maintained if one of the services is removed in the future

Basically a "MET" at each service position, with a big chunk of copper linking those "MET's" under the consumers control - which may also avoid the insanity of physically bonding to a common pipe more than once

(technically not MET's - probably earth marshalling terminals would be a better description)

To put this in perspective, it's not uncommon for a building to have a second DNO supply for such things as secondaries to fire fighting lifts, sprinkler pumps or other safety services. I've been looking at a shed recently that has 2 x 11kV interconnected rings running around the external perimeter which feeds 6 groups of 2 x 1MVA transformers unloading into 6 No split intake 400V switchboards ranged around the shed internal perimeter- so effectively 6 (or even 12) supplies in the same shed (big shed)

Regards

OMS

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Failure is always an option
 02 April 2013 01:58 PM
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Parsley

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So in smokes case as there are supplies in different parts of the building, if the site was in UKPN territory only one service should use the DNO earthing facility. The same earthing manual also states that all networks even ones using SN-E cables should be assumed to be PME.

OMS, the way I read it is that UKPN require bonding from each intake to the gas, water etc seems OTT especially if the intakes will need to be in the same room if PME is going to be used in the future?

Regards
 02 April 2013 02:15 PM
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OMS

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Agreed Parsley - but if you've linked the MET's then effectively each intake is bonded to the relevant service anyway - it's just more sensible to do it that way.

Idealy you wamt one service in the OP's case - if only to reduce the standing charges paid and to negotiate a better tarriff from the MoP - that said, I don't see anything problematic with the OP's potential solution.

Regards

OMS

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Failure is always an option
 02 April 2013 04:54 PM
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Jaymack

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Originally posted by: OMS

I've been looking at a shed recently that has 2 x 11kV interconnected rings running around the external perimeter which feeds 6 groups of 2 x 1MVA transformers unloading into 6 No split intake 400V switchboards ranged around the shed internal perimeter- so effectively 6 (or even 12) supplies in the same shed (big shed)


To put this in perspective, This is a common setup for new installations in industry and commerce, with H.V./M.V. ring mains and directional relay protection etc., for security of supplies; but there's still only one point of supply by the network supplier.

Regards
 02 April 2013 05:24 PM
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OMS

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Originally posted by: Jaymack

Originally posted by: OMS



I've been looking at a shed recently that has 2 x 11kV interconnected rings running around the external perimeter which feeds 6 groups of 2 x 1MVA transformers unloading into 6 No split intake 400V switchboards ranged around the shed internal perimeter- so effectively 6 (or even 12) supplies in the same shed (big shed)




To put this in perspective, This is a common setup for new installations in industry and commerce, with H.V./M.V. ring mains and directional relay protection etc., for security of supplies; but there's still only one point of supply by the network supplier.



Regards


Actually, in this case there is more than one Jaymack - the complex is served by 4 HV supplies, sepeately metered and arising from 2 totally independent HV 11kV supplier side rings derived from independant upstream 33kV supplies.

There is a north and south HV switchroom, spit into an A and B supply in each switchroom - ie the four incomers via metering breakers

Other than that - bog basic stuff as you say - I mentioned it to emphasise that more than one supply in a building is common - in some (many) cases even endorsed by the DNO's

regards

OMS

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Failure is always an option
 02 April 2013 07:14 PM
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slittle

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I reckon I could get a few turkeys in that sized shed OMS... Can you arrange for it to be completed but empty from July to Christmas


Stu
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