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Topic Title: Two supplies in one building
Topic Summary: But from another building
Created On: 12 June 2013 05:36 PM
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 12 June 2013 05:36 PM
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mrl200

Posts: 60
Joined: 26 January 2006

Looking around a companies buildings and noticed that they have a 125A TP supply coming from building 'a' into building 'b' serving a test bench. Both buildings have their own service head and as it turns out come from different substations.

It just doesn't seem right.

Just wondered what peoples thoughts were on this?
 12 June 2013 05:53 PM
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AJJewsbury

Posts: 10964
Joined: 13 August 2003

Provided they're bonded together with a suitable size conductor and suitably labelled, I don't see a major problem. Some DNOs might not like it in terms of being able to cut the supply for the fire brigade in an emergency or such like, but that's not a BS 7671 issue.
- Andy.
 12 June 2013 05:57 PM
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perspicacious

Posts: 7032
Joined: 18 April 2006

Below is a typical DNO requirement:

7.7.2 HV/LV Supplies to Same Customer or Building
Different HV and LV supplies to the same customer or building are a cause of concern due
to the possibility of interconnecting different earthing systems. For example, if a new HV/LV
supply is introduced into a building that already has a UK Power Networks LV supply, the
earthing system of the latter may be exposed to unacceptable fault and circulating current.
To address this situation, site specific design considerations are necessary.
If a customer supplied at HV provides LV supplies into the same building that already has LV
supplies from the distribution network, the customer shall ensure that an alternative earthing
system is not introduced within the building.
Customers are not permitted to use a PME system on their own internal LV network (ESQC
Regulation 8 (4)). However a customer may operate a TN-S network with deliberate
interconnection with the UK Power Networks' earthing system provided the following criteria
are satisfied:
 The connection between the two systems shall be sized appropriately (minimum 70mm2)
to handle diverted fault currents arising from faults on the customer's system.
 The customer shall carry out specific studies to ensure the rise of earth potential on any
part of the system is maintained at safe levels under all circumstances and that there is
no possibility of danger arising to UKPN's existing customers.
 A removable link shall be provided to separate the two systems for maintenance and
testing purposes and a warning label installed as detailed in Table 3. Note: There will be
an electric shock risk between the two earthing systems when the link is removed.
If these criteria cannot be satisfied the earthing systems shall be segregated by not less than
2m.

Regards

BOD
 12 June 2013 06:00 PM
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OMS

Posts: 18919
Joined: 23 March 2004

Perhaps in addition to both earthing and bonding you might also want to consider phasing, sequencing and rotation - particularly if there is a potential that the "secondary" supply will need to become the "primary" in certain fault circumstances. More an issue for the DNO than BS 7671 (as Andy indicated).

Put in context - many bigger buildings have multiple supplies from any number of sources (CHP, Generator, PV, UPS, DNO x N number of supplies etc etc). None of this presents a problem to BS 7671 compliance - but does need a bit of managing.

Regards

OMS

-------------------------
Failure is always an option
 12 June 2013 06:05 PM
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mrl200

Posts: 60
Joined: 26 January 2006

Thanks for the replies

Both buildings have different earthing 'b' is TN-S 200A supply and 'a' is PME 400A.

I could see that they should be bonded together but would this causes more problems?
 12 June 2013 10:32 PM
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AJJewsbury

Posts: 10964
Joined: 13 August 2003

Traditionally different buildings fed from a whole variety of earthing systems and different substations were interconnected via bonding to metallic water & gas networks - somehow we seemed to survive.

That said, there have been a few problems with interconnections between PME supplies & diverted N currents causing interference and potentially overheating some DNO conductors.
- Andy.
 13 June 2013 08:56 AM
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Parsley

Posts: 884
Joined: 04 November 2004

I'm sure UKPN will be able to advise on this one.

Regards
 13 June 2013 09:47 AM
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OMS

Posts: 18919
Joined: 23 March 2004

Originally posted by: Parsley

I'm sure UKPN will be able to advise on this one.



Regards


Just TT the lot, perhaps !! -

Regards

OMS

-------------------------
Failure is always an option
 13 June 2013 06:36 PM
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mrl200

Posts: 60
Joined: 26 January 2006

Would adding stakes suffice as opposed to changing the system type?
 13 June 2013 06:49 PM
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perspicacious

Posts: 7032
Joined: 18 April 2006

What is the lowest value you've ever measured for an electrode? The lowest I've measured is 4 Ohms to give you a clue

So, if there is say only 20 A of diverted neutral current, 20 A times 4 Ohms gives you how many Volts?

Regards

BOD
 13 June 2013 07:20 PM
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Avatar for OMS.
OMS

Posts: 18919
Joined: 23 March 2004

Originally posted by: mrl200

Would adding stakes suffice as opposed to changing the system type?


Not unless you could get the Ra down to expected levels for Ze on a TN system.

My apologies if I confused the issue with that post - it was more a tounge in cheek response to the probable advice from UKPN rather than any attempt to suggest that adding electrodes would be of any benefit.

Regards

OMS

-------------------------
Failure is always an option
 15 June 2013 08:12 PM
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mrl200

Posts: 60
Joined: 26 January 2006

Just had a look at the building with PME, I thinks its almost best reading i've seen 0.03 Ohm and on some tests at head position 14.4Ka x2 for PSSC.

So with those potential fault levels would be better not to bond the two and ensure they don't interconnect which would be hard due to the structural steel or should it be bonded because of those fault levels? Or should it be a phone call to UKPN to ask the question?
 15 June 2013 08:30 PM
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perspicacious

Posts: 7032
Joined: 18 April 2006

However a customer may operate a TN-S network with deliberate
interconnection with the UK Power Networks' earthing system provided the following criteria
are satisfied:
 The connection between the two systems shall be sized appropriately (minimum 70mm2)
to handle diverted fault currents arising from faults on the customer's system.
 The customer shall carry out specific studies to ensure the rise of earth potential on any
part of the system is maintained at safe levels under all circumstances and that there is
no possibility of danger arising to UKPN's existing customers.
 A removable link shall be provided to separate the two systems for maintenance and
testing purposes and a warning label installed as detailed in Table 3. Note: There will be
an electric shock risk between the two earthing systems when the link is removed.
If these criteria cannot be satisfied the earthing systems shall be segregated by not less than
2m.
Statistics

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