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 Topic Title: Bonding for two electrical supplies Topic Summary: Created On: 07 March 2013 09:31 AM Status: Read Only Linear : Threading : Single : Branch
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 07 March 2013 09:31 AM misterben Posts: 438 Joined: 11 June 2007 Hi , I have a job where in one premises there are 2 intake positions from the street ( both TN-S), however the gas and water supply are common to all areas of the building. It is basically split into offices , it got me thinking which earth terminal do I bond the services to? When you isolate one supply the other is still on and can introduce a potential into the pipework? Am I over complicating it and should I just bond to one earth terminal? your thoughts appreciated regards Misterben 07 March 2013 09:54 AM tattyinengland Posts: 996 Joined: 23 November 2006 When I've come accross this before, The instruction from the supply authority has been for the bonding conductors to sized to the largest supply of the two. The two systems need to be linked together with an individual earth cable and the bonding between the two labelled as such. So if you have one 100A TPN supply and one 800A TPN supply the bonding conductors must be sized for the larger (800A) supply. I'm sure some one will come up with the mathematical reasons why. 07 March 2013 09:56 AM Parsley Posts: 1359 Joined: 04 November 2004 I believe you should bond from both services. That way if one service is removed the bonding is still in place. Regards 07 March 2013 09:57 AM AJJewsbury Posts: 16215 Joined: 13 August 2003 When you isolate one supply the other is still on and can introduce a potential into the pipework? Yup, but that's the case wherever piped services are shared between different customers - e.g. terrace of houses or block of flats. - Andy. 07 March 2013 10:00 AM Parsley Posts: 1359 Joined: 04 November 2004 Originally posted by: tattyinengland When I've come accross this before, The instruction from the supply authority has been for the bonding conductors to sized to the largest supply of the two. The two systems need to be linked together with an individual earth cable and the bonding between the two labelled as such. So if you have one 100A TPN supply and one 800A TPN supply the bonding conductors must be sized for the larger (800A) supply. I'm sure some one will come up with the mathematical reasons why. Normal operational diverted neutral currents and possible loss of PEN on the larger supply could result in the bonding conductors and bonded parts etc carrying the larger installations neutral current if the installations is PME. Regards 07 March 2013 10:47 AM Parsley Posts: 1359 Joined: 04 November 2004 I was about to respond to JCM's post but its gone! 07 March 2013 12:54 PM jcm256 Posts: 2303 Joined: 01 April 2006 Here it is again, sorry; I took it off because original question was TNS supplies, the post that I had put on referred to PME. No time this morning to think, did not want to confuse. Regards This extract is taken from one of the network suppliers blog. PME YES if there is only one service into the building and equipotential bonding complies with the current edition of BS7671 Multiple services - NO - may result in high EMF due to neutral currents returning via the steel frame. See 1.3 Industrial and Commercial supplies Where two or more customers occupy the same building refer to Section 1.3.1.3 Multi occupancy buildings. Note that if two or more services were to be provided there is a danger that neutral current from the electrically remote one service(s) may return to the substation via the service cable electrically nearest the substation. This is due to the interconnection of the service neutrals by the PME equipotential bonding and other conductive routes such as metallic pipe-work. The passage of neutral current may cause high electro magnetic fields (emf) which may cause electronic equipment to malfunction. More importantly, the PME bonding and metallic pipes etc. are not designed to be load carrying and may represent a fire risk. This does not prelude the use of two incoming cables supplied from the same substation to a single metering room as per Section 1.3.1.8 Arrangement D. "Second service direct from substation". In this case both services have the same size neutral and are of a similar length. The cut-outs are positioned within the same room and the neutrals bonded together with a large conductor capable of carrying full load current. (185mm2 and 300mm2 Wavecon both have 116mm2 copper neutrals and the bonding conductor is 120mm2) 07 March 2013 01:13 PM Parsley Posts: 1359 Joined: 04 November 2004 Originally posted by: jcm256 Here it is again, sorry; I took it off because original question was TNS supplies, the post that I had put on referred to PME. No time this morning to think, did not want to confuse. Regards This extract is taken from one of the network suppliers blog. PME YES if there is only one service into the building and equipotential bonding complies with the current edition of BS7671 Multiple services - NO - may result in high EMF due to neutral currents returning via the steel frame. See 1.3 Industrial and Commercial supplies Where two or more customers occupy the same building refer to Section 1.3.1.3 Multi occupancy buildings. Note that if two or more services were to be provided there is a danger that neutral current from the electrically remote one service(s) may return to the substation via the service cable electrically nearest the substation. This is due to the interconnection of the service neutrals by the PME equipotential bonding and other conductive routes such as metallic pipe-work. The passage of neutral current may cause high electro magnetic fields (emf) which may cause electronic equipment to malfunction. More importantly, the PME bonding and metallic pipes etc. are not designed to be load carrying and may represent a fire risk. This does not prelude the use of two incoming cables supplied from the same substation to a single metering room as per Section 1.3.1.8 Arrangement D. "Second service direct from substation". In this case both services have the same size neutral and are of a similar length. The cut-outs are positioned within the same room and the neutrals bonded together with a large conductor capable of carrying full load current. (185mm2 and 300mm2 Wavecon both have 116mm2 copper neutrals and the bonding conductor is 120mm2) The above looks very similar to UKPN's latest guidance, not very useful in existing buildings that have supplies in different areas of the building that were once TN-S and are now TNC-S PME. Regards Regards 07 March 2013 09:36 PM UKPN Posts: 673 Joined: 17 January 2012 this is TNS, only one set of bonds required Regards 07 March 2013 09:43 PM UKPN Posts: 673 Joined: 17 January 2012 for PME the bonding size is related to the highest service ie 35sqmm or less = 10sqmm over 95sqmm not more than 150sqmm = 35sqmm so 35sqmm Regards 07 March 2013 09:48 PM Parsley Posts: 1359 Joined: 04 November 2004 Doesn't your new earthing manual state existing TNS is to be treated as TNCS PME anyway? 07 March 2013 10:00 PM UKPN Posts: 673 Joined: 17 January 2012 probably!! but it would be CNE/PME at the mains joint, not the service position. the jointer would "PME" at the cutout/neutral. Regards 08 March 2013 10:32 AM misterben Posts: 438 Joined: 11 June 2007 Hi, The largest supply is 3 phase 100 amp TNS feeding 3 different DB,s. The other is 60amp TNS. Although I did raise the question that the new supply looks like a TNCS, but they ran an unprotected 10mm earth alongside the hockey stick straight into an earth bar! I have bonded to the gas and water in 10mm from the largest supply but also left in place the existing 6mm to the water for good measure. It seems this is ok from your responses. regards Misterben 08 March 2013 11:36 AM Parsley Posts: 1359 Joined: 04 November 2004 I've just dug out an old London Electricity Earthing Manual, it implies that where two or more services are installed in a single premises to prevent voltages occurring under normal conditions the earthing systems of the two or more supplies shall provide the same means of earthing. It then goes on to that if the supplies are TNS or TNCS appropriate cross bonding shall be installed between the two services. And a label stating that the building has two or more sources of supply fitted at the service cut-outs. It might be worth trying to speak to current DNO/supplier to see what they now require, but they will probably just tell you it's your responsibility to assess whether you should use their earth facility.
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