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

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.

I believe you should bond from both services. That way if one service is removed the bonding is still in place.

Regards

Yup, but that's the case wherever piped services are shared between different customers - e.g. terrace of houses or block of flats.
- Andy.

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

I was about to respond to JCM's post but its gone!

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

this is TNS, only one set of bonds required

Regards

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

Doesn't your new earthing manual state existing TNS is to be treated as TNCS PME anyway?

probably!! but it would be CNE/PME at the mains joint,
not the service position. the jointer would "PME" at the cutout/neutral.

Regards

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

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.