TNS and PME connected]

lyledunn said:

Issue?

Clause 6 of BS 6907-2 Clause 6 doesn't include TN-C-S ... BUT i'd be happy to argue that, provided the changeover requirements are met, the supply to the quarry is TN-S (whether or not bonded or otherwise connected to PME).

Nothing in BS 7671 or BS 6907 series, or ESQCR or the guidance on ESQCR, appears to prohibit connection of a PME earthing arrangement (whether or not via earthing or bonding).

I guess there's the question (in terms of immediate risk) as to whether any part of the quarry is considered explosive atmosphere? Whilst again, the terminology in BS EN 60079-13 BS EN 60079-14 is unclear, there's a general professional understanding that electrical installations of hazardous areas with explosive atmospheres are not connected to PME (or, if it can be helped, exposed to diverted neutral currents).

'Whilst again, the terminology in BS EN 60079-13 is unclear . . .'

Did you mean BS EN 60079-14?

With reference to earthing systems, BS EN 60079-14 states that where a TN earthing system is used it shall be TN-S with separate protective conductor and neutral in the explosive atmosphere.

- Ross

I presume. like the quarry supply  the genset is supplying TNS and so , when running on genset, is the supply to the workshop. The problem then, if there is one, is the NE link at the office. How is the change-over from the PME supply to generator power managed ? if it is done in a way that breaks the potential current path through this link, then there is no issue. (though that does mean that simply strapping all the earths together at both ends won't quite do)
I'm kind of assuming the generator is only used when the transformer and it's hybrid earthing is not. Of course if both ever run together that is different.
One could of course TT the offices and workshop, but I can see in a rocky environment why that may not be desirable. (though there must be an electrode in the vicinity of the generator already)
Mike.

Yes, apologies, typo edited previous post.

mapj1 said:

if it is done in a way that breaks the potential current path through this link, then there is no issue. (though that does mean that simply strapping all the earths together at both ends won't quite do)

BS 7671 does permit transfer switching to also transfer the means of earthing.

BUT ... to completely isolate one earthing system from another fully in these circumstances, and maintain safety, is not easy.

Agree, its a funny one, and the fact the other end is a quarry is a complication.  If it wasn't it would be similar to a street with a mix of TNS and TNC-s service heads on the same substation with the earths linked by common water pipes etc. That common situation does introduce currents in the bonding as sort of 2nd parallel neutral,  but we don't normally worry unduly. I also suspect that in reality the additional risk would be very low. 

If it was a farm it would be all TT.... 

It does beg the question of how hard it would be to separate N and E between transformer and site offices - but I assume its not a private transformer and the metering is in the wrong place so that is a DNO owned cable.

Not for the first time the PME approach solves one problem and introduces another.

Mike

Is the office block the only customer on the PME side of the supply?

If so, I think I can see a point of view that with the change-over switch in the generator/TN-S position there would be no loads connected to the TN-C-S system and therefore not potential differences along the PEN conductor (whether due to normal voltage drop along the N in normal conditions, or broken PEN situation as there'd be no connected loads to pull up the voltage on the severed end). Thus the customer's end of the PEN conductor should be at the same potential as the transformer's electrode and, hopefully, that throughout the TN-S system. So even with the bonding to the PME situation in place, the hazards of PME shouldn't be present, in this particular situation. (Unless I've overlooked something obvious again).

   - Andy.

 GK said;

Nothing in BS 7671 or BS 6907 series, or ESQCR or the guidance on ESQCR, appears to prohibit connection of a PME earthing arrangement (whether or not via earthing or bonding)

Yes, so it would seem. However, the safety manager for this quarry company also asked us to keep an eye on the "Management of Electrical Safety in Quarries...etc", issued by the Quarries National Joint advisory Committee. They recommend that PME is not used in a quarry situation. I guess its difficult to keep everyone happy these days! 

My OP was really to get a sense of what the consequences might be and where in the risk spectrum it might sit. It is nice to be able to bounce things off the experts on this forum. Thanks to all!

The Energy Networks Association Engineering Recommendation G12 - Requirements for the Application of Protective
Multiple Earthing to Low Voltage Networks appears to agree with (or may be the source of) the position of the Quarries National Joint Advisory Committee:

'6.2.8 Mines and quarries

Provided the building’s electrical installation wiring conforms to BS 7671, a PME earth terminal can be provided to mine/quarry permanent buildings (e.g. permanent offices and canteens). This excludes amenity shower blocks unless an earth grid is installed. Supplies to underground shafts, the production side of quarries or associated amenity shower blocks shall be from a TT system having a separate connection with earth, independent of the PME earth terminal. Where a mine or quarry requires a supply both to a permanent building and either an underground shaft or the production side of the quarry, precautions must be taken to ensure that these latter supplies have an earth system segregated from the PME earth system.

If the site has a dedicated secondary substation that only supplies the consumer it will usually be possible to provide a TN-S earth terminal directly from the transformer neutral.

NOTE: The requirements for supplies to the working areas are covered by specific statutory legislation (see
BS 7671, section 110.1)'

- Ross