Pme query

[Tridentsystems]

Hi all , went to look at a job today..... Main house with PME all good bonding etc. The detached garage and workshop also has a separate PME supply. Inbetween main house and detached garage they have built a one bed granny annexe, this links the two buildings but with no access through to existing house or garage. New cu has been fitted to granny annexe supplied from garage supply. Boiler has been fitted from main house gas supply to the annexe which is main bonded to house PME. I think I should link the MET from each supply then bond new cu to boiler ? Any thoughts ? NAPIT technical didn't want to commit to an answer !

 

[SirKit Breaker]

Why am i not surprised that Napit wont commit!

Before i have a stab, just confirm that you have electric from one building and gas from another, all coming together in the middle bit.

Cheers...........Howard

 

[Tridentsystems]

Yes howard

 

[guest119]

The granny flat is a separate electrical installation in its own right and so should have the normal bonding of incoming services.
The fact that the gas and possibly water come from another property is neither here or there, still an incoming service, hopefully with their own shutoffs. Therefor bonding at incoming point, after shutoff and before any tee.

Boydy

 

[spark 68]

Well you are going to have to bond the boiler in the Granny annexe to the local CU in that annexe anyway.

If you have GN8 have a look at fig's 5.12 an 5.13 (pg 62 in new book), this sort of explains what you need to do.

 

[Tridentsystems]

Thanks guys just in process of reading bible but wanted some real world input. Just feels like an exported earth on the PME

 

[spark 68]

You are going to sort of end up with a mix of these two drawings.
with two bonds (4 if water services are metal)

1) a local bond from the annexe to the local MET also in the annexe
2) a bond from the supply MET (garage in your case) also to the annexe

A further point to consider

the house is already locally bonded, and an additional bond in the annexe is not needed.

At least according to the drawings in GN8

 

[Tridentsystems]

Thanks spark. Can't see the attachment for some reason

 

[spark 68]

I have uploaded it as a Jpeg, so you should be able to see it ?

Anyway the point is you only need bond from the electricity supply side (as if exporting as usual), not from the services supply side and of course local to the annexe MET.

The house is already bonded locally, and the house electricity supply/installation (and bonds) end there.

Edit: re-attached as a Pdf on this post

 

[Tridentsystems]

Thanks. Think it's an iPhone thing. Gonna hit the sack and look on PCM tomorrow. Thanks for help

 

[spark 68]

There appears to be two ways of doing this, if the supply cables (from the garage) earthing conductor has a large enough csa (it may do including the armourings) to satisfy both protective and Bonding duties then you would only need a local bond in the annexe (to each service) to the annexe MET.

The drawings above show MET to MET bonds, from the electricity supply (garage) to the MET in the annexe.


I suppose it may be considered as an extra risk (of been disconnected ?) by bonding the service to the garage supply instead, which I may have inadvertently implied in an earlier post.
Sorry for any confusion caused

 

[Tridentsystems]

I was thinking just advise a 16mm met to met and main bond new gas to new cu. all water is poly

 

[spark 68]

I was thinking just advise a 16mm met to met and main bond new gas to new cu. all water is poly

That may satisfy the conditions, but you need to be careful here where PME applies, that bond garage MET to Annexe MET may need to be as large as 35mm ! if it is to satisfy both bonding and protective functions.

IN GN8 showing a similar scenario they show 35mm local bonding too.

The Bonding conductors are sized against the incoming Neutral (to the garage) not the supply cable to the annexe, be careful.

 

[Tridentsystems]

Good point. Hate jobs that have already been done. This one was done by the plumber

 

[spark 68]

Here is another pic from GN8, showing exported PME.

 

[Engineer54]

That may satisfy the conditions, but you need to be careful here where PME applies, that bond garage MET to Annexe MET may need to be as large as 35mm ! if it is to satisfy both bonding and protective functions.

IN GN8 showing a similar scenario they show 35mm local bonding too.

The Bonding conductors are sized against the incoming Neutral (to the garage) not the supply cable to the annexe, be careful.

Were talking about a granny flat and a garage here not an installation such as is shown the GN8, which is clearly showing 120mm tails. This illustration is basically referring to substantial buildings (commercial/industrial), rather than domestic arrangements...

Taking it that the garage supply is a sub-main from the main house, and the granny flat a sub-main off the garage CU. So the main earth conductor to the granny flat will be identical to that provided for the sub main to the garage CU. Any bonding to services within the garage and granny flat will be the nominal 10mm. Let's hope the sub-main supply from the house to the garage, is suitable to also carry the max demand load of the granny flat!! lol!!

 

[spark 68]

What I was pointing out E54,

Were talking about a granny flat and a garage here not an installation such as is shown the GN8, which is clearly showing 120mm tails. This illustration is basically referring to substantial buildings (commercial/industrial), rather than domestic arrangements...

Was that the bonding conductor has to be sized to the DNO's incoming Neutral at the supply to the garage (the OP's actual supply source), not the Neutral that supplies the granny flat.

It may well be the case that 16mm will be ok, in parallel with the incoming Earth csa and armourings, I did say it may have to be larger if it was combined as the protective conductor as well as the bonding conductor (that may, or may not be the case here) .

As I cannot see the install, and don't know how it is supplied (armoured ?, T&E ?), I only advised caution wrt sizing.

The Picture shows the bonding is sized to the PME incoming supply Neutral.

Cheer S68

Edit: if the OP's supply is fed with armoured cable with a separate CPC core, it may have a large enough CSA t satisfy the conditions required without needing a separate bond back to the supply MET, which I also said earlier in the thread. :wink_smile:, but I don't know for definite if this is the case.

 

[Engineer54]

That's what i was pointing out, you only need 10mm bonding conductors for up to 35mm incoming supply conductors. You'll rarely if ever, see larger than 35mm on a domestic installation.

 

[spark 68]

That's what i was pointing out, you only need 10mm bonding conductors for up to 35mm incoming supply conductors. You'll rarely if ever, see larger than 35mm on a domestic installation.

True, assuming the garage is a domestic type, in the OP it just said it was a garage and workshop (with it's own supply), which again I don't know the details of, and it could be anything, so I did not want to say just do X and Y, in case I was giving false information.

Anyway I did better than Napit technical lol , who did not want to commit. :wink_smile:

 

[Tridentsystems]

Morning. Garage is a domestic double but has its own 100 A PME supply. No load to speak of on it ( 1 florrie and 2 dbls via an 8 way 100 A main switch split load 63 A 30 mA RCD. Annexe has been fed from a spare way on above db via 40 A MCB (Type B ) on a 10 mm armoured 3 core ( armour and 10 mm core for earth)

 

[Engineer54]

True, assuming the garage is a domestic type, in the OP it just said it was a garage and workshop (with it's own supply), which again I don't know the details of, and it could be anything, so I did not want to say just do X and Y, in case I was giving false information.

Anyway I did better than Napit technical lol , who did not want to commit. :wink_smile:

You did indeed!! lol!!!

It seems we both assumed wrong, but then who would have ever guessed that a domestic garage would be on it's own metered PME supply!! lol!! Wonder how long this owner has been paying 2 electric bills, or maybe it was brought in recently, specifically for the annexe granny flat because it was easier to route the cable, than supply the granny flat direct!!?? Who knows!! ...lol!!

Anyway, he still only need to bond the gas service pipe were it enters the separate annexe, no need whatsoever to link the two MET's from the two separate supplies. It's the same situation as you would have in a block of flats, bonding wise...

 

[Tridentsystems]

I believe originally there were a lot of old sheds behind the garage which were going to be let as small units for cottage industries so the supply was brought into the garage for them. Guess this never happened and so he thought use it as its there. Still think it would have been easier to have put new ccts onto existing house db. Saved a lot of SWA IMO. would the gas pipe bonded at each end to different PME supplies suffice ?

 

[Engineer54]

I believe originally there were a lot of old sheds behind the garage which were going to be let as small units for cottage industries so the supply was brought into the garage for them. Guess this never happened and so he thought use it as its there. Still think it would have been easier to have put new ccts onto existing house db. Saved a lot of SWA IMO. would the gas pipe bonded at each end to different PME supplies suffice ?

It'll be the same DNO's PME supply, maybe on different phases from the same network cable though!! Yes it will suffice, as i say no it's different to a block of flats, or even a row of houses that each bond to a common service pipe as it enters the property... No need to over think or complicate matters here!!

 

[Tridentsystems]

Haha I've been told before about over thinking !! Thanks very much for your helpful advice. Think I will on this occasion let the plumber finish what he started and contact LABC himself. Useful info all round