Outdoor Building Power

[antix_v2]

It's been a while since doing any form of installation so I thought I may ask here for clarification especially with new regs......

Detached shed/building approx. 50m from house CU. Need to run 13A sockets and lights in it. TN-CS/TNS supply

Is it best to...

A) Take from MCB, then SWA down to RCD protected garage board with localised earth rod.
B) Take from RCD protected way to garage unit with/without RCD
C) As B but with earth rod also
D) Answer not above

Any help appreciated. I think I'll stick to commercial lighting...

 

[Richard Burns]

A) is allowable
B) is allowable if there are no extraneous conductive parts in the remote building or if those parts are bonded back to the MET.
C) is allowable but it would be a nuisance to reset an RCD in the house (that has taken out other circuits)
D) this is possibly your best choice if there are no extraneous conductive parts in the building and so can do A without an earth rod.

 

[JD6400]

What is the building being used for ?

 

[antix_v2]

It's an ex chicken shed; now storage/garden type shed. But it does have metal in the fabric of the frame; it's not just a timber garden type unit.

 

[Midwest]

It's an ex chicken shed; now storage/garden type shed. But it does have metal in the fabric of the frame; it's not just a timber garden type unit.

Is you supply TNC-S or TNS. Metal work, generally has to be embedded in the ground to be extraneous, but test would confirm either way.

 

[antix_v2]

In my understanding of the definition, the metal is bolted/fixed into the ground (concrete base) therefore extraneous.

So it looks like I'll need to use a local rod and ensure the metal frame is bonded.

Any further advances???

 

[Leesparkykent]

In my understanding of the definition, the metal is bolted/fixed into the ground (concrete base) therefore extraneous.

So it looks like I'll need to use a local rod and ensure the metal frame is bonded.

Any further advances???

Test the metal work to see if its extraneous.

 

[antix_v2]

Test the metal work to see if its extraneous.

Extraneous.

So, SWA from MCB, with NO connection to MET at CU, then RCD garage board with rod at shed???

Will that provide the RCD required for regs between CU and shed, or would that only RCD the shed itself?

 

[Midwest]

Extraneous.

So, SWA from MCB, with NO connection to MET at CU, then RCD garage board with rod at shed???

Will that provide the RCD required for regs between CU and shed, or would that only RCD the shed itself?

Re my post #5, qualify what supply you have?

 

[antix_v2]

Re my post #5, qualify what supply you have?

TN-CS by the look of the pic I took.

 

[Midwest]

You mentioned TNS as well?

 

[antix_v2]

Yeah then I stopped in for a peak as I was passing...

 

[Midwest]

Did you have a test result for the suspected extraneous metal work? You'll have to decided whether to export your TNC-S supply or not to your shed. Any extraneous metal work may have an impact on your decision.

 

[Leesparkykent]

If the metal work is indeed extraneous and you've carried out a test to confirm this then you could extend the equipotential zone to the out building and bond the metal work. You could to this by running a separate bonding conductor, sizing it correctly by bearing in mind the earthing arrangement is tncs and it would also have to comply to the minimum csa for direct burial due to corrosion etc. The other/easier option Imo would be to use a 3core SWA and size it appropriately so the CPC could act as as a combined bonding/circuit protective conductor.

 

[HandySparks]

Will that provide the RCD required for regs between CU and shed...

No specific requirement for RCD protection of a distribution circuit in SWA.

 

[LeeH]

Did you have a test result for the suspected extraneous metal work? You'll have to decided whether to export your TNC-S supply or not to your shed. Any extraneous metal work may have an impact on your decision.

You're not exporting anything, just extending the equipotential zone.

 

[davesparks]

Just run a 10mm three core and bond the metal, far better than messing around with earth rods.

 

[davesparks]

Extraneous.

So, SWA from MCB, with NO connection to MET at CU, then RCD garage board with rod at shed???

Will that provide the RCD required for regs between CU and shed, or would that only RCD the shed itself?

No, to create a Seperate TT earth the armour must be connected to the earthing system of the installation it is fed from and must meet the requirements of a cpc (or have a core used as a cpc connected) this then would be isolated at the load end with a plastic gland/heat shrink etc and a separate earthing system create by use of multiple properly installed earth rods.

 

[Midwest]

You're not exporting anything, just extending the equipotential zone.

You say to-may-toes and I say to-mah-toes. Ohh, how tiresome.

 

[LeeH]

You say to-may-toes and I say to-mah-toes. Ohh, how tiresome.

You said 'exporting the TN-C-S supply'

I said 'extending the equipotential zone'

I wasn't being flippant like your reply, just simply pointing out the error.

 

[Midwest]

You said 'exporting the TN-C-S supply'

I said 'extending the equipotential zone'

I wasn't being flippant like your reply, just simply pointing out the error.

Don't understand the error; I've see both phrases used for the same procedure. Could you explain the difference for me. Always willing to learn by my mistakes.

 

[Simonslimline]

Don't understand the error; I've see both phrases used for the same procedure. Could you explain the difference for me. Always willing to learn by my mistakes.

You cant export the TN-C-S as this stops at the service head. It is then TN-S inside the property. So the circuit supplying the outbuilding/shed, from the consumer unit, or possibly a switch fuse, is a TN-S arrangement also.

Extending the equipotential zone is the correct terminology.

 

[GEOFF S]

Hi

You dont export/extend an equipotential zone, you create one, and i suggest that you do indeed export the pme, reason being is that all your exposed and extraneous condutive parts are still connected to a PME service head so needs to be treated as such.

Cheers

 

[telectrix]

it's a question of terminology. if i export something, then it goes away, nothing remaining. like if i sold something to a company in china. nothing would remain here. so i concur with extending the equipotential zone. or creating a new EZ and combining with the opriginal.

 

[GEOFF S]

it's a question of terminology. if i export something, then it goes away, nothing remaining. like if i sold something to a company in china. nothing would remain here. so i concur with extending the equipotential zone. or creating a new EZ and combining with the opriginal.

Hi

No, you create an equipotetial zone, it doesnt just appear. Export means to transfer, and in this context it is the earthing system and the neutral potential which is being exported.

Cheers

 

[telectrix]

just what i mean. the pme is not transferred, as it remains where it is. it's just extended/expanded.

 

[Midwest]

Just to add my two penneth, seeing's though I started the discussion on exporting/extending.
IMO the supply, any supply stays in it's form, wherever it is in the installation. For example, TNC-S supplies to permanent buildings on caravan sites, remains a is. The supplies to actual caravans are then converted to TT. TNC-S supplies between buildings are still TNC-S, and relevant regs apply.

As for the phrases; 'exporting TNC-S supply' & 'extending equipotential zone', I've seen both used & quoted. However, only in discussion forums etc. I can find neither of those phrases used in BS7671 (large document mind), nor guidance note 8, for example. Just reference to the correct use of earthing arrangement.

Hence in my opinion, each phrases are both right and both wrong.

 

[antix_v2]

Okay, so terminology aside. I've had a shifty of the cable route etc too now. So the final job is...

SWA approx 50m from house to shed.
Supply is confirmed PME.
Metal frame is extraneous

Option1
MCB (with RCD main)at CU, 10mm SWA, bond at shed

Option2
MCB (no RCD) at CU, 4mm SWA, rod at shed

 

[Spoon]

Option 3
Install conduit from house to shed.
MCB (no RCD main)at CU, 4mm SWA, 10mm Earth, bond at shed
Then you can add extra cables (like alarm, phone, Ethernet) at a later date.

 

[Risteard]

A) Take from MCB, then SWA down to RCD protected garage board with localised earth rod.

I would say this (but may not need localised earth rod, although you could always additionally have an earth rod), but taken from a fuse or MCCB - not an MCB. MCBs in series will not discriminate under fault conditions.

 

[Risteard]

Just run a 10mm three core and bond the metal, far better than messing around with earth rods.

True, but there isn't any harm in additionally installing an earth electrode (as opposed to implementing a TT arrangement).

 

[Risteard]

It is then TN-S inside the property.

It isn't TN-S inside the property. TN-C-S means Terra (Earth)/Neutral Combined and Separate - i.e. combined on the supply side and separate within the installation. If you had a PEN conductor within the installation that would be TN-C (not TN-C-S).