Outdoor Building Power

[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.