Discuss ECP's in Prefab Concrete Outbuilding- Damp Concrete? in the Electrical Forum area at ElectriciansForums.net

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Advice required from the more experienced and knowledgeable.
Has anyone installed power to one of these prefab concrete garages with a steel roof structure, steel clamps (on concrete panels), steel door mechanism and damp concrete floor?
My conundrum is that I intend on upgrading the current install in the garage to incorporate a small CU for a 32 Ring Final and a 6 amp Lighting circuit. I want to use this for a forthcoming assessment with a scheme provider, so I need to be 100%.
On initial investigation, it was apparent there were no obvious Extreneous Conductive Parts. Hence, as it is a short run of 25m, I was planning to utilise the TNC(S) at the house and run 6mm SWA from a 45 amp MCB to a small 3 way CU with an upfront 40 amp 30mA RCD. However, as a precaution, I thought it best to check the metal structure's resistance to the disconnected main earth by conducting an IR test at 500V using my wander lead. Here's the problem I was getting readings lower than 10kohms.
Perhaps the steel bolts holding the clamps are touching the steel rebars in the concrete, which in turn is damp, through the damp concrete base to the soil.
Has anyone worked a way around this before?
I know my options in extending the equipotential zone or installing a TT at the outbuilding, and I'm inclined towards the TT due to the damp concrete. However, even with a TT, bonding is still likely required, and I don't like the idea of bonding the steel roof structure of a coated steel roof or am I overthinking this?


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Your 30mA RCD in the garage will need to be rated higher than the over current protective device supplying it from the house which isn't the case from what you've said.
As for bonding, is the whole building steel framed, ie. is the frame bolted to the ground, or is it just the roof which is steel?
Either way I'd be inclined to bond it.
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The whole building isn't steel-framed, just the roof supports it's reinforced concrete only to the base with the exception of the garage door. My typo - the MCB at the Main CU will be 40amp.

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