Main bonding detached garage

[dunny123]

Having determined that the steel structure of a detached concrete sectional garage is an extraneous conductive part (0.34 ohms when measured between the CPC on the house downstairs ring final and the steelwork ), I know that I have to provide main bonding to the steel work from the MET .

Question is, do bonding conductors need to be mechanically protected ? There is the potential to route it under the block paving slabs in the sand into the garage without too much disruption to the driveway.

The existing supply is in 2.5mm 3 core SWA fed from a 13A FCU in the kitchen, the G/Y core providing the CPC of the distribution circuit to the garage.

Also, what is the CSA of the armour on 2.5mm SWA ? Is it feasible to use this as (a section of) the bonding conductor ?

[EDIT] I have ascertained that the armour has a CSA of 19mm (thanks wiki) Is it therefore permissable to terminate some 10mm to the armour where it leaves the house via a lug or pirhana nut, and then in the garage do the same and terminate on steelwork ?

 

[nozspark]

Why not TT it and then keep all your bonding local to the garage?

 

[dunny123]

Thanks for reply, Nozspark, I did consider the TT option, my understanding is that in this case I would not be able to take the CPC off the distribution circuit from the house, therefore I would need an RCCB in the garage to provide the RCD protection. As it stands its a fairly basic DP switch in the garage feeding a socket outlet with a 3A FCU for the lights.

 

[Spark123]

0.34 seems like a very low figure to me, is a metal clad accessory mounted onto the steelwork?
If you deem you need a 10mm MEB then the SWA armour which is a different material to copper needs to be corrected to take this into accout. Also the resistance of any MEB on a bog standard 100A TN service should not exceed approx 0.05 ohms.

 

[dunny123]

Yes, its a low figure, and thats with the suppy from the house isolated. I think its because the base of the garage is a concrete/steel cage raft, and the steel framework of the garage is in contact with the steel of the raft, and must therefore be providing a good earth.

And yes, further reading has revealed I would need a CSA of SWA to be 40mm2 for a TN system to achieve the same conductance as copper, so thats out.

So I am left with the need to provide a separate 10mm2 bonding conductor, OR treat the garage as a TT system (can I do this with only a 13A supply in 2.5mm??) , with the structural steel as the earth electrode (Am I right in thinking I can do this?)

I would have to ensure the CPC and armour of the supply SWA is isolated from the garage system and thinking aloud, I could use an RCD spur in the garage to provide RCD protection, and replace the 13A FCU in the kitchen with a DP switch ??

 

[Spark123]

You could use the structural steel as an electrode, however I am still a bit weary of the figure you get, what instrument are you using? Have you a metal water supply in the garage common to another premises? When you say you tested in isolation, did you disconnect the CPC of the circuit involved too? Is the CPC of the circuit in contact with the metallic steelwork of the garage?

 

[dunny123]

Initially I used an analogue multimeter which showed pretty much zero resistance.

I then used a fluke 1653. I used the R lo setting, zero'd leads, and measured from the earth pin of an extension socket (plugged into the house) and a bolt head on the steelwork in the garage...and also subtracted the resistance of the extension lead CPC (0.1ohm)

The armour supply is not currently connected, so there is no electrical supply to/in the garage.

There is no water or gas service in the garage.

 

[Spark123]

Just seems very low to me!! Makes me think it has some connection to the electrical supply earth which may or may not be reliable.
Is the armour connected to your earthing system even though the live conductors are isolated or are you meaning the armour is completely detached?

 

[securespark]

There must be some kind of parallel earth path, with that low reading.

 

[dunny123]

Armour completely not connected, i.e. coiled up !

It does seem low, but as I said theres a big cage of steel in the ground over an area of a double garage of say 5x8M, surrounded by damp sand of the bedding for the block paving which abutts its front and side.

 

[Spark123]

Metal water pipe?

 

[dunny123]

No water pipe !

What I find interesting is that if I treat the steelwork as an extraneous conductive part then I must connect it to the house earthing system by means of a bonding conductor.

If I treat the steelwork as an electrode for a TT system, then I must isolate it from the earthing system in the house !