Earthing/bonding questions re. garden shed

[Fusewirey]

I have an existing 3-core armoured cable from the house to an external junction box that then powers several lights, all properly wired up. The cable is more than adequate to supply higher current, and is protected by a RCBO in the house.

I want to add a spur from the junction box to a new (fully-weather-proof) shed.

I will fit a four-way consumer unit on a plywood panel, with a main switch and two RCBOs, one for lights, one for sockets.

That all seems straight-forward (agree?) but the shed is a METAL FRAME, built rather like a shipping container, which arrived as a flat-pack, and rests on bricks. I want advice on earthing/bonding, please.

Shall I just add an earthing rod to the shed, and that ' shed earth' is always separate from the 'power supply earth'?

Or, shall I ALSO connect the two earths together? Without doing that, would a (line)-(shed earth) fault always guarantee to trip the RCBOs?

(I presume NOT having an earthing rod would always rely too much on the quality of the house wiring.)


No-one dies from adding too many earths or bonding, so no doubt I am making a mountain out of a molehill. But I am interested in the underlying issues from experts.

 

[binky]

Any idea what the house earthing system is? Sometimes there's a lable next to main fuse.

 

[SPECIAL LOCATION]

That all seems straight-forward (agree?)

Bit difficult to agree or disagree with limited information given..

What are the earth arrangements coming into at the house?
Cable sizes & lengths?
Type & rating of protective devices?
Are the cable and length also adequate for volt-drop?
Why would to plan on having RCBO's in series?
How are you achieving discriminations between house & shed?

 

[Murdoch]

Why a CU in the shed?
What rating of breaker is there protecting the existing cable?

Also fitting a CU falls under part P - who is testing and certifying this ?

 

[Tim Howard]

But I am interested in the underlying issues from experts.

In general terms, a low impedance earth path utilising a supplier provided earthing system is preferable as a live - earth fault should allow plenty of fault current to flow and an over current protective device to quickly operate (if the circuit is properly designed).

That said, there are sometimes also reasons not to do this. For example if it were a caravan it wouldn't be permitted to connect a certain type of supply earthing to the metalwork. If you research Open-Pen supplier faults you will see why.

IMHO your case does need some careful thought and risk analysis by someone experienced on-site.

 

[Chewbacca]

Without knowing what else is going on, we need to know the construction of said shed. A metal frame tells us nothing about the rest of the shed.