Earthing/bonding questions re. garden shed

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.

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

That all seems straight-forward (agree?)

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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?

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 ?

Fusewirey said:

But I am interested in the underlying issues from experts.

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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.

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.

Fusewirey said:

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.

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While the cable may be capable of carrying the extra current there are other factors that need to be considered when deciding on the size of cable to be used

Fusewirey said:

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.

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To the untrained person it may appear straight forward but could very quickly become complicated by other factors that may not have been noticed or properly assessed

Fusewirey said:

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

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Knocking in an earth rod isn't an instant fix and needs to be properly installed and tested to prove it is providing and adequate earth

Fusewirey said:

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.)

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This requires very careful consideration IMO as it could become dangerous in some situations

Fusewirey said:

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.

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There are times when you might only find out how good your earthing is when it is too late, earthing and bonding needs just as much consideration as other parts of the installation if a fault current flows the bonded items could become live and the person touching them when a fault current is flowing could provide an extra path to earth and get a nasty shock in the process

It begs the question. The RCBO in the house protecting the supply cable, what is protecting the lighting. Is that already off a spur?