Shed office build nearly done but not sure about earthing the consumer unit

Finally got my shed office wired up last month and went through exactly this head-scratching exercise. Worth getting clear on a few things before you proceed.

First — is your supply coming from the house via an armoured cable (SWA)? If so, the armour itself can serve as the earth path back to the house CU, but you still need to check your main earth arrangement is up to scratch. Don't assume it just works.

The key question with a detached building is whether you're doing a TN-S arrangement (earth via the armour back to the house) or installing a separate earth electrode at the shed and running TT. Most domestic installs here in the UK end up going TT for outbuildings because:

• The earth fault loop impedance via a long SWA run can be too high for MCBs to trip reliably
• TT with an RCD + earth rod is often cleaner and more straightforward

If you go TT, you'll need a decent earth rod driven in (copper-clad, minimum 1.2m ideally deeper) and your consumer unit must have RCD protection on all circuits — a dual RCD board or RCBO per circuit.

Victron kit plays nicely with either arrangement if you're running any solar/battery into that office too, just worth noting.

Honestly though — if you're not confident on this bit, get a sparky to at least check and certify the earthing. The wiring regs (BS 7671) are quite specific on detached building supplies and an EICR will be needed if you ever sell.

What's your cable run like — how long and what size SWA did you pull through?

Hey @LizStewart, good thread topic — earthing for outbuildings trips people up more than almost anything else!

One thing worth flagging that often gets missed: if you're running an armoured cable from the house, your earthing arrangement at the shed end depends heavily on whether you're using the armour as your CPC or running a separate earth core. Many folks assume the armour alone is sufficient but the cross-sectional area needs to properly match up with your supply cable.

Also worth mentioning — if the shed is a separate structure with its own earthing electrode (TT arrangement), you'll almost certainly need an RCD at the origin rather than just relying on the main house protection. Most modern consumer units have this covered but it's worth double-checking your prospective fault current values are actually achievable with a rod earth before you sign it off.

Get an electrician to verify if you're unsure — Part P applies here!

Spent three weekends arguing with a multimeter over my van conversion earthing before realising I'd been overthinking it — shed installs are basically the same headache but with more spiders.

Key thing nobody mentions: if you're running a TN-C-S supply (most UK properties), you cannot just extend the PME earth terminal out to a separate structure — regs prohibit it because of the potential for dangerous touch voltages if the neutral goes open-circuit.

Your options are basically:

• TT earthing with a proper electrode and RCD protection on the outbuilding consumer unit
• Or get a sparky to confirm your supply type first, because guessing wrong here isn't a "oops" situation, it's a "someone gets hurt" situation

My shed runs TT with a Victron setup and a dedicated earth rod — not glamorous but dead compliant. Worth every penny having an electrician sign it off too.

Classic thread. Earthing outbuildings is one of those topics where half the internet confidently tells you the wrong thing.

The key question nobody's asked yet: is this a TN-C-S (PME) supply or TT?

If you're on PME — which most UK properties are — you cannot extend the PME earth to a separate structure containing metalwork (gas, water, exposed metalwork). Regs are clear: Part P and BS 7671 both flag this. A fault on the DNO network can put dangerous potential on everything connected to that PME earth.

For a shed office, a TT earthing arrangement with its own earth rod is almost always the right answer. Pair it with a proper RCD at the consumer unit and you're protected.

@VoltMax the multimeter rabbit hole is real — but here the answer genuinely does require understanding the supply type first, not just measuring things.

Get an electrician to confirm your supply type if unsure.