Do I need to install a local ground point for a secondary consumer unit in my shed?

Question

I've just installed outdoor sockets on the side of my brick shed, where previously we just had a double socket inside the shed itself. The original socket in the shed is on a spur from the ring main for the downstairs sockets. This is hooked up to a breaker in the main consumer unit in our basement.

Instead of just splicing that spur into two for the shed socket and outdoor socket, I installed a secondary consumer unit in the socket's old location and ran the shed socket and outdoor socket from that, on separate breakers.

The new consumer unit has an external screw terminal for attaching a ground point. The manual mentions that it is an optional feature. I do have a clad copper water pipe going through my shed, to which I could potentially attach a grounding strap, but I'd rather avoid doing so if possible because it's not really near the consumer unit (it's about 4ft away) and the area around the pipe is currently inhabited by an annoyingly large and confident spider. I'm pretty sure she's been eating the others and I am not a fan.

Everything currently works. I've verified ground continuity all the way from the incoming feed to the outdoor sockets. I'm just not sure if I need the additional ground connection. Must I face the spider, or can I shy away from battle?

Answer 1

From your reference to "socket ring", I assume you are in the UK. I shall use UK terms.

From the additional information you have provided in the comments, you have the easy case. There is no long run from house to shed, so you don't have to worry about resistance in the cable reducing the short-circuit current (either live to neutral or live to earth) to the point where the fuse in the consumer unit doesn't blow.

Just use the earth wire in the cable, and export the earth from the house. Do not attach another earth in the secondary consumer unit in the shed.

Do make sure that there is an RCD in the circuit - either in the secondary consumer unit, or at the point where the spur comes off the ring-main. (Personally I think where the spur comes off the ring main is best, but in the shed is certainly "good enough".)

Do make sure that the spur to the shed is adequately protected by a fuse. Note that ring mains can go up to 32A, despite the fact that they are wired in 20A cable - this is because they are only rated for 32A on the assumption that it is reasonably evenly divided over the ring - so each leg of the ring shouldn't go above 20A. With a spur there is only one leg, so you can't rely on that - you need to limit it to 20A. There really ought to be a 20A fuse in the FCU where the spur comes off the ring, but if the spur doesn't come off the mains in an FCU and it's difficult to install one, bunging a 20A fuse as the master fuse in the secondary consumer unit won't be strictly according to the regs, but it will get you most of the protection.

Answer 2

Being well grounded saves you from the lightning (bug)

There are two facets to the earthing (grounding) of an electrical system:

• Protective Earth wires and the earth-to-neutral bond in your main consumer unit provide a path for errant utility electricity to get back the utility before it gets the chance to shock you. (Hopefully, this path is low enough in resistance that the breaker trips -- if not, that's why RCDs were invented.) You already have this taken care of with the earth wire in the twin-and-earth feeding the shed's consumer unit.
• The earthing electrodes and associated wiring to the main panel earth bar provide a path for errant natural electricity (such as induced currents from lightning strikes) to get back to nature, and that's what you need to bolster by putting in earth electrodes at the shed. This protects the insulation on wires and in devices from electrical surges induced by lightning storms and other natural events.

Furthermore, since you are bonding the consumer unit to a water pipe, this allows the breaker to trip and cut off power if electricity gets onto the water pipe system, say through an errant wire, which is also important for safety's sake.

So, I'd pack a few juicy caterpillars to bribe the local spider with while you fit the earthing wire from the secondary consumer unit to the water pipe.