Electrical diy

[Peter Johnson]

Installed an external socket off a (new) fused spur from an an existing socket. Surprised myself when it worked first time. Pictures:

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[SH]

I would have used a switched FCU rather than a FCU for the siomple reason one can then electrically isolate the external socket in case a weather related fault occurs.

That way you can re-energise your indoor sockets having isolated the outdoor socket and then fix it when the weather improves.

[ARW]

I would have made a switched fused spur.

And put it at the same height at the 2g socket:-)

[John Rumm]

That looks like a better design of MK masterseal than some of the ones that are fitted here...

Yup double pole switched FCU would probably have been the preferred option, but there is not much scope for the outside bit to cause too many problems with just the one socket an no extra wiring.

[newshound]

Agreed, but at least he can pull the fuse to isolate.

[SH]

but a switch can be operated by hand, whereas he's got to rummage somewhere for a flat blade screwdriver to pull open the fuse carrier

[williamwright]

Hits you in the eye dunnit?

Bill

[williamwright]

Suicidal slugs are always getting into mine. I don't know how they do it.

Bill

[John Rumm]

A fuse is single pole, and that is not isolation.

For example water ingress into the external socket causing a low resistance path between earth and neutral would likely lead to a RCD trip on the circuit that pulling that fuse would not clear.

[Scott]

As an aside, am I right in thinking at one time both the live and neutral were fused but the rules were changed because failure of the neutral fuse could leave the entire circuit energised; and therefore the practice was banned?

[Steve Walker]

And removing the fuse only isolates the Live, while a Neutral to Earth leak could keep tripping the RCD.

[Fredxx]

Is the fuse necessary? I can understand a DP switch for safety and to stop neighbours stealing power :-)

[John Rumm]

Yup. If you look inside some old metal clad main fuse enclosures like this:

You can see the original design allows for fusing of both poles:

(the one pictured with a bridge bar in place of the neutral fuse)

[John Rumm]

It's not actually required since this is in effect just a single socket on a ring circuit - probably wired as a short spur.

A fuse is often included by default since a switched FCU is convenient and readily available accessory that includes a high current dual pole isolation switch.

[Owain Lastname]

Pulling the fuse only isolates the live. A neutral-earth fault outside would still trip the RCD.

Owain

[Scott]

On Thu, 30 Sep 2021 01:54:34 +0100, John Rumm [e-mail] wrote: [snip]

Why did they do that? Was it not an accident waiting to happen? I assume it was on the basis that two fuses are better than one, but why not put both fuses in series on the live?

[Owain Lastname]

You would, but would your apprentices?

I know I've had back boxes which mysteriously seem to move off-level between screwing them in and fitting the accessory.

Label's a bit slanty as well, which is an easier thing to correct.

Owain

[Adrian Caspersz]

Seen some filled with water and corrosion. Make sure the drain hole is open / not blocked.

[Owain Lastname]

At that time, neither side of the supply was reliably tied to earth, so double pole fusing was routine.

Owain

[John Rumm]

It may have been the logic that in the event of a L/N reversal in the supply, The double pole fusing would ensure you can still clear a phase to earth short[1].

Which is true... however at some point they realised that in adding protection from a very rare fault, they were creating a risk of making a far more common fault (overcurrent on L to N) more dangerous, since you then had a 50/50 chance the circuit/installation would be left live after the (wrong) fuse blew.

[1] Logic that fails with the common modern practice of TN-C-S earthing, but may have had some scope with older TT earthing systems relying on a connection to a gas or water pipe for an installations earth.