Ring wiring question

[leenowell]

Hi all

I have extended the ring main into a cupboard in my bathroom to power some shaver sockets. As part of the ring I need to power some 12v led strip lights.

I was planning on using a standard light switch with the following wiring

- connect the 2 neutrals from the ring to the neutral of the transformer

- connect both lives of the ring to one side of the switch

- connect the live of the transformer to the other side of the switch.

- transformer does not require earth so connect the 2 earth's of the ring together.

For some reason I think I have missed something so wanted to double check with you guys just in case.

Thanks in advance

Lee.

[charles]

This should be done through a fused spur.

[Andy Burns]

You won't have any fuse between the (32A MCB?) protecting the ring and the transformer. I'd use a switched FCU instead of a light switch, there will be separate load L/N terminals to feed the light transformer.

[leenowell]

Thanks both. Funny how gut feel sometimes saves us :) My gut said this wasn't right but couldn't see why. Glad I checked as you are spot on. Thanks.

[Roger Hayter]

Should not the already-fitted shaver socket have a fused switch too? Bad style, but could not the both transformers be on the same spur if they are adjacent?

[Dave Plowman (News)]

It's more usual to run a shaver socket from the lighting circuit. Cable for that is cheaper - and often a shorter run than from a ring, which wouldn't usually be already in the bathroom.

[Brian Gaff]

Unless the transformer alread has a fuse as some seem to these days. One thing that has annoyed me though about transformers is how badly some of them buzz, particularly when mounted on a woooden surface. Can we really not make normal mains transformers that are silent any more. I don't like switch mode as then they run they kick out crap and when they fail you find they tend to go bang! Brian

[Brian Gaff]

But are the shaver outlets fused too? Most shaver adaptors have fuses after all.(Except the one I recently bought which is crap) Brian

[leenowell]

The shaver sockets are these

Wired directly into the ring main with no additional fuse other than the one in the MCU

[Max Demian]

Maybe the transformer limits the current so a dead short won't do any damage to the supply or itself.

[Roger Hayter]

Unless it has an actual fuse that can safely break a current of tens of Amps (unlikely) then it can't be relied upon to do this.

[Max Demian]

I was thinking of a transformer with low regulation (for example by having the primary and secondary coils on different arms of the core), so the voltage would drop as the load increased.

[Roger Hayter]

Shorted turns in the primary (common transformer fault) might dissipate several kilowatts, to be replaced rapidly by a low resistance carbon skeleton of the plastic parts which will continue to dissipate several kilowatts without upsetting the 32A MCB, nor necessarily giving any earth leakage. By the time the studs are burning it probably doesn't help if the wires fall off.

[leenowell]

Unfortunately I had to break into the ring as this also supplies a whirlpool bath in the same room

[leenowell]

I ended up using a fused spur as suggested above

[Harry Bloomfield]

It happens that Max Demian formulated :

and if the primary or the supply cable develops a short?

It needs a suitable fuse before branching from the ring.

[Dave Plowman (News)]

I'm actually surprised the terminals in a shaver transformer are big enough for 2 x 2.5mm.

[leenowell]

Hi All

I got a bit confused as to which transformers we are talking about the shaver socket ones or the led light one? Are you saying that both need to be fed from a fused spur? I have never noticed shaver sockets been done this way before.

Thanks

Lee.

[Roger Hayter]

That's probably because they are usually supplied from the lighting circuit, which is fused at 6 or perhaps 10 amps. I rhink they should be on fused spur if supplied from a 32A circuit (though I am not an electrician). Equally, if they are near together, I would supply both transformers from the same fused spur. I thing that is, at worst, a mild stylistic fault, especially if their combined maximum current is well under 3A.

[Mike Clarke]

AIUI distance doesn't come into it. Downstream of the FCU you can have any number of accessories and any length of cable providing the total load doesn't draw a current exceeding the fuse rating and the cable is adequate for the current.