Illegal to take a feed for a ceiling light from ring main?

[Andy Wade]

I am not in favour of feeding the main lighting for a room from the ring circuit.

[Andy Wade]

Uh? All Part P says, basically, is:

"Reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury."

If main lighting is fed from a RCD-protected socket ring circuit have you made reasonable provision to protect persons from injury?

[David Hansen]

On Fri, 22 Sep 2006 09:35:22 +0100 someone who may be Jake G wrote this:-

Probably not. What is wrong with a one-gang accessory?

[John Rumm]

It would be arguable depending on the circumstance I would have thought. If the light were the only light over the stairs on a main escape route, then no it would obviously be an unacceptable solution. If it were just a light for a conservatory or the cupboard under the stairs then probably less of an issue.

[John Rumm]

What is the room to be used for? Are there other sources of light in it, or that spill into it? Is it part of a main escape route?

Having said that, if you can run a new circuit for it, then that would certainly be better.

[Jake G]

It is actually a loo. Light from the hall spills into it if the door is open. It's not part of an escape route.

I could do that, but it means taking floorboards up which is a real hassle. Access to the hall's ring main is dead easy though.

Jake

[Jake G]

No... It is an extra fuse box devoted to the flat with one fuse for the electric shower and one fuse for the ring main in the flat. The lighting however, comes from the main fuse box. It's a house that has been convrted into flats, with separate coin meters for the ring main in each flat. The lighting for the whole building, however all comes from the main (original) fuse box.

Yes, it would be possible, to make a T connection to the lighting circuit for the flat, if I take floorboards up in the room above. If I do that and find a ceiling rose, is there an easy way to determine which cable is coming directly from the main (rather than going to or from from a light switch)?

Thanks again,

Jake

[David Hansen]

On Fri, 22 Sep 2006 13:21:00 +0100 someone who may be Jake G wrote this:-

Yes. Look at the markings, which will often be on the plastic bar covering the terminals when you unscrew the rose. One of these will be the neutral bar, probably marked "N", to which one of the cables coming from the lamp holder will be connected (hopefully the blue cable) which will have one or more black cables connected to it. The other cable connected to the lamp holder will be the switched live, which should be tagged red but may be black. The brown cable from the lamp holder will hopefully be connected to this.

The other terminal will be the permanent live, often marked loop-in, which will have two or more red cables connected to it (one the feed and one to the switch).

[Andy Wade]

Oh well, in that case the answer's fairly obvious, isn't it?

[meow2222]

I've never seen a mini architrave sized FCU, dont know why. Other than that it sounds fine.

NT

[meow2222]

Its quite normal to not supplying everything on one floor from just one feed. Just how much of a future idiot are we supposed to 'protect' against? If theyre unable to switch the power off before working on something, or to determine whether power is on or off, surely they should not be touching the electrics in the first place.

NT

[David Hansen]

On 22 Sep 2006 07:56:18 -0700 someone who may be [e-mail] wrote this:-

I thought there might be something in the Gridswitch range, but didn't see anything when I skimmed the MK site.

[Jake G]

Many thanks for that. Could save me a lot of faffing around!

Thanks also to all who gave opinions on this one. I'll certainly feel better with it wired into the lighting cirquit... A bit more work, but worth the effort... I can sleep at nights from now on, hopefully. ;-)

Jake

[robgraham]

Andy Wade wrote: (except for TT installations, where it should be

Can you expand on that please ? One reading of that comment is that the whole system, lights and all, should be protected by the rcd for a TT system.

Having just replaced an old fusebox (TT earthing) with a modern split CU, and done what I thought was correct - power circuits on the rcd and lights directly off the isolator switch, the throwing in of this 'TT' variable has me concerned.

Rob

[Andy Wade]

In a TT earthed installation all circuits need to be RCD-protected. With no metallic earth path back to the supply transformer the external earth fault loop impedance (Ze) is high - it's allowed to be as high as

21 ohms. However good your own earth system, there's no way you will get enough earth fault current to trip a high-rated MCB. If you're lucky the loop impedance might just be low enough to trip a 6 A MCB, but this can't be relied on. Even a circuit with a 6 A (Type B) MCB needs Zs to be under 8 ohms.

Oh dear. All your circuits on the non-RCD side will have no earth fault protection. In the event of a L-E short the MCB may not trip, and all your earthing will be up at close to 230 V (I hope your equipotential bonding is in order!). If the fault doesn't clear then your earth electrode will get very hot which will tend to dry out the soil and make matters worse. There are consequences for the electricity bill too.

Presumably with the old fusebox there was an old ELCB ("earth leakage trip") which you have removed as part of the replacement?

Clearly this is a very dangerous situation requiring urgent attention. One option might be to convert to TN-C-S (PME) earthing, which is now available on many overhead supplies.

Otherwise you need to adopt one of the three options shown in the On-Site Guide. These are:

(a) Tails into a DP main switch, then split out to two RCDs: a 100 mA RCD for the fixed equipment circuits which in a TN system would be on the non-RCD side, and a 30 mA RCD for the socket circuits;

(b) Tails into a 100 mA Type S (time delayed) RCD acting as main switch, then as per a normal TN split-load arrangement;

(c) Tails into main switch, then individual 100 or 30 mA RCBOs for all circuits.

Option (b) will probably give the simplest solution - just replace the existing main switch with a 100 mA Type S RCD, provided you can get a

2-module one with an adequate current rating (i.e. not less then the present main fuse rating).

Act quickly.

[John Rumm]

'kin expensive, but try RS part number 264-9584