lights on ring mains??

[Thornwhite]

Hello,

I want to get the opinions and advice of the forum as to what should be done do to rectify what I think is a problem electrical situation.

Our house was extended in 1980 and it looks like a rough job. I am intending to insulate the ceiling (flat roof) so pulled the plasterboard down to expose the joists...

What I am seeing is the ring main running in the ceiling through the joists. It is also running to/through the base of the two light sockets. In each case a T&E spur is running off to the associated light switch.

I have confirmed it is the ring main by seeing what else isn't available when its 32A MCB is tripped.

My questions:
How good/bad/otherwise is it that lights are being powered through the ring main in this way?
If bad, then what should be done to rectify it?

I am not intending to do the work myself, but would like to understand what I should be asking a sparky to do - and maybe why.

Any comment would be appreciated.

 

[PrenticeBoyofDerry]

My questions:
How good/bad/otherwise is it that lights are being powered through the ring main in this way?

It shouldn't really, the correct method would be to spur via a fused connection unit and down fuse to possibly 3A.

If bad, then what should be done to rectify it?

As above the use of fused connection unit.

 

[Thornwhite]

Does this mean it is just a case of replacing the original switch with a 3A fused connection switch?.

if so great!

Thanks for this.

 

[AndyPRK]

No that would be fairly pointless.

You need to see if there is one obvious place where the light circuit is connected to the ring and put an fcu in there.

 

[PrenticeBoyofDerry]

Does this mean it is just a case of replacing the original switch with a 3A fused connection switch?.

if so great!

If you only have a single gang switch then it will do, if you have multi gang switch then the FCU can be positioned along side them or a grid switch system could be used (this would included a fused module.

I will just go back a couple of steps, what do you mean by light sockets, sockets that allow a lamp to be plugged in or are we talking light fittings?

 

[PrenticeBoyofDerry]

If 5 amp sockets they should be okay as a fuse would protect from plug onwards, but the issue would be the switch as it will likely only be rated at 10Amps, so either a switched FCU or a higher rated switch.

 

[JohnW2]

How good/bad/otherwise is it that lights are being powered through the ring main in this way?

It shouldn't really, the correct method would be to spur via a fused connection unit and down fuse to possibly 3A.

Kind Regards, John

 

[winston1]

If 5 amp sockets they should be okay as a fuse would protect from plug onwards, but the issue would be the switch as it will likely only be rated at 10Amps, so either a switched FCU or a higher rated switch.

What fuse? 5 amp don't normally have fuses.

 

[AndyPRK]

I'm confused whether its ceiling roses or sockets switched by light switches

 

[eveares]

Could you please post a picture of the light sockets as is ambiguous to what you are talking about.

Are your ceiling sockets ceiling roses or old fashion round pin lighting plugs?

Also as other people have said, you need to put a switched fused spur in with a 3 amp fuse in before the "light sockets"; see here for a diagram:

http://img150.imageshack.us/img150/1984/fcu2ea0.gif (note your wire colours may differ from the diagram)

 

[PrenticeBoyofDerry]

What fuse? 5 amp don't normally have fuses.

You can get them fused, whether you consider that normal or abnormal, I will leave that opinion to you!

 

[davelx]

Could you please post a picture of the light sockets as is ambiguous to what you are talking about.

Are your ceiling sockets ceiling roses or old fashion round pin lighting plugs?

Also as other people have said, you need to put a switched fused spur in with a 3 amp fuse in before the "light sockets"; see here for a diagram:

http://img150.imageshack.us/img150/1984/fcu2ea0.gif (note your wire colours may differ from the diagram)

Sounds to me as if the RFC has been looped through the ceiling roses and that the "spur" he refers to is the switch drop. i.e. standard loop-in/out way of wiring the rose, but on the RFC instead of on a lighting circuit. Quite likely there are other socket outlets on the downstream side of the second rose

So probably not just a matter of installing an FCU on the feed to the lights.

 

[ericmark]

Putting it simply the ceiling rose is rated at 5 or 6 amp so the fuse or MCB or RCBO feeding the ceiling rose needs to be 5 or 6 amp.

The standard way to wire a light is to take the supply to the ceiling rose and from there to the light switch with this configuration the fuse would need to be before the ceiling rose.

There is an alternative method to wire (USA method) where power goes to switch first. Using a fused connection unit (FCU) it would be easy to include a fuse with the switch. Where more than one switch then using a grid switch will allow fuse and multi-switches in one plate.

There is a limit of 16A for lighting but due to ceiling roses being rated at 5 or 6 amp that is really 6A but as well as fuse size the problem is should something trip the power you will also be left in darkness. As an additional lamp using a FCU is OK but for main lights it really should have it's own dedicated supply. One way around the problem is to use emergency lights so if tripped your not plunged into darkness.

I have had problems with a 16A MCB feeding an outside lamp. When the lamp failed it went short circuit and welded the bulb into the holder meaning whole fitting needed changing. It would be even worse with a 32A MCB so you really do need to fuse to correct size.

 

[JohnW2]

Putting it simply the ceiling rose is rated at 5 or 6 amp so the fuse or MCB or RCBO feeding the ceiling rose needs to be 5 or 6 amp.

Some would probably say that the fuse, MCB or RCBO was there to protect the cable, not the accessories.

I have had problems with a 16A MCB feeding an outside lamp. When the lamp failed it went short circuit and welded the bulb into the holder meaning whole fitting needed changing. It would be even worse with a 32A MCB so you really do need to fuse to correct size.

As you know, the MCB will not affect the current that flows during fault conditions - so the magnitude of the current would be as high with a 6A MCB as with a 32A one. A 6A one would obviously reduce, at least to some extent, the duration of the current (as compared with a 16A or 32A MCB) before the device operated and disconnected the supply - but whether that would be enough to prevent the sort of outcome you describe is, I presume, far from certain.

Kind Regards, John

 

[ericmark]

I will admit there is an anomaly with lights and fuses. The ceiling rose is rated 5A and works as both junction box and feed to light so yes 5A fuse to feed 5A junction box.

However most BA22d bulb holders are rated at 2A yet we don't fuse to 2A. Theory is inside every bulb is a built in fuse which if when the bulb blows there is ionisation will rupture stopping the main supply fuse from blowing.

But where we use a MCB rather than a fuse the magnetic part of a B6 MCB will operate at 30A in around 0.01 of a second far faster then the built in fuse so when bulb blows it takes out all the lights.

What I am saying is with a B32 MCB the current needs to be 160A to trip in 0.01 of a second and unless the earth loop impedance is below 1.44 ohms it will not trip on the magnetic part of the trip and although ionisation may be regarded as near short circuit it is not quite and 160A is a lot of current.

So likely with a B6 MCB 100A will flow for 0.01 of a second but with a B32 MCB it will flow for 12 seconds before disconnecting. That's 1200 times the energy which will need dissipating as heat to that using a B6 MCB.

So with a fault current between 30A and 160A the heat generated under fault conditions will be 1200 times the expected which will melt the solder on any normal bulb welding it to the holder. As I have said in an ideal world there should be a fuse fitted in the bulb but we don't live in an ideal world and to have to change the bulb holder is a real pain when a bulb blows.

Since what happened to me I have not bought any more Ikea bulbs these clearly don't have the built in fuse. It was a 6W CFL so I suppose ionisation was not a consideration but since BA22d lamp holder is rated at 2A that would mean fusing lighting to 2A which UK lighting is not designed that way.