Spur security light

wtf69 · 15 Feb 2010

Hi,
I guess from reading a lot of these posts I am just about to get abused from left right and center, still if i dont ask i dont learn, so here i go!
you are able to spur of a socket with 2.5 cable as you can only pull off supposingly 13Amps from a single / double socket.
therefore why cant you use 1.5 cable (rated 15A)?

main question is though, I have just bought a house and there is a security light in my back garden and this has been attached striaght into the back of a socket on my ring main, which is ran from a 32A breaker protected by a RCD, the cable is 1.5mm.

now i know this is deffo a no no, still...... this security light will never be able to pull more then 2 amps (it only has a 150w bulb in but guess it could go to 500w), so this cable will never overload. so what is the real problem.

Hopefully some one will be able to explain the actual problem and not just state BS numbers, firstly because i dont have the book and secondly because i have asked for a general explanation.
Hoping this doesnt come across as aggresive as I would like my question answered.

Thanks

Chri5 · 15 Feb 2010

A twin socket spur via a ring main requires 26 amp cable, thus 2.5mm

A 13 a fused spur can be run on 1.5mm since it's limited to 13 amp.

As for a fixed device such as a lighting spur run from the back of a ring socket should a) be fused down- hence the need for a fused spur b) needs seperate isolation so you don't have to switch off ring main each time- hence a fused spur as isolation c) under 17th ed should be via an RCD device (either circuit RCD or a rcd fused spur).

wtf69 · 15 Feb 2010

Thanks Chris,
Guess the 2.5 cable may kick up a fuss but as said, better to ask and learn then to stay dumb and just do it!
still, from reading the wiki 2.5 cable is only rated at 20A, hence been able to plug in two items of 13 A takes it above there. Reading the posts I understand that you will very rarely, if ever, see this happen. the fact is it could happen and I think this is one area that the standards are let down.
and for the security light (which is main reason) I understand your points, however, surely if it is a fixed device as a security light the only thing that will ever be ran off this cable is a security light. agian maximum 2A?
Therefore the cable is fine? I also do agree with you about the isolation but then again you can turn off the MCB? not ideal but you can.
So, do you agree that it is safe and not an electrical risk? but it is wrong in experience of electrical wiring?

cleggy · 15 Feb 2010

correct me if i am wrong.
But a qualified electrician did my outside light this way

he took a cable and put a plug on one end and into a socket,other end went into a fused switch with a 3 amp fuse,from fused switch to outside light.

he said you definately do not connect direct to back of a socket,
reason is you have to change bulb when it goes and unless you turn off all the power the light is still powered up and could be dangerous.

Therefore definately use a fused switch

holmslaw · 15 Feb 2010

wtf69 · 15 Feb 2010

well I agree, the correct way should be spur of a socket (behind it) with 2.5 cable upto a FCU with a 3A fuse inside. Then you can use 1.5 cable upto the security light. You have isolation etc so all is good!

however, this question is not how to wire a security light up

) lol

I want someone to tell me how it is wrong connecting a 1.5 cable into a fixed appliance (security light) that should not pull more then 2A max.

Or will someone stick there neck on the block and tell me there is nothing electircally wrong with it, it is just not correct practice and a complete plonker way of wiring lol, and if i wanted to leave it this way I would not end up damaging my house due to faulty wiring lol.

chapeau · 15 Feb 2010

wtf69 said:
I want someone to tell me how it is wrong connecting a 1.5 cable into a fixed appliance (security light) that should not pull more then 2A max.

Because a security light is a device designed to give off light and heat and not a protective device designed to protect a cable from overcurrent? Do I win £5 ???

chapeau · 15 Feb 2010

Spark123 · 15 Feb 2010

The 1mm CPC in the cable may not be protected against fault current.

wtf69 · 15 Feb 2010

What is the rating for fault current in a 1mm^2 piece of wire?

Also, the circuit is protected by a 30mA RCD and therefore this would trip.

Spark123 · 15 Feb 2010

It isn't fault current on its own which you need to look at as such, it is the energy let through of the protective device.

Steve · 15 Feb 2010

wtf69 said:
Also, the circuit is protected by a 30mA RCD and therefore this would trip.

That depends on the nature of the fault.

Taylortwocities · 15 Feb 2010

Firstly an RCD is additional protection and is not to be relied on to brovide basic or fault protection (I'll not confuse things by talking about TT - OK guys?).

You have to look at two things. Yes the light will pull 2amps so a weedy buit of cable will do that. BUT the circuit, the cable and the protective device is chosen to cope with fault conditions.

If you put a weedy bit of cable into the back of a socket on a ring final, everything will be just fine until the weedy bit of wire gets damaged. At that point the 32amp fuse/whatever will probably sit there still allowing 230volts to assist with the melting of the weedy cable and the resulting fire....

wtf69 · 16 Feb 2010

Thanks for all the replies, I love this site!!!!

so... I totally agree with the cable not been upto the 32A (MCB) and therefore is totally under rated for this! and yes you are correct, on installations you should use cable that is suited for whatever breaker you use. so in this situation, it is completely wrong. I agree

).

But... what fault could occur, and damage the cable allowing the cable to get so hot that it could, if it was surrounded by materials that could easily catch fire, do exactly that? surely what ever fault could occur on this actual cable supplying a security light would trip the RCD? in a normal world. or can you describe a scenario?

same as you would very rarely see two high powered appliances plugged into a double socket spured off the ring with 2.5 which would in fact do the scenario you are talking about

and is much more likely, even to very rare!!!

chapeau · 16 Feb 2010

The 2.5mm spur (which I assume is rated at >26 amps as otherwise it was badly selected for the environment it is installed in) is protected by the two 13 amp fuses in the plug, these are proper protective devices.

You are protecting your 1.5mm spur with a light bulb, this is not a protective device. I think you now owe me a tenner

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Spur security light

holmslaw

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