Spur what if question

[blacksheep]

Hi all

I have run a spur to a double socket in another room, all done properly its from the main ring off a previously unspurred socket.

This is all in theory..

There isnt much on the new socket as such, but of recent I have acquired a portable heater which can run 2kw at full power (8amps yeh), plus some other stuff on a multiway. I dont think im likely to get to 13amps but what would happen if I did (I think a typical double socket is rated at 13 amps combined not 26?) Is the socket going to blow or should I, for safety, install a FCB or a fancy double socket with a fuse if they exist (seen rcd ones).

Thanks guys

 

[ricicle]

Or now you know the issues with overloading double sockets don't overload it !

 

[blacksheep]

I dont intend to overload it but just think it should be protected against all eventualitys.

I was thinking of putting a 13A FCU inline (maybe switched for convenience of switching the room off), but as the socket is 13A wouldnt it be more correct for it to be 12.5A or something (not that a fuses like that exists) or does the 13A socket die at bit more?

 

[ban-all-sheds]

Twin sockets are supposed to be able to cope with at least 20A.

 

[blacksheep]

Ok thanks for the info..
so you think that a FCU is just overdoing it as this problem could exist on a ring too, and my radial can handle 20 odd amp (2.5mm) ?

 

[ericmark]

2.5mm cables rating alters according to how installed Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface) just 13.5A but Reference Method C* (Clipped direct) 27A that's for 70degC thermoplastic insulated move to thermosetting (90degC) then 33A. All these are continuous ratings and most of what we use is intermittent. So there is no fixed answer.

In the main Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) which gives 21A is considered as normal and although a double socket could reach 26A it is unlikely to maintain that figure plus they are only rated at 20A.

So although one could exceed the value it is unlikely to do it for long enough to cause damage. So if it complies so no longer than 3 meters as with regulation 433.2.2 then a risk assessment is likely to show a very low risk.

I have fitted many RCD FCU active type to give RCD protection and to ensure the power does not auto return after a power cut in some cases with 4 or more sockets and the fuse blowing is very rare. Where something has happened like a squashed extension lead often the B32 MCB has tripped before the fuse has blown. This will largely depend on the loop impedance of the circuit so if correctly installed with a B32 MCB or RCBO and a loop impedance of 1.44 ohms or less there is unlikely to ever be a problem.

 

[blacksheep]

Yeh, its pretty much above the original socket into the attic so about 2Metres. Its installed in plastic trunking up wall then straight through ceiling past some rockwool for a few inches then terminated.

No MCB for Rings though, just wired fuses in an older style CU

 

[ericmark]

Although the humble fuse needs a better loop impedance than the B type MCB the MCB has a very marked cut off point where it fails to protect where the fuse has a gradual reduction in time it takes to open.

So to change is not a DIY job. To be able to supply a new socket (under 20A) from a re-wire-able fuse it would need to be done before 2008 and where there is an electrician on site.

Not sure of the date when re-wire-able fuses were outlawed in premises with ordinary persons in control but it was after 1980 but before 2001.

After 2008 one had to have RCD protection on new sockets so only real way would be to use a RCD FCU to supply the new socket.

converters for Wylex are easy to fit to change from fuse to MCB but really it would need the loop impedance to be measured and also still lacking the RCD.

 

[blacksheep]

There is one RCB in the Wylex for another circuit but the remains are Wired. I could get some more to modernise it a bit at some stage.

I guess the house could do with a new CU altogether, ideally.

So can you get FCU RCD sockets, or are they just RCD sockets?

 

[blacksheep]

This badboy do ..

http://www.screwfix.com/p/volex-13a-rcd-fcu/83049

Can you get Wylex MCBs like above but with built in RCDs (RCBO) that plug into the fuse section? I would like to protect my ring that runs the washing machines, dryer, dishwasher etc.. and this would be a quick route if possible.

 

[ebee]

Ericmark,

"So if it complies so no longer than 3 meters as with regulation 433.2.2 then a risk assessment is likely to show a very low risk."

Are you saying that a spur from a ring (all 2.5/1.5) should not exceed 3 metres length?

PS A twin socket just like a single socket is actually rated at 13 amps although it is type tested to 20 amps - although I did know someone who insisisted they were 30 amp sockets because the went on a 30 amp ring

 

[davelx]

Ericmark,

"So if it complies so no longer than 3 meters as with regulation 433.2.2 then a risk assessment is likely to show a very low risk."

Are you saying that a spur from a ring (all 2.5/1.5) should not exceed 3 metres length?

PS A twin socket just like a single socket is actually rated at 13 amps although it is type tested to 20 amps - although I did know someone who insisisted they were 30 amp sockets because the went on a 30 amp ring

If you stick to the 3m rule per 433.2.2.(ii) (, you don't have to worry about/bother doing the adiabatic calculations to ensure compliance with 433.2.2(i)->434.2.2->434.5.2 (don't you hate the way the regs have multiple linked references you have to follow to get to the actual requirements!)

 

[ebee]

the 3 metre rule of 433.2.2 need not apply to spurs from rings because overload protection is via the plugtop fuses

 

[davelx]

the 3 metre rule of 433.2.2 need not apply to spurs from rings because overload protection is via the plugtop fuses

You still have to ensure that FAULT protection is provided as I described above, otherwise there is still a 3m limit under 434.2.1

 

[ebee]

Absolutely and you would not have unlimited lengths on spurs (or rings themselves for that matter) for that reason (& volt drop considerations too) but it still does not mean a need to apply the 3 metre rule