Re: 10 amp lighting circuit.

[Fredxx]

>> I duly ordered one (Hager) from TLC - they list them but the branch >> doesn't stock them. After two weeks of phoning and it hadn't came in I >> got >> one elsewhere. And went to fit it today. Only to discover the lighting >> MCB >> was a 10 amp one, and I'd got a 6 amp. I suppose I should have checked - >> but the CU is so high it requires steps to read that sort of thing. >> >> But I'm slightly concerned about just fitting a 10 amp C type. The wiring >> is original so about 25 years old. It's a small two bedroomed town house >> with only one lighting circuit. But lots of low voltage lighting. The >> lighting wiring is definitely 1mm at the CU. The house is timber framed >> with brick cladding, and well insulated. The CU is pretty new and 17th >> edition. > > Well 1mm is good for 16A if not in insulation etc. So that may still be > ok. Have you counted up the total actual load? Could it be that the > nominal load is actually under 6A and the installer was just trying to > cure surge problems by over speccing the MCB? > > (note also on a 10A circuit some smaller lamp fittings like SES, will be > inadequately protected) >

All light fittings I've seen are rated at 6A. I therefore thought the MCB rating should match this, but am I wrong here?

After all the rating of fitting would be associated with the load and not fault current?

[Pete Cross]

I had this problem with a new light fitting, 300va toroidal and umpteen 12v halogen bulbs on end of umpteen stalks. Blew mcb every time. I had a couple of VA1104 inrush current suppresors ( ntc thermistor ) from my tv repair days and one of those in series with the light cured it, it's been working now for 2yrs. Charles Hyde used to stock them for about £5 each. Dunno why light manufacturer doesn't build one in.

Pete

[John Rumm]

There are occasions where protection at 10A is permitted - but it does preclude some types of fittings, or may require additional fusing to protect them.

Well both really - it obviously needs to cope with the load (not usually a problem), but also fault currents caused by either bulb failures or botched attempts at bulb changes with the power on etc.

[Dave Plowman (News)]

Sounds good. I did wonder replacing the transformer with a soft start electronic one - but they seem to be a max of 250 watt. And splitting the fitting into two lots wouldn't be easy to achieve neatly.

[ARWadsworth]

I can find electronic transformers that will power 300va. They are about £100.

Adam

[Dave Plowman (News)]

Right - I couldn't unless looking for a general purpose PS. But I think I can sort things for rather less than that. ;-)

FWIW, a 300 Va transformer costs about 40 quid.

[ARWadsworth]

I am hoping the 6A C type MCB will sort it:-)

Adam

[Tim S]

Tim S coughed up some electrons that declared:

Although - there is another consideration, that I've not been totally clear on:

What about the max load capacities of accessories? Some switches are 10A, some, eg fan isolators, some light switches and dimmers are stated at 6A. Technically most dimmers would overload beyone 1-2A.

One could argue that the switch load only need be sufficient for the design load of the sub-circuit it controls. Others might argue that the switch max current should be

[Tim S]

Some interesting info:

Regulation 559.6.1.6: "Lighting circuits incorporating B15, B22, E14, E27, E40 lampholders shall be protected by an overcurrent protective device of maximum rating 16A"

And, from the Green IEE Electrican's Guide to the Building Regs, the "standard lighting circuit" is described as a 10A Type B or Type C circuit.

Looks like the IEE/IET is strongly pushing for 10A being the norm, and allows upto 16A.

Whilst the regs have a nice override on the standard lampholders, irrespective of the lampholders design rating, one thing it doesn't address is ancilliary equipment that wants a lower level of protection, like the fan I saw that stated 6A.

Bit of a mess the whole thing...

Then again, as I said before, I doubt it would make any difference in practise if the fan when dead short whether the breaker is a 6A or 10A, type B or Type C as long as the circuit is designed for it.

If the fan (etc) developed a peculiar L-N fault where it managed to draw 7A, I'd expect it to be on fire fairly quickly and neither device would afford any protection. Ditto any other ancilliary equipment like smoke alarms. Perhaps the answer is to plug all ancilliary equipment in via fused 1A clock connector plugs? But this is dodgey with smoke alarms, as there'd be no indication if the fuse had blown.

My inclination, in the light of the regs is to say sod it and use 10A, provided the disconnection times for the core wiring is met.

Cheers

Tim