Lighting from fused spurs

[Ian1981]

Hi all. Just wanted to resurrect this one as I've found something in the regs that would mean a 13A BS1362 fuse may not be appropriate for protecting lighting in many cases, and wouldn't have been suitable in this instance. It's to do with the co-ordination between conductor and overload protective device.

433.1.1
The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:
...
(iii) the current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit.

For a 13A BS1362, it appears I2 is 26A.

26/1.45 = 17.9A

So for 1mm² T+E, the 13A fuse would not comply in any case, and for 1.5mm², it would only comply if ref. method C.

View attachment 57198

A single light or lights wouldn’t necessarily require overload protection by nature of the load fed from a fused spur

 

[Pretty Mouth]

A single light or lights wouldn’t necessarily require overload protection by nature of the load fed from a fused spur

I was just beginning to think I'd got this sussed

 

[lozarus]

Hmm. There's certainly a few sides to this. I must admit on the odd occasion I've been in situtions like this (for example in my own under stairs cupboard) I have taken a 13a FCU from adjacent RFC > 2x Sockets > 5a SWFCU acting as the light switch itself.

I think for me, in that instance it just acted as 'neat' way of changing conductor sizes down to the 1mm I used for the light. And just because it seemed logical. The source FCU was some distance away, it may "look wrong" if someone were to see a light fitting coming straight off the back of a socket (albeit fused 13a at the source FCU). The FCU for me acted more as a light switch and differentiation point.

 

[Dave OCD]

I just think it's always good practice for the OCPD to be rated lower than the ccc of the circuit cable,regardless of the 'fixed load' and it's no bother in a loft /outbuilding or similar to fit a sfcu instead of just a normal light switch. Always done it this way and I see no reason to change now.

 

[pc1966]

For a 13A BS1362, it appears I2 is 26A.

Where did you find the current/time plot for the BS1362 fuses? I have not seen that in any (obvious) place the few times I looked.

 

[Ian1981]

Where did you find the current/time plot for the BS1362 fuses? I have not seen that in any (obvious) place the few times I looked.

Maximum Zs for a 13 amp bs1362 fuse is 2.30 ohms 0.4 disconnection time , table 41.2
For a 5 second disconnection time it is 3.64 ohms table 41.4
Or 13 x 1.45= 18.85 amps
[automerge]1587045045[/automerge]

Where did you find the current/time plot for the BS1362 fuses? I have not seen that in any (obvious) place the few times I looked.

Maybe try this

 

[pc1966]

Maybe try this

I saw the graph, just wondered where it came from as I have not seen it in any other book, etc, so far.

 

[Pretty Mouth]

I saw the graph, just wondered where it came from as I have not seen it in any other book, etc, so far.

It's a secret, I'm not telling you
[automerge]1587063076[/automerge]
I have no idea where I got this from, but here it is:

 

[pc1966]

Thanks, great to actually see it!

 

[7029 dave]

I just think it's always good practice for the OCPD to be rated lower than the ccc of the circuit cable,regardless of the 'fixed load' and it's no bother in a loft /outbuilding or similar to fit a sfcu instead of just a normal light switch. Always done it this way and I see no reason to change now.

Coz we are old school Dave.

 

[lozarus]

Sometimes you need to simplify things I say. Is adding an FCU before a light switch make the situation dangerous. No? Does it break any regulations? No. Would it make sense to a future electrician? Sure, even if they don't agree with it. So therefore it's not worth worrying about IMO.

 

[Pretty Mouth]

Sometimes you need to simplify things I say. Is adding an FCU before a light switch make the situation dangerous. No? Does it break any regulations? No. Would it make sense to a future electrician? Sure, even if they don't agree with it. So therefore it's not worth worrying about IMO.

Indeed, it is a very minor point. But I enjoy chatting carp about the minutiae of our trade

 

[lozarus]

Indeed, it is a very minor point. But I enjoy chatting carp about the minutiae of our trade

Oh absoloutely (I can get into pretty boring in depth detail about 13a plugs!) I just felt it a point worth mentioning. I'm guilty of getting wrapped up in thought on a job and often find myself having to strip it back a bit when in a job situation.