Lighting from fused spurs

[Pretty Mouth]

Evening all. Your thoughts please:

I was helping out another spark today. The job was to spur from a ring final to a 13A fused spur, to feed several sockets and some lighting in a loft conversion. For the lights (regular common or garden pendant sets) he asked me to fuse down further to 3A, via another fused spur. It's his job so I did it, but I wonder if the 3A fused spur was really necessary?

The relevant reg seems to be 559.5.1.204:
Lighting circuits incorporating B15, B22, El4, E27 or E40 Lampholders shall be protected by an overcurrent protective device of maximum rating 16 A. To me, this is achieved by the 13A BS1362 fuse, but perhaps there's something I'm missing.

 

[Midwest]

I’m thinking he was thinking a typical domestic lighting circuit is normally on a 6A ( mcb). Only thing, that might have bearing, is installation method for ’lighting’ circuit of the new install?

 

[SJD]

Aren't many light switches only rated to 10A?

 

[Pretty Mouth]

I’m thinking he was thinking a typical domestic lighting circuit is normally on a 6A ( mcb). Only thing, that might have bearing, is installation method for ’lighting’ circuit of the new install?

I'm thinking the same Midwest. the lighting part was in 1mm T+E, ref method was 102 (the lighting was for the loft eaves/storage area, accessed though a hatch) so should have been ok.
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Aren't many light switches only rated to 10A?

Good point SJD, but would that matter? The only current flowing through a switch would be from the lights it controls wouldn't it?

 

[SJD]

If a lamp fails short circuit, it would be better not to put more than the switch's rated current through the switch.

 

[Midwest]

Aren't many light switches only rated to 10A?

Although, I did read something somewhere that they are rated to 13A for resistive load?

Or is it inductive

 

[Pretty Mouth]

If a lamp fails short circuit, it would be better not to put more than the switch's rated current through the switch.

Aha, I think I see what you mean. I'm glad I asked the question as I wouldn't have thought of that. Thanks!
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And some switches are just 6A:

British General 6A 2-Way Pull Cord Switch White - Screwfix

Order online at Screwfix.com. Ceiling pull switch. Controls lighting circuits. FREE next day delivery available, free collection in 1 minute.

www.screwfix.com

 

[davesparks]

It's normal/traditional to have domestic lighting circuits on 6A circuits and some people will just not deviate from this through stubbornness, lack of understanding
Or even if they do understand they know that the next electrician to look at it will likely be someone who will shout loudly about how wrong and dangerous it is.

 

[Pretty Mouth]

Just thinking out loud about this one. @SJD , if a lamp failed short circuit, quite a large current could flow in the brief time it would take for the fuse to blow. We are able to show if a conductor could withstand the resulting let through energy using the adiabatic equation, but for obvious reasons we can't apply this to a light switch.

I wonder if there is something in BS EN 60669-1 that says a switch must be able to withstand a minimum I²t?

 

[SJD]

Just thinking out loud about this one. @SJD , if a lamp failed short circuit, quite a large current could flow in the brief time it would take for the fuse to blow. We are able to show if a conductor could withstand the resulting let through energy using the adiabatic equation, but for obvious reasons we can't apply this to a light switch.

I wonder if there is something in BS EN 60669-1 that says a switch must be able to withstand a minimum I²t?

To be honest, if I'm using switches rated at 10A, I just make sure the circuit is on 6A or 10A MCB, or a 3A or 5A fuse. In your original example, I'd have just included the 3A FCU without really thinking if I could get away without it.

Actually the comment from davesparks is true, it is a worry if the next electrician along is going to suggest something I've done is non-compliant, and I'm not around to defend it.

Once in a while I come across a failed switch where the customer has been replacing a light fitting, mixed up the wires, and put the switch directly across L & N, and a 6A MCB doesn't protect it.

 

[Pretty Mouth]

Or even if they do understand they know that the next electrician to look at it will likely be someone who will shout loudly about how wrong and dangerous it is.

Actually the comment from davesparks is true, it is a worry if the next electrician along is going to suggest something I've done is non-compliant, and I'm not around to defend it.

Even with my limited experience, I see this sort of thing a lot in our industry, and it is a concern because the customer won't know who is right. Classic example is light switches in bathrooms outside of the zones. Perfectly compliant, but no one ever does it, and is assumed to be regulation.

So yeah, I can see a case for doing the 'done thing', even if it's not perhaps necessary

 

[Mark Wright]

You got A pretty mouth

 

[Pretty Mouth]

You got A pretty mouth

Squeal like a pig boy etc

 

[Pretty Mouth]

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.

 

[Pauls0212]

what about a 3 amp fuse