Lighting circuit off a fused spur

[Jupiter01]

I’ve seen on a few occasions that lighting circuits are fed off a fused spur running off the RFC as opposed to the lighting have a dedicated circuit with a protection device at the consumer unit.

It’s also been mentioned for shed lighting in here - I think I recall from previous posts.

Q. If this is fused up to 6amps, that’s fine I suspect but what happens when an unsuspecting householder replaces the fuse with a 13amp?
Would this put the 1mm t&e cable at risk or does it have protection up to 13 amps if it is buried in insulation and all the usual stuff?

It feels like a bad practice but interested in your thoughts.

 

[EFLImpudence]

I’ve seen on a few occasions that lighting circuits are fed off a fused spur

Fused Connector Unit - the cable is the spur; fused or not.

running off the RFC

Did you mean Ring Final Circuit or Radial Final Circuit?

as opposed to the lighting have a dedicated circuit with a protection device at the consumer unit.
It’s also been mentioned for shed lighting in here - I think I recall from previous posts.

It depends whether there is a lighting circuit to use or not.

Q. If this is fused up to 6amps, that’s fine I suspect but what happens when an unsuspecting householder replaces the fuse with a 13amp? Would this put the 1mm t&e cable at risk or does it have protection up to 13 amps if it is buried in insulation and all the usual stuff?

Nothing; as long as the cable is adequate to cope with any fault current until the 13A fuse blows.
As 1mm² T&E is good for 16A (method C) it will be.

It feels like a bad practice but interested in your thoughts.

There are lots of things that are done out of habit which are not really necessary.

 

[ericmark]

The old idea was sockets and lights come from diffrent circuits so if a fault happens, you don't lose lights at same time as sockets.

If there is alternative lighting like a window, then there is clearly no problem, but using that idea having sockets and lights on same RCD would also be a problem.

So looking at a risk assessment, having a rechargeable torch which auto switches on with a power failure would remove the risk.

1 mm² will take 13 amp, however ceiling Rose's typically rated 5 or 6 amp, so normally we look at a 5 amp fuse.

But nothing really wrong using a fused spur.

 

[Jupiter01]

Did you mean Ring Final Circuit or Radial Final Circuit?

Sorry, I meant a Ring. Would it be any different if it were a Radial?

Nothing; as long as the cable is adequate to cope with any fault current until the 13A fuse blows.
As 1mm² T&E is good for 16A (method C) it will be.

Thats very clear. Thanks

 

[333rocky333]

Sorry, I meant a Ring. Would it be any different if it were a Radial?

Not really, traditionally Rings and radials are fused at 20A or 32A, whereas lighting 6A or occasionally 10A, hence the reason the so called "spur units are sometimes used to fuse it down."
The "spur" unit if switched, can also negate the need for a normal lightswitch.

Similar idea with the units sold as garage consumer units.
They often come with a 32A mcb and a 6A mcb fitted, which does a similar job of fusing down the lighting circuit.

 

[EFLImpudence]

Sorry, I meant a Ring. Would it be any different if it were a Radial?

No, they both begin with 'R'.

 

[ericmark]

Sorry, I meant a Ring. Would it be any different if it were a Radial?

The ring final is normally 30 amp fuse or 32 amp MCB/RCBO using 2.5 mm² cable (1.5 mm² is allowed with mineral cable but that is rare). A radial final can have a range of cables sizes, with 2.5 mm² one would expect to see a 20 amp overload, with 4 or 6 mm² a 32 amp over load, but there is nothing to stop a 16 amp overload feeding a radial final with 1.5 mm² cable.

The point is a 2.5 mm² radial can have a branch from it looking at the socket exactly like that of a ring final, but because supplied by a 20 amp fuse, it does not require a fused connection unit to feed more than one socket.

The problem with the radial is volt drop, it is considered (although no hard and fast rule) that a ring final can have 106 meters of cable, when first told I had a problem emulating the calculations.

The design current is taken as 20 amp centre and 12 amp even spread, and the tabulated mV/A/m is corrected, but do same calculations for a radial

and one drops from 106 to 32 meters, so to replace one ring final you need three radials using 2.5 mm². It also means three RCBO's instead of one, so more expensive to use radials to ring finals. Also it is less able to take the inrush, I have seen many a 110 volt yellow brick that worked without a problem on a ring final, but would trip the B20 MCB on a radial final due to inrush.

The problem that I found was the calculations for volt drop are not easy, which is why I wrote a java script program to do it for me, main point the results for a ring final were known, so I could test the program gave me good results.

As to volt drop on a spur, I have never written a program to work it out, and today with RCD protection and switch mode power supplies, not sure the loop impedance is quite as important as in the old days, things like solar panels and EV charging points are often limited to the 207 to 253 volt range to ensure they trip with a loss of PEN, but neither are really DIY jobs so no need to explain further.

However many people ignore the inspecting and testing, and the line - neutral loop impedance is not recorded, only the line - earth, and this is calculated as a B = 3 to 5, C = 5 to 10 and D = 10 to 20 times the thermal cutout setting, plus 5% safety margin, so B32 need to be able to supply 32 x 5 x 105% = 168 amp, so 230/168 = 1.37 Ω, the problem is the cheap plug in testers are designed for 20 amp as used with rest of Europe, so the pass figure is often 1.9 Ω great for when using a B20 which needs 2.19 Ω but not really much good with a ring final.

So instead of a tester costing £50 you need a tester costing around £200, OK should also test the RCD and insulation resistance etc, but it means to inspect and test a ring final you need a low ohm ohmmeter (must use 200 mA so multi meter no good) or a loop impedance tester. So the fused spur you can test with the cheap £50 tester and most radials, so for the DIY man the fused spur seems the best option.

The Martindale EZ365

does a reasonable job, at around £65.34 both loop and RCD, although it does not measure the time, and 40 mS is not really some thing one can measure with a stop watch. But good enough for most radial finals to test them.

We are looking at near enough here, and can't really say they all OK, but unlikely you will have a problem if it shows OK, and if we expect the DIY to buy the full test kit, I think we are living in cloud cuckoo land.

 

[ebee]

and if we expect the DIY to buy the full test kit, I think we are living in cloud cuckoo land.
and if we expect the All So-Called Electricians to buy the full test kit, I think we are living in cloud cuckoo land too.

 

[ericmark]

When I used the LABC route the inspector wanted to see my test equipment, and I would hope the scheme providers also want to see the test equipment, can't force electricians to use it, hard to prove the results have been fudged, but should be spot checks, as to how far out measurements can be is some thing else.

The program shown was written as I was worried about getting caught out. Fit a ring final with a loop impedance of 1.5 Ω and in theory you could have to correct it, swapping cable runs for 4 mm² or reducing the MCB/RCBO to 20 amp, some way some how as the installer it needs correcting.

This could work out very expensive, so if one tests on every job, you get a feel for when running close to the wind. My worry was doing an EICR and missing it, and next guy finds it, and the customer wants you to correct as you missed it. However after writing the program I realised the readings with an earth loop impedance meter are around 0.02 Ω either way, so needs to be 0.04 Ω out to really show an electrician has made an error.

As the DNO supply can also vary, in real terms hard to prove the electrician has made an error unless well over the limits, I have see a DNO supply at 0.24 Ω which the DNO by changing which part of their ring is in use, could move to 0.35 Ω meaning the reading can be out by 0.11 Ω and you can't prove the electrician has made an error.

 

[ebee]

." My worry was doing an EICR and missing it, and next guy finds it, and the customer wants you to correct as you missed it." Not sure I`m with you on that one Eric lad. If you missed it and that in itself caused expense further on then yes I`m with you but if you missed it then someone else found it then what would you likely be liable for? you had not caused it.

I once had occasion to comment on things I found after another electrician had done some work. OK a couple of items he did not too good at all but also he did not correct main bonding to water. The cable was there to see at the mains end and it disappeared in the kitchen seemingly towards the stop tap area but I did a continuity check and got bad result. I pulled the kitchen end as it disappeared into plaster and hey ho a cut end plastered into the wall leading to nowhere. I agreed price with him and replaced it. He was a bit miffed off with this other electrician and said "He even left the rubble on site too", turns out I knew well this first electrician. A year or two later I was on a job and he was there, we had a catch up and I mentioned this job by saying "Whoops I might have inadvertently dropped you int it" meaning the none too good items I had seen. He explained he had since corrected them free of charge then admitted he had not checked bonding continuity so was fooled by a purely visual aspect. Turns out this cheeky customer had insisted he reimburse the cost of renewing the bonding. I said "Cheeky git, that was not your omission that it was not present but merely you`d not noticed it but if you had you would have corrected it and charged him for the correction accordingly so he was no worse off with your omission anyway. He did moan about you not disposing of the rubble". He told me he had said to this customer at the outset that he would bag the rubble up (which he did) and that the customer would have to dispose of it before he even costed the job.
When I had been on the job the customer had also mentioned an outrageous amount he`d paid for numerous downlighters installing, I asked him how much then I exclaimed "Oh so you had already cut the holes in the ceiling prior to the electrician fitting them, wow it is a very cheap price (about half of what I would charge) " he said "no he cut the holes too". Blimey not outrageous at all, far too cheap in my opinion.
Thing is, the customer rang me a few months later asking me to do another job. I had originally done his job as a help to his appointed electrician (call him Phill) and he had not even asked Phill in the first instance in the first place, he tried bypassing Phill by asking me directly. Needless to say I told him I was too busy, I had decided this was a customer I could do without.

some folk just get up yer nose.