DIY Doctor


Postby kbrownie » Sat May 17, 2008 3:52 am

Cheeky question.
If discrimination is used when cables of a different type/csa are connected together. Why is there not a protective device at the ceiling rose between the circuit and the lampholder flex/light fitting wirering.
Especially if circuit cable is in 1.5mm.
Rank: Project Manager
Posts: 1995
Joined: Mon Mar 05, 2007 9:36 pm


Simply Build It

Postby ericmark » Sat May 17, 2008 11:25 am

This is why we are limited to 5/6 amp on lighting circuits although the BS7671:2008 says 16 amp. It also says we must comply with manufactures instructions which limit the rose to 5/6 amp.
If you look at lighting track the plugs all contain fuses so a 16 amp supply is not a problem giving 72 HF ballasts on a three phase 16 amp supply switched together on a C type breaker.
4 Curcuit_Protective_Devices.pdf from Hager (Spelt as Hagar did) is very handy in working out how many lights can go on one circuit.
Even using Klik sockets with florescent lamps although no fuse in plug most fluorescents have fuse in the lamp. The Klik are rated 6amp.
The ceiling rose is the only item where you can access line terminals without the use of a tool a special case in BS7671

Postby ericmark » Sun May 18, 2008 1:21 am

I have looked hard at 314 and due account shall be taken of the consequences of the operation of any single protective device together with take account of danger that may arise from the failure of a single circuit such as a lighting circuit and then one wonders what we can really do with RCD’s and still comply.
I would consider if the load is likely to exceed that which the single circuit can supply and as a result we split it then there is no need for two RCD’s the same if we split to reduce volt drop. If we split to avoid danger which may arise separate RCD’s would be required. So if the house was supplied with a single lighting circuit it would neither exceed the volt drop of become overloaded and yet we still split in into two then we would need separate RCD’s. The same goes for other circuits so where a 10 way board is used and the total of all the MCB’s adds up to 200 amp yet the main incoming fuse is only 80 we can hardly claim we have split it into 10 circuits purely to overcome volt drop and overload considerations and although I would agree we would not need RCD’s on every circuit neither could I justify just 3 RCD devices as shown in the Hagar diagram. We all know the problems with sockets and cooker supplies and to comply with 314.1(iv) each would need their own RCD device. But to provide so many devices would require the use of RCBO’s and the PFC is lower than MCB’s typically 4.5kA instead of 6kA so may not be suitable. The chances are what ever you install in first few months of 17th Edition will pass without too much scrutiny but as we get to doing the first PIR’s the trade will have decided what is acceptable and should a mistake be made on a housing estate the results of a fail could be really expensive. 537.2.1.5 and 537.2.2.4 also gives rise to concern as I have not seen a method of locking consumer units or individual MCB’s 537.2.2.5 as I read means using a single phase MCB is not accepted and the twin pole RCD or Isolator must be used again I have not seen any means of locking these off. I had this problem in one factory that I worked in the moulded breakers had add on bits that would allow locking off when mounted in a multi-breaker board but not when in a single breaker unit and we had to remove the lid and weld on a hasp and staple so it could be locked off. Years ago it was easy fuse carriers and fuse when in your pocket until finished not so easy with MCB’s. The problem is once one person gets hurt it’s not just the one installation that’s altered but all they can find with same system and someone has to pay for the alterations. Once first PIR is done you are reasonable safe but that could be 10 years away. And all Electrical Installation Certificates are held by building control but they don’t hold PIR’s so proving you were not the last person to work on the system may be hard. As electricians should we be designing house electrical systems or should an electrical engineer design the system and should we only follow his instructions as done with commercial installations? Maybe its just a case of having right answers? “I was told a cupboard was being built to enclose the consumer unit my honour which was to include a lock.â€

Postby kbrownie » Sun May 18, 2008 8:34 am

Hi ericmark, thanks for reply not really what I was getting at,
If we had a lighting circuit in a domestic property and the circuit cable was 1.5 T&E and the protective device was rated at 10amps.
At the ceiling rose the flex cord is 0.5mm csa.
A 10amp fuse wire is, I believe is 0.5mm.
Discrimination asks for a protective device to be placed where cables of a different csa are conected together, is that not right.
I have yet to come across a device in a domestic install at the ceiling rose.
Now is it because the length of the flex offers less resistance. i.e. normall length apporx. 300mm and a little longer for pendants.

I never really trusted the method of removing a single fuse, unless the enclosure could be locked off. As it was easy for someone to put another fuse in or swap them about. Scares me.

PS how did the The exam go? Not been on hear that much of recent, so never picked up how did.
Rank: Project Manager
Posts: 1995
Joined: Mon Mar 05, 2007 9:36 pm

Postby ericmark » Sun May 18, 2008 11:41 am

Exam was disappointing I got one wrong in Appendix's a pass of course but I wanted 100%.
And yes I agree that it does seem as if a fuse should be fitted. Although in panels it is allowed for fuses to be on appliance rather than supply side and I seem to remember it is dependent on cable length and since there is suppose to be a fuse built into each bulb I would think we can get away with it.
But the protective device was rated at 10amps does present a problem as most ceiling roses are rated at 5 or 6 amp.
416.2.4 has exemption for ceiling rose. (412-03-04)
559.6.1.1 (553-04) starts a whole series of regs on lighting and 559.6.1.6 (TABLE 55B) gives the 16 amp rule.
611.2 says must follow manufacturer's data. Also 134.1.1 as does 421.1 and 510.2.
This all means in most houses we still have 5 or 6 amps max on lighting circuits as that is manufactures limits placed on ceiling roses. I will agree I have seen 10 amp MCB's on lights but in the main on domestic lighting there is something somewhere which is only rated 5 or 6 amp. And with 0.5mm² being only rated at 3 amp one wonders if we should not be using 0.75mm² ratted at 6 amp? Looking at Table 52.3 I think 0.75mm² is the smallest cable we are allowed to use? If that is correct then with a limit of 6 amp on lighting circuits we have no problems.
As to current at which copper will fuse I look at Table 53.1 but this gives diameter rather than cross sectional area. Using =PI()*POWER(A1/2,2) where A1 = dia I get.
Amps Dia CSA
3 0.15 0.017671459
5 0.2 0.031415927
10 0.35 0.096211275
15 0.5 0.196349541
20 0.6 0.282743339
25 0.75 0.441786467
30 0.85 0.567450173
45 1.25 1.22718463
60 1.53 1.838538561
80 1.8 2.544690049
100 2 3.141592654
So your 0.5mm cable would be more like 25 amps before it fused.
Am I now answering the question? I don't have a BS7671:2001 only a BS7671:2008 so I can't cross ref all I find.

All best Eric

Postby kbrownie » Sun May 18, 2008 6:06 pm

Hi Eric, Thanks agian for reply and congragulation on pass 1 wrong a fine effort I would say, even if you are a little dissapointed. I take mine next month any tips, thinking about going back in to the trade. It was 15th ed'n when I was last involved, I have been going to college on an evening to oil the rusty bits in the old grey matter though. but not strictly for 17th ed'n requirements.
Thanks agian for answer did come up with some usefull info and without you knowing also answered a something that's been puzzling me.
Rank: Project Manager
Posts: 1995
Joined: Mon Mar 05, 2007 9:36 pm

Postby sparx » Sun May 18, 2008 9:16 pm

Hi both, I think you will find reason for pendant not requiring fusing is the possible total load on one lamp is so low you could not possibly over load flex. Largest lamp available is 100w so even fitting with 3 lamps nowhere near current rating of 0.5mm2 flex, any larger fitting would break pendants weight limit before overloading electrically!
3 X 230= 690w.....load limit 2 kG.....
regards SPARX
(well passed ERIC)
Rank: Project Manager
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

Postby ericmark » Mon May 19, 2008 9:17 am

The IET web site (Was IEE) has some corrections for the book of questions and answers which may help. I also have some but I did up-grade not full 17th Edition so was only asked questions on the changes. Try typing Eric Mark Palmer into google and you should find links either to my family tree or CV and that should give you my email address and I can forward questions I have. Or look on IET web site I use same log in name and you can send personal message. We are not allowed to give email address on here.

Postby kbrownie » Tue May 20, 2008 9:16 am

Hi Eric,
Thanks for the offer of help, found your CV and tried the email a couple of times but thw messages are failing to be sent.
I'll try it from home tonight just in case the pc at work is not doing as it should.
Rank: Project Manager
Posts: 1995
Joined: Mon Mar 05, 2007 9:36 pm

Postby ericmark » Tue May 20, 2008 2:28 pm

My login name here at

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides
  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!