17th Edition - Interpretation Ask questions and find answers to many subjects relating to electrics and electrical work

[ericmark]

Three questions:-

What is a circuit? An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s). is the 17th Edition definition so where we have an RCD feeding a number of MCB’s is the circuit what is protected by the RCD or what is protected by a MCB if the former then Part P will not be required when a MCB is added to the RCD circuit?

The second is rather more of a problem. Where a central heating boiler uses an SELV supply to the thermostat how do you wire this in a normal house?
We are not allowed earths on a SELV supply so surrounding with any earthed protection is not an option and neither is the use of an RCD and an IMD will not work as the circuit is never deactivated. Although there is not a danger of getting a shock generally for SELV which could also include extra low voltage lighting there is a problem with high currents and fire so the rules 522.6.5 to 522.6.8 can not be dismissed as not referring to SELV systems as well. Alarms will also have the same problems and again one can see why burying a fire alarm cable within a wall where damage to the cable could go unnoticed until it completely fails may be something requiring redress and may be it is intended to prevent to burying of such cables in the future. This will suit post build alarms but can’t see the site agent on new builds liking the idea of exposed alarm cables?

Last question is when. “BS 7671:2008 Requirements for Electrical Installations was issued on 1st January 2008 and is intended to come into effect on 1st July 2008. Installations designed after 30st June 2008 are to comply with BS 7671:2008.â€

[kuzz]

My interpretation would be 1)mcb protect against over-current rcd does not. 2)would happily bury selv cables in correct zones. 3)yeah could still be working to the 16th in 2 years.

[kbrownie]

Hi,
would like to point out to my understanding that RCDs are not for protecting the circuit but indentify earth leakage over the devices permitted amount.
KB

[ericmark]

That would be a RCM or IMD not RCD although I have seen combined units with bar graphs for RCM function and wired into a mounded breaker for RCD function. I am sure it is intended that fitting a extra MCB into a consumer unit will be a new circuit but if that is the case it should also include a fused spur so either a fused spur will require Part P or and extra MCB on an existing RCD will not. As yet no one has asked me to do it but it is always as well to work it out before the crunch. Although as it is the responsibility of the person ordering the work to ensure building control regulations are complied with not really my problem. Kuzz answer is more like the answer required RCD only protect from over current to earth not through circuit so yes that would make sense still leaves fused spur question though.
As to being happy at burying SELV cables so would I but could an inspector then use that to refuse a completion cert?
I see on another post Sparx has also talked about when 16th Edition work will cease and has pointed out it would need a code 4. On an inspection and test I would agree but not so sure on a Installation certificate so long as 16th Edition forms were used on 17th Edition regs alone it would be OK but if for example electricity at work act says current version of BS7671 should be used it could override the 17th Edition's own rule or could it?
Its like the question in exam where does 17th Edition apply and supply authority is not included yet the supply authorities own regulations says BS7671 must be complied with.
I think the main problem is they are very good as recommendations which is what they are intended to be but not so good as law which Part P and the Electricity at Work act has turned them into.
It is the same as trying to make the AA hand book law! The highway code is a guide to the law with a few extras but the law does not say the highway code has to be followed. The HSE with electricity at work act is not a problem as they use common sense and consider monitory considerations as well as rule books. But the County Councils building controls have too many jobs worth working for them who if not sure will fail rather than use common sense.

[kbrownie]

Hi again, I know you are a man of great electrical knowledge and I bow to that, but I was always told that the RCD orginally ELBC (earth leakage circuit breaker) operates on a fault to earth and is not there to protect the the circuit but to save lives.
If this statement is untrue my college lecturer need to be had a word with!
KB

[ericmark]

Please accept I am being pedantic, but they do trip on overcurrent although to earth rather than line to neutral. I do in essence agree with what you are saying, but should it suit to consider it as protecting against over current to earth I can’t really see a way of saying that’s not true. Although I made one big mistake. The “back to the consumer unitâ€

[kbrownie]

Hi ericmark,
It's a good point well made and in the most I agree with you, Part P as well as it is mean't and intended. As regards to the electrical safety of domestic installations etc.. Fantastic idea! But surely BS7671 2001/2008 should be made the legal document along side the ewar and part p a guide to the domestic side of the electrical installations. As it stands it's a little crazy that Part P is a legal document and BS7671:2001/2008 is not.