SWA supply to garage consumer unit

[Dambo]

Ask away, all questions are legitimate. Sometimes we all ask things and then realized we knew the answer all along . Just confirming what we think we know. The MCB is overload protection, the RCD is earth leakage protection.

If you had a run out to a garage protected by a 20A MCB in the house consumer unit then [in simplistic terms] all that can be drawn down that cable is 20A. If you fit a garage consumer unit with a RCD rated @ 100A and a 16A breaker for sockets and a 6A breaker for lights it doesn't matter. You can't suddenly pull 22A (16+6) because the MCB in the house won't let you.

Make sense?

Aye, that all makes sense Vet What I don't completely understand is, if for argument sake the RCD in the garage was 40A / 30mA, then surely the MCB at the main CU only has to be able to deal with the garage load (16+6) i.e. 32A MCB or bigger would be sufficient.

 

[ianmacd]

Im all for helping people out if they are stuck, however, these questions are very basic, plus the fact the NICEIC have passed him for registration!!!!! W T F?????This trade just cant be learnt from the internet and forums such as this (as good as it is) and can only be learnt from experience.

Yes i would prefer someone ask rather than just guess and go for it.

This is the whole reason i am no longer directly involved with this industry.

We have got all these "electricians" attending courses, whatever ones they are, getting qualified and then working on peoples home "via the 'net".

All i can say is im glad i work on my own house and dont have to rely on the services of an electrician.

Well I hope that you are either registered with a little club, notify LABC or do not undertake notifiable work then.

 

[ianmacd]

It might silence the critics if you could fill us in on your qualifications and experience... as someone who is considering becoming qualified and registered myself, but finds the huge amount of knowledge required is both frightening and daunting, I would personally be very interesting to find out.

No. Why should he need to defend himself? He clearly knows enough to know he needs to know more and thats more than most people not on this forum. If someone thinks the question is 'too basic' for them then they should just ignore the thread.

 

[Dambo]

It might silence the critics if you could fill us in on your qualifications and experience... as someone who is considering becoming qualified and registered myself, but finds the huge amount of knowledge required is both frightening and daunting, I would personally be very interesting to find out.

I trained as a domestic electrician through New Career Skills. It took me about 18 months to complete the course. I passed my C&G 2377, 2382 and 2392 and the EAL certificated domestic installer scheme.

I came into the industry with no prior experience and worked hard to learn as quickly and as thouroughly as possible. I done some voluntary work while doing the course to try and build up my experience. Once qualified I tried to get in with a firm to learn on the job while getting paid but vacancies were scarce and the ones that were available required at least 2 years practical experience (which I don't have). So, after have spoken to course tutors and local electricians, I decided to go it alone but work on smaller jobs to try and build up my confidence and experience, which i've been doing.

It probably sounds stupid but I actually end up already knowing the answers to a lot of the questions that I ask on here, but it's good to get the opinions and advice of the long-serving electricians on this forum.

There are a lot of things to learn and take in but I know i'm getting there

 

[kme]

What I don't completely understand is, if for argument sake the RCD in the garage was 40A / 30mA, then surely the MCB at the main CU only has to be able to deal with the garage load (16+6) i.e. 32A MCB or bigger would be sufficient.

The amperage rating of an RCD is the maximum it can safely handle. Think of it like a speed limit on a road - the maximum speed classed as safe is not necessarily the speed you do all the time.

Similarly, the RCD MUST be able to carry the maximum current the MCB could let through. It doesn`t matter if it higher - i.e.MCB20A, RCD 32A, 100A, 320A - Any one is fine.

Yes??

KME

 

[Dambo]

The Rcd must be capable of carrying the current allowed to flow by the MCB in the main CU, i.e. if supply 32 A MCB then RCD must be

that, normally 40 A / 30 m A type suituable, but a 25 A / 30 m A woud NOT be.This is basic stuff mate

.

If I use a 50A MCB in the main CU sparkytim, then surely a 40A / 30mA RCD would be sufficient provided that the garage load didn't exceed the RCD maximum load?

 

[kme]

NO.

`cos the RCD isn`t capable of carrying the current which the MCB would allow through.

Choose the MCB from cable size / design calcs. Make sure the RCD rating is HIGHER than the MCB rating - always.

KME

 

[ianmacd]

NO.`cos the RCD isn`t capable of carrying the current which the MCB would allow through.

Choose the MCB from cable size / design calcs. Make sure the RCD rating is HIGHER than the MCB rating - always.

KME

Indeed. What if the fault occured between the mcb and the rcd in the garage then the rcd would have had an unsafe current though it and should be replaced but it may never get noticed until it failed to opperate as expected a week later when someone bridges line to earth with their body and it fails to trip within the correct times.

As KME said, always design the overcurrent protective device to be lower than the max working current of everything its protecting up until the next ivercurrent protective device.

 

[Dambo]

NO.`cos the RCD isn`t capable of carrying the current which the MCB would allow through.

Choose the MCB from cable size / design calcs. Make sure the RCD rating is HIGHER than the MCB rating - always.

KME

Indeed. What if the fault occured between the mcb and the rcd in the garage then the rcd would have had an unsafe current though it and should be replaced but it may never get noticed until it failed to opperate as expected a week later when someone bridges line to earth with their body and it fails to trip within the correct times. As KME said, always design the overcurrent protective device to be lower than the max working current of everything its protecting up until the next ivercurrent protective device.

Thanks guys, that really does make sense

 

[Manator]

I would just like to thank all those who have shown why this is the best forum by far. I would hazard a guess that all electricians when they first qualified would still doubt their skills when left alone for the first time. Experiance and knowledge comes on the back of time and good sound advise.

 

[initforthemoney]

Well I hope that you are either registered with a little club....

Yep and have been for many years. Guinness