New consumer unit

bibibobo

Hi,
I bought a new consumer unit for €65 and it came with various mcb's and one rcd.

Q1: Why are the lighting MCB's rated at 10A? - Should they not be 6A to protect the 1mmsq and 1.5mm sq cable in the event of overload? Or does it really matter that much?

Q2: Why are light circuits not wired via an RCD? You could potentially give yourself a shock if your being careless with rewiring lamps or doing drilling work on a floor above a ceiling. And sure some switch plates are metal, same for some light fittings. Would it not be best practice to run these circuits from the main RCD that protects sockets? Or is it to ensure that lights remain on for basic safety in the event of a ring main fault to earth?

Q3: Why are 40A shower MCB's protected with there own dedicated RCD/MCB switch? Is this for reaction times being that bit more critical in a shower situation or is it because a shower potentially pulling 37A (8.5kW Triton) if shared with load on ring main circuits could cause an unbalance in the circuit thus tripping the RCD when not required?

fishdog

Why are the lighting MCB's rated at 10A?

The 10A MCB will operate before the 1.5 cable becomes overloaded.

Why are light circuits not wired via an RCD?

When an electrician wires a house he/she can ensure that permanently connected equipment IE cooker, lighting is safe by testing it. However for the electrician there is no way to control what faulty equipment a person may plug into an installation, so it was decided to give socket circuits additional protection. This additional protection comes in the form of an RCD.

Why are 40A shower MCB's protected with there own dedicated RCD/MCB switch? Is this for reaction times being that bit more critical in a shower situation or is it because a shower potentially pulling 37A (8.5kW Triton) if shared with load on ring main circuits could cause an unbalance in the circuit thus tripping the RCD when not required?

You could potentially give yourself a shock if your being careless with rewiring lamps or doing drilling work on a floor above a ceiling.

True, but in theory you are only doing this type of work if you are a qualified electrician and therefore know how to do it safely.

Would it not be best practice to run these circuits from the main RCD that protects sockets? Or is it to ensure that lights remain on for basic safety in the event of a ring main fault to earth?

It is not normal practice to put lights on an RCD. You could put them on their own RCD if you are really concerned.
In some countries RCDs are not used in domestic installations at all.

Why are 40A shower MCB's protected with there own dedicated RCD/MCB switch?

Electricity and water are a bad risk, therefore it was decided to ensure that a person having a shower would have additional protection. RCDs are generaly rated at 40A, the shower draws close to that so it is better to be on its own.

Also the RCD is designed to operate when the current imbalance is greater than 30mA. The more items & cable on the RCD the higher chance there is of the RCD tripping when there is no fault.

if shared with load on ring main circuits could cause an unbalance in the circuit thus tripping the RCD when not required?

A ring circuit has nothing to do with unbalance.

Hope that answers the questions for you. Be safe!

sparkyjo

just wondering if you were going to put this in yourself ?

the reason i ask is because you sound like you are and if so you should not do it as it is not just a case of changing cablesmost of the times there faults on cables and you don't seem to have any experience and also if you break any seals on the like of your meter or switchfuse the esb will fine you now

and anyway it should only cost about 120-150 to get changed by a real electrician and between 1 to 2 hours of time

1.5mmsq pvc pvc cable will carry around 15.4 amps thats why there is a 10 amp breaker

Stoner

sparkyjo wrote: »

just wondering if you were going to put this in yourself ?

the reason i ask is because you sound like you are and if so you should not do it as it is not just a case of changing cablesmost of the times there faults on cables and you don't seem to have any experience and also if you break any seals on the like of your meter or switchfuse the esb will fine you now

and anyway it should only cost about 120-150 to get changed by a real electrician and between 1 to 2 hours of time

1.5mmsq pvc pvc cable will carry around 15.4 amps thats why there is a 10 amp breaker

I agree, it does sound like you are about to do something that you are not experienced in. WRT replacing boards, there is a method, there is proper cable management, additional earthing requirements and sleeving etc. this is a job for someone who has training in the field. Obviously if you are only trying to get to grips with what a sparks is going to do then fair enough.

fishdog

You would also need the proper test equipment and training to undertake a job such as this in a safe manner.

However it is good to have an understanding of what is going on.

bibibobo

sparkyjo wrote: »

just wondering if you were going to put this in yourself ?

Yes. will be doing this myself. I have rewired the entire house and will be installing the new Consumer Unit together with moving over all existing cabling, testing rings. Will not move over the mains myself - will leave this to a spark friend for his expert skills. Installing the consumer unit is not rocket science. It is dangerous and care needs to be taken. Basically watch what your doing, label everything correctly, if unsure what to do then don't proceed until you find out.

fishdog

Installing the consumer unit is not rocket science

No, its not. But it is amazing the mess that some people make of it due to lack of proper training. They assume that just because everything "works" that it is safe.

Here are some faults I have seen alot of:

A "C" type RCBO supplying an instantanious shower that due to the lenght of cable run the earth (in a T&E) is too small resulting in the disconnection time being too long. "B" type RCBOs seem to be harder to buy off the shelf, and many DIY types do not know the difference.

2.5 and 1.5 cable used as busbar.

Neutrals mixed up, so the RCD keeps tripping, so the RCD is linked out!

RCD neutral bar earthed to metal enclosure for the distribution board, again RCD linked out. This was done by a RECI electrical contractor while I was present, I made him change it!

Loose connections in one case burning down a switch room and in another melting the distribution board!

Permanent feed and switched feed for emergency light fed from different circuits.

Conductor sizes incorrect, 1.5 T&E fed from a 32A MCB

Mains voltage lights wired in alarm cable.

Poor terminations, strands cut out etc.

No understanding of earthing, bonding, neutralising etc.

16 sq. flexable panel wire used without boot lace type crimps on the ends resulting in poor quality connection.

Will not move over the mains myself - will leave this to a spark friend for his expert skills

I would do that live in just a few minutes. That is the easiest part if you know what you are doing.

.......if shared with load on ring main circuits could cause an unbalance in the circuit thus tripping the RCD when not required?

I am not trying to put you down, clearly you have some understanding of what you are doing, but the above statement would worry me. It demonstrates that you have little or no understanding of how an RCD works. We all had to start somewhere, but electricity kills!

Sparky

bibibobo wrote: »

Yes. will be doing this myself. I have rewired the entire house and will be installing the new Consumer Unit together with moving over all existing cabling, testing rings. Will not move over the mains myself - will leave this to a spark friend for his expert skills. Installing the consumer unit is not rocket science. It is dangerous and care needs to be taken. Basically watch what your doing, label everything correctly, if unsure what to do then don't proceed until you find out.

I assume then you will be getting your sparks to do the line earth loop and impedance tests, doing a completion cert for you to have the Esb connect up the new supply to your new consumer unit along with him coming back to preform RCD trip times?

For somebody asking the questions that you have asked above, I'm wondering what else you may not know!