Consumer unit wiring query

[bobo]

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?

 

[Hoagy]

A 10amp MCB is normal for lighting circuits, an RCD is only required for circuits feeding portable appliances and water heaters.
RCD tripping on lighting circuits would be a hazard.
The shower gets its own MCB/RCD primarily because the RCD in the consumer unit is only rated at 40amps.

 

[bacchus]

RCD tripping on lighting circuits would be a hazard.

Why?

 

[Hoagy]

Why?

`
The obvious answer, really. RCD trips, all lights in the house go out, not just one circuit.

Similar argument continues in the UK about MCB's versus fuses. Fuses being less sensitive are considered by some to be safer than MCB's for lighting circuits.

 

[Leo]

Q3: Why are 40A shower MCB's protected with there own dedicated RCD/MCB switch?

Showers should be on their own circuit, and this should be protected by an RCBO now, these cover both leakage and overcurrent protection in the one unit.

Please tell me you're not considering installing this yourself???
Leo

 

[bacchus]

`
The obvious answer, really. RCD trips, all lights in the house go out, not just one circuit.
Similar argument continues in the UK about MCB's versus fuses. Fuses being less sensitive are considered by some to be safer than MCB's for lighting circuits.

I don't think this is that obvious.... IMO, RCD offer extra protection, full stop. Now, it is questionable as to whether being out of light is a greater hazard than not having a RCD protection...
It is very common from MCB to trip when a bulb flows... It does not mean that MCB should not be used to protect the circuit...

We talked about RCD (though for bathroom) in this thread.
You said the RCD for bathroom light protection did not make it into the final version.. Would you happen to know the reason?

 

[Hoagy]

You said the RCD for bathroom light protection did not make it into the final version.. Would you happen to know the reason?

Well spotted. I've now gone back and read the RECI newsletter properly and it actually refers to the 4th Edition of the Wiring Rules, which is in production at the moment so I haven't seen the final version.
The 4th Edition was out for public comment around Sept 2006 and I don't recall seeing this requirement, but I may have missed it. Apologies if I've misled you.
In any event the requirement would only deal with lighting circuits in the bathroom zones, rather than all bathroom lighting.