dual rcd?

[garlicbread]

where in the regs does it state that a dual rcd board has to be fitted in accordance with the 17th?
if you have only say a 4 way board ie, lighting, power, shower, and say water heater would you still need to split the circuits or could you still use a single rcd to cover everything?

 

[Spark123]

The 17th edition doesn't say you have to fit a dual RCD board.
One part of it basically says that the installation should be divided into circuits to avoid hazards and inconvenience.
One RCD covering the whole installation can't really be seen to be taking this into consideration hence the reason for splitting between two or more RCDs.
As an alternative you may opt to use 4 RCBOs in a 4 way board so each final circuit has its own separate RCD protection.

 

[Steve]

where in the regs does it state that a dual rcd board has to be fitted in accordance with the 17th?

It doesnt.

if you have only say a 4 way board ie, lighting, power, shower, and say water heater would you still need to split the circuits or could you still use a single rcd to cover everything?

You could use a single RCD, but the regs ask you to consider how one circuit fault will affect the other circuits.

On a 4-way board, RCBOs might be more appropriate, since you'd only need 4. It may also work out cheaper than 2 RCDs and 4 MCBs.

 

[Steve]

BLAST! You beat me to it.

 

[ban-all-sheds]

As an alternative you may opt to use 4 RCBOs in a 4 way board so each final circuit has its own separate RCD protection.

314.1(iii) and one lighting circuit?

Better put some emergency lights in.

 

[garlicbread]

ok cheers for the info, it was more to do with the regs side of things and what was stated.
so would installing a single rcd board now, contravene the regs then.
just curious as to how the regs interpret it.

 

[Spark123]

It depends what is on it.
In a domestic dwelling with a single consumer unit it won't comply.

 

[Steve]

so would installing a single rcd board now, contravene the regs then.

Yes. Because it creates a single vulnerable point of failure.

A single lighting circuit is also not considered wise, even in small properties, unless emergency lighting is in place. Where multiple lighting circuits are used, they should be spread across the RCDs.

 

[Taylortwocities]

Go read 314

Specifically 314.2


Why is it important? - consider this often used example:

"You are 79 years old. The time is 9.30 p.m. It is December, it's dark and the weather is cold. You come from the bathroom on the first floor, just on top of the stairs. The thermostat of the central heating comes on, a neutral to earth fault on the pump trips the residual current device on the distribution board that has been fitted as the incomer and all supply is lost in the home. No lights. What happens next..."

 

[ericmark]

ok cheers for the info, it was more to do with the regs side of things and what was stated.
so would installing a single rcd board now, contravene the regs then.
just curious as to how the regs interpret it.

If it does not trip out without cause then no you can have a single RCD this is standard arrangement for caravans and boats but they nearly always have some 12 volt lighting so tripping does not cause danger.

However if it does trip out then it does not comply and nearly impossible for an electrician to know if it will or will not trip.

This is what it says:-

314 DIVISION OF INSTALLATION
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.
314.2 Separate circuits shall be provided for parts of the installation which need to be separately controlled, in such a way that those circuits are not affected by the failure of other circuits, and due account shall be taken of the consequences of the operation of any single protective device.
314.3 The number of final circuits required. and the number of points supplied by any final circuit, shall be such as to facilitate compliance with the requirements of Chapter 43 for overcurrent protection, Section 537 for isolation and switching and Chapter 52 as regards current-carrying capacities of conductors.
314.4 Where an installation comprises more than one final circuit, each final circuit shall be connected to a separate way in a distribution board. The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated.

So unless one uses RCBO's one could still not comply. Although in many cases twin RCD's will comply there is no guarantee and the electrician has to make a judgement.

 

[ban-all-sheds]

And bear in mind that in his judgement he should take into account all reasonable requirements for the installation, the needs of all people who might reasonably be expected to be using it, etc, for the rest of its life.

Which might be longer than his.

 

[garlicbread]

so its basically a lighting issue until the fault is repaired then, so proving you install emergency lighting throughout the house you could in theory use a single rcd board and comply with the regs.

 

[Steve]

so its basically a lighting issue until the fault is repaired then, so proving you install emergency lighting throughout the house you could in theory use a single rcd board and comply with the regs.

What about heat? And ensuring there are some sockets still available to use (even if it means running an extension lead for a while)

If it were my house I'd make sure the essential circuits are not RCD protected, such as a boiler radial in SWA or split con, and the lights would have emergency modules built in.

Dont forget, EM lights only last 3 hours on a full charge. They are only for "making safe" a building until its occupants are removed from danger, for example by evacuating or in this case providing plug-in lamps.

 

[garlicbread]

314.1 (i) then
cheers

 

[AndyPRK]

I would buy a RCBO C6 for the lighting, and fit everything else on the RCD side.

In practice, should be ok.