Limitation of 63A RCD

[seasickstevie]

Notice a lot of CUs come with dual 63A RCDs, while others tend to have one 63A and one 80A.

And it's not coverned (it seems) by the number of ways, or even the size/combination of MCBs in the 'pack'

Seen some 12-ways with six breakers including 32A & 40A breakers fed by a 63A RCD ... don't seem right to me.

 

[EFLImpudence]

May only have a 60A fuse in the supply.

It's up to the designer to use the correct parts if it (63A) is considered to small.

 

[westie101]

One word


Diversity!

 

[JohnW2]

One word ... Diversity!

I'm not sure the concept of diversity is applicable in this context - because I imagine that the "63A" is the maximum instantaneous current it is 'rated' to break, rather than the 'average' current it can carry over a period of time.

If, say, a 10.5kW shower, a 3kW immersion and a 3kW kettle all happen to be drawing current at the moment something trips the RCD, that's over 70A that the device would have to break, even if (per 'diversity') the total current demand averaged over half an hour (or whatever) might be only a small fraction of that.

I suspect that the 'maximum current ratings' of RCDs are probably very 'conservative'.

Kind Regards, John

 

[AndyPRK]

What brand did you notice this in op?

 

[JohnW2]

What brand did you notice this in op?

If you look at what TLC has to offer, as the OP says it's a bit of a mixture:

Hager: 2 x 63A RCDs
MK Sentry: 1 x 63A, 1 x 80A RCDs
Wylex & Contactum: 2 x 80A RCDs

Kind Regards, John

 

[AndyPRK]

As I suspected apart from hager.

 

[JohnW2]

As I suspected apart from hager.

Yes, I was very surprised that the only one with 2 x 63A ones was Hager - quite the opposite of what I would have guessed!

Kind Regards, John

 

[seasickstevie]

What brand did you notice this in op?

http://www.screwfix.com/p/bg-10-way...ed-insulated-consumer-unit-dual-63a-rcd/72088

Like JohnW2 says ... plenty likelihood of shower, I/H, kettle, even a couple of oil-filled radiators on one RCD rated at 63A

But whether the ratings of the cheap brands can be assumed to be 'conservative' is probably another matter.

 

[ericmark]

The Screwfix linked board is a "High Integrity" unit so the person fitting the board can decide which circuits are protected by each RCD and which have either no RCD protection or are RCBO's.

It is supplied with 10 MCB's by Screwfix of fixed sizes but through most whole sale outlets you are given the options of what MCB sizes you want.

Since every consumer unit I have seen is rated 100A in theroy two 63A RCD's should cover but as already said diversity does not really count for the RCD and 4 x 32A and 1 x 40A can never be fitted to limit the current to 63A.

I would expect with most installations not all the MCB's are used but as already said it is down to the installer to decide what to place together. Having the kitchen ring and the cooker on separate RCD's would seem to make sense so that if for example the RCD is tripping it would be easy to swap one item at a time to the cooker outlet socket to identify which appliance had the fault but this would also mean lights and power on the same RCD unless some RCBO's are used.

314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(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

Is the RCD considered as forming a circuit? From the above I would say yes.

Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).

If you exceed 30 mA to earth is that "overcurrent"? If so then the RCD's form a circuit.

So the question is "Can we fit a CU with only 2 RCD's?" and to answer that needs a risk assessment. My youngest daughters house has no natural light into the bathroom so for her to lose power to bathroom light is a hazard. In my house the stair case is central so again to lose light above the stairs produces a hazard. (In my case I have an emergency fitting above the stairs.) But the point is the electrician fitting the CU has to decide how to split and if the provided MCB's and RCD's are appropriate or if he needs to change any for RCBO's or a larger RCD.

Be it an extra £20 for the 80A RCD or an extra £20 for a RCBO that is up to the person fitting the unit. We near always get 13A plugs with 13A fuses and we often replace that 13A fuse for a smaller one and never buy 13A fuses as we always have an excess. If you are fitting a CU on a regular basis then likely you will also have an excess of MCB's and 63A RCD's where you have swapped them.

I would say that since the CU linked to is a "High Integrity" unit the installer is expected to replace some of the MCB's with RCBO's.

The ring final when protected with a 32A MCB would be counted as being 32A but a 20A hob even if supplied from a 32A MCB would be considered as 20A so when adding the current ratings of the circuits it's not just a case of adding up the value of the MCB.

I do question the 60A DNO fuse. I know my house has a 60A and with a gas powered shower that's enough, but the DNO fit the fuse and all we really know is it's no larger than 100A so as a domestic electrician can we say protected with 60A fuse so 63A RCD is enough?

This of course comes up again and again the question "If some one in the future does X." Be it swap a 20A hob for a 32A cooker or fit a 100A fuse instead of a 60A fuse the question is the same do we have to install with the idea some one may change things? I would say no the person changing it is responsible for insuring it is suitable.

So with a populated CU it's not up to the supplier to ensure correct components are used it is up to the person fitting it.

I do agree there is not much chance of using a 40A MCB and 4 x 32A MCB's in the same unit and be within the 63A for the 2 RCD's but it's only £20 extra to correct this. And when designed you could have used one of the permitted cables and supplied the cooker with a 40A MCB without RCD protection.

 

[seasickstevie]

... you could have used one of the permitted cables and supplied the cooker with a 40A MCB without RCD protection.

Thanks eric. By 'permitted cables', are you suggesting cooker on SWA (or equivalent means of mechanical protection).

And by looking at that CU I've linked to, I hadn't previously noticed it's 'High Integ' with three N busbars. But then there's no provision (as far as I can see) for RCBO ways

Btw, I'll probably be using a 'superior' CU than this. It's for an HMO, and I can imagine all sorts of unexpected load scenarios!

 

[JohnW2]

I do agree there is not much chance of using a 40A MCB and 4 x 32A MCB's in the same unit and be within the 63A for the 2 RCD's but it's only £20 extra to correct this.

If the design loads were all equal to the MCB ratings, there would theoretically be no chance of getting the figures to work even with two 80A RCDs (what you mention adds up to 168A) - and, in practice, there would obviously normally be at least one or two 6A circuits, and quite probably at least one 16A circuit, in addition to what you mention.

In terms of the general issue, big showers are the main problem. Once one has a 10.5kW shower on one RCD, there's very little scope for much more on that RCD if it's 63A, and not even a lot more if it's 80A.

63A is really not all that much. If one is going to split hairs, a 63A RCD cannot (quite!) even support two 32A ring finals, let alone more.

Kind Regards, John

 

[JohnW2]

And by looking at that CU I've linked to, I hadn't previously noticed it's 'High Integ' with three N busbars. But then there's no provision (as far as I can see) for RCBO ways

Indeed - as eric has said/implied, unless one had permitted non-RCD-protected circuits, one would have to discard some of the supplied MCBs and replace with RCBOs. The (I assume L) busbar is described as 'versatile' - which I suspect means that they supply one long piece which one can cut into two or three pieces, as required.

Kind Regards, John

 

[seasickstevie]

... I suspect means that they supply one long piece which one can cut into two or three pieces, as required.

I take your point. I hadn't thought that.
I think I'm right in saying the experience I've had so far of High Integ CUs, the first 'tail' or 'tooth' (whatever the term is) on the busbar has been slightly off-set / out of line / wider / different to the others, so doesn't have the total flexibility in making an extra third busbar if not provided that way.

But maybe with some makes, the busbar comb is all equal.

 

[JohnW2]

... I suspect means that they supply one long piece which one can cut into two or three pieces, as required.

I take your point. I hadn't thought that. I think I'm right in saying the experience I've had so far of High Integ CUs, the first 'tail' or 'tooth' (whatever the term is) on the busbar has been slightly off-set / out of line / wider / different to the others, so doesn't have the total flexibility in making an extra third busbar if not provided that way.

Yes, some (most?) have a different/offset/whatever 'thingy' for the RCD or main switch. If that were the case, they could obviously only have 'versatility' by supplying three bits of busbar. If, as per that picture, it comes pre-populated with no non-RCD ways, they presumably would have to supply a third bit of busbar in case one wanted to change it to have some non-RCD ways.

Kind Regards, John