RCDs, 100A vs 80A vs 63A

[NorthEastSpark]

Domestic CUs-

I can't remember where this bit of info came from, but since the latest update I am 'under the impression' we can no longer utilise cons units where RCDs are rated below the rating of the main fuse.

E.g. can't use a dual RCD consumer unit with 80A or 63A RCDs if the cut out harbours a 100A fuse- OR if the rating of ocpd is unknown (assume it's 100A), even if you would not expect overloading.

This isn't normally an issue for us when using RCBO boards or such as hager dual RCD where they have 100A RCDs, however the smaller hager (such as 3+ 3 ways) and the cheaper stuff (MCG, BG, CGD etc) usually have 63A or 80A rated RCDs.

A few wholesalers have told me I'm the only one who has ever questioned it.
Have I got the wrong end of the stick?

 

[loz2754]

The regulation is about whether the RCD is rated for the total current of the downstream devices.
However it's not just a case of adding up the total of all the circuit breaker ratings. Otherwise it could run to 100 or more amps. For example, maybe 3x 32 amp, 1x 16 amp and 1x 6 amp would total 108 amps. You can apply diversity to the downstream circuits.
Hager have jumped on the easiest way of complying by making all their RCDs for domestic use rated for 100 amps.
There's a useful guide by Beama:

BEAMA Technical Bulletin Ed 2 Mar 2019 - Overload Protection of an RCCB or Switch BS EN 61439-3

www.beama.org.uk

 

[NorthEastSpark]

The regulation is about whether the RCD is rated for the total current of the downstream devices.
However it's not just a case of adding up the total of all the circuit breaker ratings. Otherwise it could run to 100 or more amps. For example, maybe 3x 32 amp, 1x 16 amp and 1x 6 amp would total 108 amps. You can apply diversity to the downstream circuits.
Hager have jumped on the easiest way of complying by making all their RCDs for domestic use rated for 100 amps.
There's a useful guide by Beama:

BEAMA Technical Bulletin Ed 2 Mar 2019 - Overload Protection of an RCCB or Switch BS EN 61439-3

www.beama.org.uk

This guide is exactly what I needed.

I wonder how many BG screwfix boards are being put in that comply with these requirements?

 

[MJPD29]

This guide is exactly what I needed.

I wonder how many BG screwfix boards are being put in that comply with these requirements?

Any self respecting spark wouldn't install BG anyway. Made the mistake of doing it once I'm my life. 1 faulty 63a 30ma RCD later and a further 2 faulty replacements and i got their eventually. 4th time lucky

 

[loz2754]

I have fitted over 30 BG boards without any issues.
Having said that, I have heard several complaints about faulty RCDs.

 

[NorthEastSpark]

Any self respecting spark wouldn't install BG anyway. Made the mistake of doing it once I'm my life. 1 faulty 63a 30ma RCD later and a further 2 faulty replacements and i got their eventually. 4th time lucky

I have fitted over 30 BG boards without any issues.
Having said that, I have heard several complaints about faulty RCDs.

I didn't mean that they were poor quality. We've fitted LOADS of BG boards, and the only problems experienced were VERY stiff and difficult to tighten N & E bars when they first appeared (~2010ish?) and one RCD fail on a re-test five years later.

I'd certainly rate them higher than CGD/Lewden/MCG/Chint etc.

My reference to BG screwfix boards in this case was due to the standard populated 10W boards for sixty odd quid they sell.

£56.66 (+VAT)

• 100A Mains Switch
• 2 x 63A 30mA RCDs
• 10 x MCBs (3 x 6A, 2 x 16A, 4 x 32A & 1 x 40A)

Stick one of these in an average 3 bed semi with a 9kW shower, it's not always going to comply with the recommendations of this BEAMA guide!

The hager 3 + 3's still incorporate dual 63A. But they aren't populated, and I can think of a few recent scenarios where if I'd used these instead of RCBOs, they would have complied with these requirements.

 

[stevethesparks]

Apart from the reasons of profit (having to upgrade) I could never understand Screwfix/BG supplying less than 100A RCD's in the first place as they are all a very similar price. Unless I'm being naïve, it must cost more to produce the three rather than just the one 100 Amp unit?

 

[loz2754]

Apart from the reasons of profit (having to upgrade) I could never understand Screwfix/BG supplying less than 100A RCD's in the first place as they are all a very similar price. Unless I'm being naïve, it must cost more to produce the three rather than just the one 100 Amp unit?

The cost of the 63 amp and the 80 amp RCDs is the same, however it costs about £5 more for the 100 amp. Screwfix and the like are in business to make profit and no other reason.

Also, the regulation that says we need to consider potentially overloading an RCD is only a recent regulation.

 

[marcuswareham]

I have often thought how stupid it is to have a split board with say 63a RCDs and a 100a main fuse

In my opinion diversity is not a good solution as yes the likelihood of overload is low, but if one day the heating packs up and they plug in loads of electic heaters, switch immersion on, and then start cooking dinner, etc etc. There is no control and nothing to stop overload

I found it crazy how you were allowed to have RCDs and the cables to them inside a consumer unit completely unprotected against overload (and therefore thermal effects) when we are so ---- about protecting every other cable against overload

In my own house I have a 16th edition board with a single RCD, and a 100a mains fuse, when i got the house this RCD was underated and I swapped it for a 100a one, along with other upgrades including rcbos etc. A friend of mine a few weeks ago sent me a picture of his consumer unit asking which breaker the cooker was as it is unlabeled, it is a BG split load unit with 63a RCDs on one of the RCDs there is a 40a, 3x 32a, 2x 16a, and on the other side 2x 6a and a 16a, and its a new compliant install with a 100a DNO fuse crazy!

 

[pc1966]

Another reason for metal enclosures

I guess more of a concern is an overheated 63A RCCB might not burn out in a twisted fire-starter sense, but very likely to have the trip electronics fail prematurely and maybe even have the trip mechanism stick as a result of the thermal stress.