Why has RCD type changed in 18th edition?

[spud1]

Does anyone know why RCDs have changed in type in the 18th. Someone told me it was some5hing to do with DC faults? What type were they and what are they now?

Thanks

Spud

 

[spud1]

A supplier asked me about 100Amp rcds due to 18th edition, i told him i was unaware of this, which i am. Does it only apply to single upfront rcd on 100A supplies not split?

This reply from Essex to my question earlier in the week pretty much covered it :

Ok so this was covered in my 18th update course. Your supplier is 100% correct.

Reg 536.4.3.2 and 536.4.202 are the relevant regulations and basically say that RCCBs must be selected so that they are greater than or equal to the downstream circuits after diversity has been taken into account. However it also goes on to say that diversity alone cannot be used to determine RCCB ratings and that we must take into consideration manufacturers instructions.

BEAMA have told us that two methods can be used:

Method 1:
ensure that the rated current of the sum of the downstream MCBs do not exceed the rated current of the switch or RCCB when installed within the consumer unit.

Method 2:
ensure that the rated current of the switch or RCCB in the related assembly circuit in the consumer unit (Inc) stated by the assembly manufacturer, is not less than the rated current of the upstream overcurrent protective device (OCPD). For a domestic installation this is likely to be the cut-out fuse rated at 60 A, 80 A or 100 A.

So if you have a small 2 way DB with two 6A lighting circuits on you can use a 63A main switch or RCCB regardless of what the upstream OCPD is as the total rated current of the downstream devices are 12 amps and the 63A RCCB is greater than that and the DB cannot be added to.

However if you had a 10 way DB with 3x 32A then this means the total current of the downstream devices is 96 amps so the RCCB would not be compliant with BS7671: 2018 if the upstream OCPD was over 63A.
If the upstream OCPD was 60A you could put whatever you wanted downstread as the 63A RCCB or main switch is protected by the 60A OCPD.

 

[spud1]

It was a mic drop moment for him.

 

[Nebu1a]

Maybe the suppliers could go back to doing OCPD in ratings of 5amps. Would make it easier to use method 1.
30A+30A+15A+5A= 80A on each RCD and then any other circuits on an RCBO.

So how would we code a 17th edition board with a bedroom RFC, a living room RFC, dedicated boiler cct, a lighting circuit and a spare way on a 63amp RCD?

 

[Ian1981]

Maybe the suppliers could go back to doing OCPD in ratings of 5amps. Would make it easier to use method 1.
30A+30A+15A+5A= 80A on each RCD and then any other circuits on an RCBO.

So how would we code a 17th edition board with a bedroom RFC, a living room RFC, dedicated boiler cct, a lighting circuit and a spare way on a 63amp RCD?

No more than a C3 in my opinion if the rccb shows no signs of thermal damage which ,in probably most cases will show none after all we used to apply diversity alone of the final circuits protected by a single rccb.
It’s hard to imagine that on a single phase domestic dwelling that 80 amps plus will be drawn at any one time.