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spud1

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Hi chaps

A Wholesaler told me me this week that the RCDs in the new '18th' split load boards have to be rated at 100a because the 100a service fuse is only means of upstream overcurrent protection. But I was once told that service fuses, although marked as 100A on the cutout label are actually only actually 80amp?

Any thoughts guys?
 
If it says 100A on the label it should be 100A in the fuse holder.
Some labels say 60/80A which is pointless, as you don't know if its 60 or 80 without a visit from the seal fairy.

The RCD's are normally included with split boards and can be 63 or 80 depending on the size of board.

If the boards are connected properly, with near equal load share between the 2 RCD's, there shouldn't be a problem. Meaning an electric shower on one, and an electric cooker on the other, for example.
The number of times I've come across boards with shower, cooker, kitchen sockets all on one side and the lights, and another rfc on the other.
 
Don't base your technical knowledge base on what you hear over the wholesale counter. It's pretty much always guaranteed to be rumour or BS.
The rating of a RCD must be equal to or more than the load drawn through it, same as any equipment.

The only way to verify the size of a DNO fuse is to look at it. The 100A marking is the rating of the fuse carrier.
 
As long as the load doesn't exceed the rating of the device then there's no problem,
 
On the 100A fuse, 100A isolator and 100A RCD thing, I heard this too. Reg 536.4.202 gives guidence and allows for diversity to be applied and states we shall follow manufacturer's instructions for fuse selection. So if the manufacturer approves 100A fuse for their CU with 80A RCDs (say) that all seems fine to me.
 
Reading around on this - it does seem (some) manufacturers are saying their 63A RCDs can only be used below 60A fuses (for example), regardless of how we may load them. I don't see this in BBB so I'm assuming it must be from BS61439 (?).
 
At present it’s the withstand rating, not the load rating that needs to be confirmed.
i.e. if the PFC is 2500A, then the withstand rating should be 3000A.
Yes there have always been concerns about this, ever since the first split board was manufactured.
 
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.
 
Reading around on this - it does seem (some) manufacturers are saying their 63A RCDs can only be used below 60A fuses (for example), regardless of how we may load them. I don't see this in BBB so I'm assuming it must be from BS61439 (?).

536.4.202 is the Reg you need to look at, last paragraph.
 
Some helpful info there Essex. :thumbsup:

Just read through the reg in full and cannot see any reason for me not to keep using diversity calculations though. The method of using the total current rating of all downstream devices is pure idiocy, and further evidence of the dumbing down nature of regulation changes.

If that approach gets taken on it will completely contradict the efforts made to divide up an installation to provide useful and safe installations mentioned in section 314.

I'm doing a domestic board change tomorrow. They have 4x32A circuits, 1x20A circuit, 3x16A circuits, and 3x6A circuits. I'm 100% sure the 100A main switch will be fine and not subject to current past it's rating in my lifetime.
 
Some helpful info there Essex. :thumbsup:

Just read through the reg in full and cannot see any reason for me not to keep using diversity calculations though. The method of using the total current rating of all downstream devices is pure idiocy, and further evidence of the dumbing down nature of regulation changes.

If that approach gets taken on it will completely contradict the efforts made to divide up an installation to provide useful and safe installations mentioned in section 314.

I'm doing a domestic board change tomorrow. They have 4x32A circuits, 1x20A circuit, 3x16A circuits, and 3x6A circuits. I'm 100% sure the 100A main switch will be fine and not subject to current past it's rating in my lifetime.

I tend to agree but the Reg does state manufacturers info must be adhered to and they say you cannot take into account diversity. Why the IET just never wrote the Reg to state this and keep it simple is beyond me.

In your case if the OCPD is 100A or less then what is installed downstream is irrelevant.
 
I tend to agree but the Reg does state manufacturers info must be adhered to and they say you cannot take into account diversity. Why the IET just never wrote the Reg to state this and keep it simple is beyond me.

In your case if the OCPD is 100A or less then what is installed downstream is irrelevant.

So for example (bear with me here as I'm not sure I have grasped this nonsense fully), In a single phase domestic situation the DNO fuse is 100A, CU main switch is 100A. A dual RCD load board is installed with 80A RCCBs. One would have to keep the total rating of all downstream devices for each RCCB less than 80A unless the manufacturer agrees that diversity can be used for that group of circuits ? But the total for the CU could be up to 160A as the main switch is in theory protected from overload ?
 
So for example (bear with me here as I'm not sure I have grasped this nonsense fully), In a single phase domestic situation the DNO fuse is 100A, CU main switch is 100A. A dual RCD load board is installed with 80A RCCBs. One would have to keep the total rating of all downstream devices for each RCCB less than 80A unless the manufacturer agrees that diversity can be used for that group of circuits ? But the total for the CU could be up to 160A as the main switch is in theory protected from overload ?

Spot on - although all UK manufacturers have said, through BEAMA that they will not permit diversity so the 95% of the regulation is void and a pointless exercise. The IET should have consulted the manufacturers before writing the reg and then just wrote what the manufacturers state.

The other alternative would be to have 100A RCCBs installed with the 100A main switch.
 
Spot on - although all UK manufacturers have said, through BEAMA that they will not permit diversity so the 95% of the regulation is void and a pointless exercise. The IET should have consulted the manufacturers before writing the reg and then just wrote what the manufacturers state.

The other alternative would be to have 100A RCCBs installed with the 100A main switch.

So the OP's wholesaler was wrong with their assertion that all RCCBs will need to be 100A rated.

My example is how I used to arrange all my dual RCD boards anyway, but not installed any for a while now.
 
I tend to agree but the Reg does state manufacturers info must be adhered to and they say you cannot take into account diversity. Why the IET just never wrote the Reg to state this and keep it simple is beyond me.

In your case if the OCPD is 100A or less then what is installed downstream is irrelevant.
What is the Reg No of the one that states Manufacturer’s info must be adhered to?
 

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