RCCB & Switch Overload Protection (536.4.202)

536.4.3.2 includes The rated current of a switch or RCCB may also be based on the application of diversity factors to the downstream

circuits according to Regulation 311.1 , and the rated current of the OCPD shall be selected according to the

manufacturer's instructions. See also Regulation 536.4.202.


Which at first sight contradicts.

so we need the rest of  536.4.202
The rated current of a switch or RCCB (InA and Inc) shall be based upon:

- the sum of final circuit current demand after any applicable load diversity factors, or

- the sum of final circuit current demand after any applicable load diversity factors together with allowances

for diversity between final circuits, or

- the sum of the downstream OCPDs/circuit rated current multiplied by a diversity factor.


which pretty much says you can have 100A total of final circuits on a 63 A RCD, as diversity will sort it for you. Equally if the house fuse is 100A, then to assume a 63:37 split or better on a two RCD board is probably a reasonable assumption.

The last clause is really saying before you  this, check with the makers instructions that it is OK.

I think we can assume that if a ready built board from say Wylex uses one of their own RCDs in this way, the makers are indeed OK with this.

Well that's my reading of the tea leaves anyway - I agree it would benefit if it was clearer, and terser.

The attached BEAMA document shows two methods to determine the sizes, shown also in the Hager document along with more diagrams. 


The tech dept. of a leading manufacturer told me a few weeks ago that they still provide what could be undersized RCCB's in CU's (undersized if having a lower rating than the upstream OCPD and the sum of the cb's exceeds the RCCB rating) because they supply what suppliers/wholesalers ask for, rather than base sizes on the two methods. The simplest thing to do is as Hager say, make them all 100A.


F

Thanks for the replies, I got this reply from BG

Hi, thank you for your product enquiry. Some manufacturers have interpreted this regulation stated in BS 7671 about the diversity being used solely. Our board is still compliant as it complies to BS 61439-3 regulations as testing is conducted using 100A as a maximum load and this is split between the two RCCBs inside the board. This has also been tested and verified by third parties.

Hager's interpretation is easier to understand, but the cynic in me thinks this simply to differentiate their product to justify their higher prices. That said I do like working with the Hager boards. I am waiting for a reply from MK as well.

So it's sounding rather like things haven't changed much. If you're assembling things yourself (e.g. RCCBs in separate enclosures) then you follow traditional BS 7671 rules and have to ensure overload protection using an appropriately rated overload protective device (or possibly the sum of several downstream devices). If however you're using a pre-manufactured assembly (e.g. to BS EN 61439 series) than it's the rules of the equipment standard that apply and then one permitted option is complete reliance on diversity without an explicit overload protective device - provided the manufacturer can justify that. Or have I overlooked some subtlety again....?


  - Andy.