RCD Rating and Cooker MCB Location

[TheScullster]

Hi all

I am looking at the options for reconfiguring a Clipsal CU with Hager MCBs and RCD. Currently the box is split with 7 non-RCD circuits and 3 power circuits protected by one 63A RCD.

To comply with the new regs (17th) I was considering moving the RCD 2 units to the right, to make room for the cooker and kitchen lights on the RCD side of the box.

So here come the questions:

Is moving the cooker MCB well away from the incomer OK - traditionally the cooker tends to be the first "user". With 3 power circuits a cooker feed (all 32A rated) and a lighting feed, do I need to uprate the RCD? The power circuits are upstairs ring, downstairs ring and kitchen in 4 bed house - nothing too far out of the ordinary.

TIA

Phil

[Tim Watts]

TheScullster wibbled on Friday 23 April 2010 15:02

I've heard of people who like to put it near the incomer, but if you look at the size (CSA) of the busbar, it think it is irrelevant.

There is a case for spacing out RCBOs (I have big current, small, small, gap where possible) as RCBOs do heat up a bit more than MCBs in theory - not seen any proof, but if possible, it does no harm.

I'll refrain on that one as it will involve digging out teh OnSite Guide and doing a diversity calc and I'm only in for an orange juice between clearing the garden :_)

[ARWadsworth]

It certainly mattered on the old Wylex fuse boxes with their small cross sectional area bus bars. On a new CU the bus bar is quite substantial. Any voltage drop (shown by the diming of lights etc) when a large load it used is down the the impedance of the supply cable not due to a voltage drop across the bus bar.

I know someone who had a 9kW water heater that caused a voltage drop when it was powered up. He was able to prove that it was not voltage drop on the CU bus bar:-)

Page 96 of OSG. It is important to ensure that distribution boards or consumer units are of sufficient rating to take the total load connected to them without the application of any diversity.

What a load of c*ck. If I pugged everything I owned in at once, switched on the electric shower, the tumble dryer, washing mashine and dish washer it would in theory blow the main fuse.

Common sense has to be applied.

The worst case would be a boiler failure and you used electric heaters as a temporary backup. As you have 3 ring main circuits you could plug in more heaters around the house than most people without tripping a MCB and possibly overloading the 63A RCCB.

I have no idea what happens if you shove 70A through a 63A RCCB. My best guess is nothing would happen.

Adam

[Andy Wade]

The footnotes at the bottom of page 96 only apply to the corresponding (starred or daggered) rows 5 to 8 in Table 1B - i.e. to stated the water & space heating loads.

Yes - Table 1B used literally does tend to overestimate the MD, IMHO. e.g. in the case of this thread we have 32 A for the first ring, about

25 A for the remaining two (at 40%) and say 22 A for a 12 kW cooker (first 10 A plus 30% or remainder), giving a total of ~79 A - so the 63 A RCD really should be uprated. In practice though, in 99.9% of cases, it will be quite OK.