3 phase RCD MCB questions....

Dan_

Hi all
i'm putting together a 3 phase board for a job i am doing at the mo...its my first 3 phase domestic, i'v mainly been doing larger cable works and the likes...

its a retail unit wit only 3 phase connections the rest are all single phase

3phase cooker through a 32a isolator 5X6sq
3phase cooker through a 25a isolator 5x2.5sq
3phase socket 5x2.5sq

the rest is just sockets and lights....

Can i get a 3phase RCD and feed all my single phase MCB's off that for my socket circuits? for the lights can i feed all my lighting MCB's from my main switch fuse?


For the rest i was going to a RCBO for my 3 phase socket and two £phase MCB's for the 2 ovens....?

Thanks for any help or advice...Dan:)

davelerave

i use only 2-pole rcds for single-phase sockets(there's a reason i can't recall atm).
if it's 1 3-phase whole-current meter just balance the single-phase loads on it

brightspark

One reason (though it may not be the only one, I don't have the regs handy) is that by putting them on seperate rcds you won't have all your sockets trip out together.

You don't want the cash register, pc or any other vital equipment loosing power just because the kettle element has blown!

2011

Can i get a 3phase RCD and feed all my single phase MCB's off that for my socket circuits?

Single phase sockets can not be fed from a 3 phase RCD as this is against regulations. Normally with commercial installations RCBOs would be used for each socket circuit.

davelerave:

i use only 2-pole rcds for single-phase sockets(there's a reason i can't recall atm).

+ 1 dave. It has something to do with a broken neutral as far as I recall.

for the lights can i feed all my lighting MCB's from my main switch fuse?

Yes this would be normal.

i was going to a RCBO for my 3 phase socket

Yes the 3 phase sockets must be fed from a 3 phase RCD (& MCB).

two £phase MCB's for the 2 ovens....?

I guess you mean a 3 phase MCB for each oven, this would be correct as these are a fixed appliances it does not require RCD protection.

Dan_

2011 Thanks for your info..apart from having 3 rcd's my only other option is all rcbo's..which is not really an option as to the size of the board in place, its a job i have taken over after the last builders went bang.

i will look into getting a larger board , but his funds may not allow, so 3 rcd is an acceptable way of connecting all my socket circuits?

again thanks

davelerave

Cash registers might need dedicated circuit.i would keep any deep freezers away from rcd if possible.

Dan_

thanks lads...i have it all sorted not ..i went with 3 rcd's.....but i was told there is no such as a 3phase rcbo,,,so its a rcd and mcb beside each other????

davelerave

good luck with it.i think one advantage with separate rcd/mcb for socket circuits is troubleshooting occasional trips.

Bruthal

2011 wrote: »

Single phase sockets can not be fed from a 3 phase RCD as this is against regulations. Normally with commercial installations RCBOs would be used for each socket circuit.

davelerave:
+ 1 dave. It has something to do with a broken neutral as far as I recall.
quote]


yes a 3 phase rcd feeding multiple single phase socket circuits would create serious problems if the neutral feeding the rcd broke, the socket circuits would then be like a star connected inbalanced load with no neutral and so the voltages across items plugged in would depend on the impedence of the load, if nothing was plugged in and then a lamp is plugged in it would`t light, but if you then plug a kettle in on a different ciruit but also on this 3 phase rcd it would now be in series with the lamp and the lamp would have the majority of the 400 volts across it, probably over 350 volts, maybe the same problem is present on 3 phase socket circuits also, but a 3 phase socket is more likely to have a balanced 3 phase load fed from it