MCCB as Main Switch - 3-pole or 4-pole?

Question

Hi, a standalone 630A MCCB within an enclosure in a locked intake room adjacent the CT metering panel has been proposed as the main switch. The main LV panel is 20m away. 
 Does it specifically need to be a 4-pole MCCB or is 3-pole sufficient please? 
 An earthing arrangement (not sure but either PME (TN-C-S) or SNE (TN-S)) is being provided by utility company. 
 Solar PV is present as part of the elec installation and set up to export in case that makes a difference. 
 Thanks

mapj1 · Accepted Answer

On a TT supply you would pretty much always want 4 pole isolation, as the NE potential is not guaranteed to be less than 50 (though it usually is of course). However TNx of any flavour, 3 pole is generally fine, and as above, sometimes desirable. 
 Generally if you are breaking the neutral unless you are very sure the downstream load can take it, you need switches that are constructed to always break neutral last and make it first, so there is no transient where the phases are alive without the neutral connected. - so when one of the terminals has an 'N' sticker, do not be tempted to use it for anything else.~ Mike.

AJJewsbury · Answer

I always went for 4pole then a nice chap called OMS told me off as I might be upsetting the arrangements for UPS downstream! 
 
 Although a UPS the depends on an upsteam N-PE link doesn't entirely meet BS 7671 requirements - and will be in trouble when the mains cable feeding the UPS has to be entirely disconnected for maintenance/alterations. 
 - Andy.

MCCB as Main Switch - 3-pole or 4-pole?

Join us to get the best from IET EngX.

Community Rules & Guidelines