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Incoming devices 4P or 3P

MS01

Just a question that comes up a lot and I’m interested in people’s opinions are on what your default position is. Is the preference to isolate the neutral or leave it connected? I understand there may be circumstances where either is the correct choice, but this question is on a standard design what do you go for in a TN system 

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  • As a broken N on 3-phase systems can cause havoc (feeding 400+V to things expecting 230V) the traditional policy is to keep N as reliable as possible - and as moving parts are generally less reliable than solid joints, the usual approach is to use 3P incomers except when it's necessary to open N for isolation (typically in TT systems). Fashions differ though and there can be other consideration - e.g. if there are RCDs upstream.

       - Andy.

Reply

  • As a broken N on 3-phase systems can cause havoc (feeding 400+V to things expecting 230V) the traditional policy is to keep N as reliable as possible - and as moving parts are generally less reliable than solid joints, the usual approach is to use 3P incomers except when it's necessary to open N for isolation (typically in TT systems). Fashions differ though and there can be other consideration - e.g. if there are RCDs upstream.

       - Andy.

Children
  • in reply to AJJewsbury

    So 4 pole isolator incomers should not be used on dist boards fed by 3 phase PME supplies? I wondered about this when I looked at an installation with multiple dist boards - should I have bonded them together with some 16mm earth?

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