Suitability of 3-p MCCB instead of 4-p MCCB for supply sorcers

In the UK the Electricity at Work Regulations 1989 deemed the Neutral as a Live conductor and the installation main switch must break all live poles of the supply.

2 two pole for two wire supplies and 4 pole for four wire supplies

Prior to this there was a phobia in the industry about the broken neutral scenario and all main switches tended to be 3 pole + neutral with the neutral being a solid bar link.

There is no absolute requirement to replace these unless the installation is being altered.

The requirement for verified electrical testing has led to more installations being tested by both professional and some not so professional people and it is very easy to forget to replace a neutral link after the necessary disconnection for testing.

It is therefore also more desirable for the main switch to be a 4 pole switch action to improve this situation.

I don't know which country you are talking about and the situation may different else where.

Dear Sir,

I'm writing this from Sri Lanka, where we mainly follow IEE standards. I basically want to know, whether there are any theoretical reasons for using 4-pole breakers instead of 3-pole breakers for supply sources. Could you please help me on this regard?

Dear Asitha,

Your question is about sizing and protecting neutral conductor. It depends on ; - Earting system (TT,TN, IT, etc), harmonic current, and method of protection against indirect contact.

Liaison between pole of breaker and earthing system for 3-phase ; in TN-C scheme neutral must not be opened in any circumstances because it constitutes a PE as well as neutral conductor, usually connected all to neutral bar, then use 3-poles breaker. But in TT, TN-S, and IT schemes, use 4-poles. All of these are for the purpose of personnel safety against indirect contacts.

Check on IEEE std 1015-2006 (revision of 1015-1997), or IEC 60364 series.

I don't have any idea wheter IEE is one of international standard organization?

Dear Sir,

Thank you very much for your valuable thoughts.

IEE stands for Institution of Electrical Engineers, London. In our country (Sri Lanka) we mainly follow IEE wiring regulations as a standard in electrical installations.

Still, I do not fully understand the danger of using 3 - pole Breakers instead of 4 - pole breakers for source feeders when several source feeders are there (Utility & Diesel generator). Because 4 - pole breakers are very much expensive than 3 - pole breakers. As far as indirect contact is concern, we still can use a 3 - pole breakers to detect earth fault protection even in TT system. So, I would be very much grateful to you, if can explain this to me.

Dear you can refer to the attached site were you can find "maybe" for what you are looking http://www.cumminspower.com/www/literature/technicalpapers/PT-6006-GroundingAC-2.pdf

Fadi

Dear Sir,

Thank You very much for your help. I found your link very much interesting.

dear ,

in most of the mccbs neutral makes first and breaks last.

that means when you switched the mccb on , the neutral makes first so that if any fault would be present the neutral will break and thus the saftey for your main poles .

now when we switch off the MCCb , the neutral breaks last again saftey to your main poles .