3 pole or 4 pole breakers

[victa1288]

I am working on 400V system. 4 wire system is required for the incoming section. so the 4 pole circuit breaker is needed for the main incomers.

IEC also recommends that 4 pole circuit breakers shpould be used if SWGR or MCC feeds equipment in a hazardous area. But I am not sure about the theory behind it. Is it really necessary to do so? I noticed that a lot of people just simply 3 pole circuit breakers.

 

[waross]

I'm not sure about IEC land, but the conventions in NEMA land may give you an interesting point of view.
Except for a few fairly rare exceptions, the neutral is rarely switched in NEMA land. If the neutral is switched, the codes require the neutral to be either simultaneous operating or early make late break.
In the event of the failure of one pole of the switch or breaker to close, loss of one line of a three phase supply rarely causes damage, but an open neutral connection often causes expensive equipment damage.
My rule of thumb is to avoid neutral switching if it is at all possible.
respectfully

 

[jraef]

The only time I see neutral switching is when switching from one power source to another, where the neutral may be grounded at a different location and thereby set up a ground loop problem. Normally it is not switched in a standard 4 wire system. But like waross, I am in North America so I can't speak for your part of the world.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[ScottyUK]

I am working on 400V system. 4 wire system is required for the incoming section. so the 4 pole circuit breaker is needed for the main incomers.

Why? The neutral is normally left solid, unless a source transfer arrangement is present. If the neutral is switched then the breaker must be a 4-pole device, but normally a 3-pole device with a solid neutral is adequate.

The main reason I can see for a switched neutral in a hazardous area would be to prevent a possible ignition source if the neutral conductor made contact with earthed metalwork. I don't believe switching the neutral on a standard TNS or TNC-S distribution service is a requirement in the UK, or by implication in Europe because ATEX is a standard for the whole EU. There may be a requirement if you are using some unusual earthing system, e.g. an IT source or a high resistance grounded source where a significant voltage between neutral and earth could exist. Both of these are almost unheard of in the UK for LV systems, so I can't offer much guidance.

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[redha123]

In the case of unbalanced system the neutral is not neutral ..!

 

[waross]

In the case of unbalanced system the neutral is not neutral ..!

I think that you mean an unsymetrical system such as a 4 wire delta system or a network service (two phases of a star system). Even though the line referred to as the neutral may not be a true neutral by some semantic deffinitions, it is still the grounded circuit conductor.

 

[alehman]

My understanding is that in some parts of the world, four pole breakers are required by code for applications other than those mentioned above. If someone familiar with those requirements could provide detail it may be helpful here.

 

[ruggedscot]

If you are running a system utilising multiple power supplies then you have to be very careful here - Some generator systems can only function properly if you are utilising a four pole breaker - using a three pole and keeping in the neutral may lead to protection issues if your earthing points differ. Im sure that if you complete a survey of what you have then you can work out at just how your system utilises the neutral.

Rugged

 

[unclebob]

Are we talking about 400 Vac or dc?

 

[ougrad1999]

yes, 4P breaker is required per IEC 60364-1 and SquareD tech. paper

http://www.schneider-electric.com/cahier_technique/en/pdf/ect172p.pdf

 

[ScottyUK]

The referenced link contains no reference to any requirement for 4-pole circuit breakers instead pf 3-pole devices. It is a discussion of the various earthing techniques used in European practice. An interesting document for sure, but not really relevant.

Could you cut & paste the section of IEC 60364-1 which prohibits use of a solid neutral? I don't have the standard but I'm interested to have a read of that part because it is significantly different to the requirements of BS7671. The British wiring regs are different to the European equivalents: that's why the British Standard doesn't bear an EN number and won't until conformance with (assimilation into?) Europe has occured.


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Sometimes I only open my mouth to swap feet...