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3 Pole vs 4 Pole Circuit Breaker Usage 2

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romanskat

romanskat

Electrical
Dec 17, 2002
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Hi Mates,

kindly illustrate the difference between 3 pole circuit breakers with 4 pole.

one quite significant difference i aware is on the price between 3pole vs 4 pole based on same rating.Because 3pole cheaper, so we always facing recommendation to use 3 pole rather than 4 pole.

Therefore need your help to understand more on the difference in application between 3 pole with 4 pole.

thanks.
 
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Use 4-pole when you need to open the neutral along with the three phases. Application is usually based on local electrical code requirements and/or system grounding and protection design.
 
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4 pole seems to be an IEC thing. I have never found myself looking for a four pole breaker. If readily available and not much more expense than a 3 pole breaker they could have simplified a few grounding schemes over the years, but given the base cost of a 2000A or larger low voltage power breaker (or any medium voltage breaker) a premium of only 33% for the fourth pole for three or four breakers means it is less expensive to arrange the system such that all required neutral-ground bonds happen at the same location. 4 pole breakers would seem to be effective if they could be had for less than a 5% premium and I doubt that will ever happen.
 
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They are needed in TN-S systems, where all current-carrying lines (that includes the Neutral) need to be switched. In such systems, it is not a question of a certain cost limit if they shall be used or not.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Thanks guys.

But Skogsgurra, why is TN-S systems must switch the neutral pole at the same time? Kindly elaborate more on the technical side?
 
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There are several reasons.

One is that the N conductor often has a smaller area than the L1-L3 conductors and that means that the N conductor can be overloaded if it is not protected and can be opened in case of an overload.

Another reason is that the N conductor usually has some residual voltage on it. If you want to make sure that an installation is completely "dead" (=no potential at all), then you need to break also the N conductor. This is often the case in hospitals.

This Schneider link has more information.
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Gunnar, what is a TN-S system? If what is shown in figures 9 and 10 of your referenced Schneider paper is an example of a TN-S system, it looks dangerously improperly grounded, violating the basic requirement that systems be grounded exactly once. With both source breakers closed all of the problems of multiple grounds are present and it is only after one breaker opens, opening the neutral and reducing the number of neutral grounds to one that the ground relaying could accurately determine what is going on and where the fault might be. Looks to me like there are better ways of designing the system than what is shown in those two figures.
 
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Hi David.
TN-S system, that mean : Neutral conductor (N) and protective conductor (PE) are separated.
BTW, additional systems:
TN-C : N and PE are combinated , now is PEN.
TN-C-S, you have combination of both of system
And, what I remambare: N-is blue color and PE is yellow/green color. And additional systems are TT and IT ( isolated system or HI sytems)
Regards.
Slava
 
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I'm also understand, whay you ask about Fig9 and Fig10,
it's some very schematic drw. I try found my old papers with "better" drw.
Regards.
Slava
 
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The neutral would be switched if there is considerable
amount of Triplen Harmonics. This would be a result from
a Delta-Wye transformer connection. The triplens are not
cansuled in the neutral as other currents,but they add
as much as 1.73 X phase current. Single phase computer
systems are well known for this (non-linear loads).
 
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davidbeach,

There is more to grounding than you think. What is the ultimate truth in the US may not be the truth in Europe. And, without being overly partial, I dare say that we have analysed these problems a lot more thoroughly than you have on your side of the pool. Read and learn!

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Gunnar,

I hope you are hiding in a bunker!

Slava,

Thanks for that IEE download. I have seen it but lost my copy. - I want to send it to a few guys at work.


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Gunnar, I know the IEC world does things differently, some of which seem pretty strange. I'm not sure how if >6mA is considered lethal here it would take 30mA there to become a hazard; I sure someone can provide all sorts of studies to support both numbers.

I'm not convinced that switching neutrals is a viable means of getting around the problems created by having two neutral-ground bonds when one is trying to detect ground faults using a residual connection.

I don't know which side of the pond may have done the most research on the topic, but regardless both sides of the pond seem to be comfortable with what they do and rather uncomfortable some of the practices on the other side of the pond.

In the example of the figures cited, I would leave the wye points ungrounded and bring in four wires, landing the neutral on the switchgear neutral bus and then forming a single neutral-ground bond in the gear. That way there is only one return path for ground faults and they can be readily detected and cleared.

But if you are satisfied with the safety and efficacy of the protection for your system designs then I won't argue with you.
 
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Yes, what you are not used to does always seem strange. But strange doesn't mean wrong. There are horror stories about your open delta, sometimes corner grounded, being told in Europe. And I am sure there are similar stories being told over there about our seemingly silly "two grounds".

The big conceptual problem seems to be that we make a very definitive difference between Neutral and Ground in TN-S installations. During my visit this October to some drive installations in Elizabeth, NJ, I was told by the foreman that everything had been done wrong in those cranes. Mainly because of the way the Neutral and Ground were connected and used.

It takes some exposure to a system to fully understand it. The TN-S system is not just another way of doing it. It is a way of keeping installations safe in a lot of fault situations. But, surely, it can be a can of worms for those that are not used to think "outside the NEC".

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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I'm in favour of four pole breakers in circumstances where multiple sources can feed one load, e.g. in a source transfer arrangement, especially where the transformers are physically separated. Earthing around source transfer systems gets much more complicated if there are multiple sources which can operate in parallel.

As David alluded to, residual connected earth fault detection becomes unpredictable with multiple neutral-earth bonds, but multiple neutral-earth bonds are not permitted under UK regulations except in a PME installation. PME installations are explicitly prohibited in the UK except for utility distribution systems so for most people the problem of multiple neutral earthing does not arise if a four-pole breaker is used to open the neutral when disconnecting the source, thus removing the neutral-earth bond at the origin of that supply. A three pole breaker would leave an additional neutral-earth bond connected which would potentially upset any residually connected earth fault detection relay such as those commonly fitted within ACBs, in addition to being a code violation. I guess the US solution would be to use an isolation transformer for each supply and locally earth the transformer secondary neutral point on the switchgear?


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I agree with Scotty. Isolation (service)transformers with sec neutral grounded solves the "one-ground at source" code stipulation.
David, in his post could have been wary of correcting problems related with too many ground paths for residual-connected earth fault protections!

Also, newer systems configurations on the other side of the pond require newer electrical protection equipment! Makes millions!

I prefer more simple and cheaper systems!
 
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