low voltage incoming breaker 2

[144x]

when the incoming breaker in a low voltage switchboard
shall be 3 pole and when it should be 4 pole?is there any
danger in using 4 pole breakers instead of 3 pole ones?
thanx.

 

[electrifire]

It is not normal to break the neutal in a 4 wire system except in cases where the neutal must be disconnected to prevent ground fault tripping problems such as on a generator in a hospital and similiar enviroments. It is also acceptable to interrupt neutrals in certain branch circuits in hazaradous atmospheres such as gasoline service stations.

Using a 4 pole device as the service disconnect is not normal and could cause problems when the neutral is disconnected but the phase circuits are not, floating neutral problems.

 

[144x]

by your answer I conclude that if all phases break together,then it is okey to break neutral.is that right?

 

[jbartos]

Suggestion: For complete galvanic isolation, the neutral should be disconnected. However, this is applied in justified cases only since it is not required normally. The 4-pole circuit breaker may be used instead of three-pole circuit breaker; however, it is more expensive, it is bigger, and it is more difficult to get off the shelf.

 

[giuseppe]

In a TN system the Neutral is normally treaten as phase conductor, and for that shall be isolated TOGETHER the phases. The TN-C-S system is expecting the Neutral to be also the Protective Conductor, and so shall not be isolated.
Four poles Breakers are bigger, more expensive, and normally avalable out of shelves from ABB Sace and Merlin Gerin/ Schneider. Square D is also a great source of this Breakers.
Good luck in choosing the Breaker...

 

[144x]

I'm confused with the last two answers.our system is a double infeed that only one of them is in service at any moment.the neutral of the AUX transformer is connected to earth at the transformer.if the neutral is disconnected
while doing maintenace on one of the incommings why protection is lost?

 

[kitha]

Hi 144x, 4-pole breakers are not at all dangerous and really better than 3 poles if we really need to isolate the total system(in most of the case we don't).Jbartos is correct. You can use a 4-pole breaker. But usually it is not the case as 4 pole breakers are bulky, bigger and expensive as jbartos said. If you need a complete isolation of any cable/busbar etc, use a 4-pole breaker. They are used for separately derived systems like gensets in order to isolate from the utility supplies as an example.

TN, TN-S, TN-C, TN-C-S terminologies are used by european electrical codes to describe different systems of earthing(grounding in North America). Giuseppe is partially correct. But in TN-C-S(Combined Separate) system the neutral is separated from the earth only at the service entrance panel and downstream of the distribution. If you are from european region we can discuss about it in more details.

By your wordings I guess you are in the North American region.In short you do not have to worry about using a 3-pole breaker at all. But if you wish you can use 4-pole as well.

 

[144x]

thankx kitha,
that was a relief .in fact I have used a 4 pole breaker and
was worried about safety concerns of breaking the neutral.
if you have any more detailed information I would be happy
to listen.

 

[kitha]

According to your description I imagined that your system is a loop feed(primary) and grounded neutral(secondary of the trans) and you are using a 4 pole breaker to disconnect low voltage feeders when you do the maintenance of any of the loop feeds. If that is the case you do not have to worry about anything. You could have used 3-pole which is cheaper.

You have to worry about breaking the neutral while the three phases are at work(energized). That may deliver line-line voltage to a single phase load as the neutral potential is floating as electrifire explained. Therefore never break the neutral while the phases are energized. 4-pole breakers break the neutral at the same time with the phases and no worries.