Neutral in Automatic Transfer Switch

[pukar12345]

I saw automatic transfer switch with 4 pole where neutral of both source connected to one pole of switch and neutral of both switch never mixed.

I saw automatic transfer switch with 3 pole where 3 phases are connected to 3 poles and neutral is connected to common neutral bus. In this case neutral of two different source is connected together in common neutral bus.

I think neutral of two source should not mix together.
Dose CSA or ANSI code says about automatic transfer switch ?

I appreciate comment on this issue.

Thanks to all

 

[alehman]

It depends on how the system grounding is arranged. If the neutral is grounded at each source, then a 4-pole switch may be required. If only one source has its neutral grounded, then 3-pole may be needed.

We would need to see a system 1-line drawing to provide a more definitive answer.

IEEE Std 446 "Orange Book" has a lot of information on this subject.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rbulsara]

Under NEC/ANSI (and also from proper engineering point of view too), there are two type of systems. Separately derived systems and non-separately derived systems. The former will have two independent sources, each grounded individually. They employ 4-pole transfer switches, where neutrals are overlapped only momentarily and also first to make and last to break.

Non-separately systems, where the neutral is interconnected and grounded only at one point (at one of the sources). These system would permit 3 pole transfer switch with solid neutral. For more look up NEC code book. The idea is not to ground the neutral of the source at more than one point on customer premises.

4-pole transfer switches are also useful on premises with more than two sources, such as one generator but multiple utility services/substations.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

The neutrals may mix if there is only one connection from the combined neutral to ground. This is the simplest safest transfer switch arrangement.
However, there may be other considerations such as ground current detection schemes, long distances to an alternate source such as a standby generator or other issues that are best addressed by using a four pole transfer switch.
An example may be a standby generator that is a long way from the transfer switch and it is desired to ground the neutral at the generator rather than with a common ground at the main panel. This issue may be addressed by switching the neutral in a four pole transfer switch.

Bill
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"Why not the best?"
Jimmy Carter