4 pole switching neutral 1

[a10jp]

This is a question on 4 pole switching nuetral. In our specification we called for fusible manual transfer double throw switch rated for service equipment application, 225A, and a switching neutral. However, I have resreached several companies, they do not make any fusible switch that comes with 4-pole. Why is 4-pole option not avilable? Moreover, I was told these profucts are not UL rated? Is it becasue when you switch or separate a bonded neutral that becomes a concern? How do you get a switching neutral for bonding to a separately derived system per NEC that will satisfy the UL requirement?

 

[davidbeach]

Yes, the assumption was transfer switch as service equipment. If you have a service with just a main, then the ATS, then all of the service related distribution you might well need a 4-pole switch IF you need ground fault sensing. As the system in the original question is only 225A there is no NEC requirement for ground fault sensing in any form other than overcurrent in the faulted phase, nothing zero-sequence. We've really gotten far away from the original question. In his simple system, the service could be grounded ahead of the MTS, use a 3-pole MTS, solid neutral at the MTS, no neutral ground bond at the generator, and only the neutral ground bond at the service equipment. Once you get to 1200A, 480V the whole game changes.

 

[alehman]

I agree completely.

 

[a10jp]

Hi, this is an old subject several months ago, and I am revisiting it again.

I am beginning to see the benefit of a 3-pole switch in this particular application. Although I ask the contratcor to provide a 4-pole switch only to comply with the spec, he made a mistake a got a 3-pole switch, so I have drew up a diagram to for a 3-pole switch connection.

Becasue this is for a protable generator, I am wondering several things.

For example, when the maintenance staff do their periodic testing, while the commercial power is on, and you do not want to de-enegerize everything, is it safe to connect the neutral of the generator to the neutral service bus (in the disconnect switch in my diagram) while power is on? For this reason, is a 4-pole a better setup than a 3-pole and when the connection scheme seems clearly?

Also, any comment to the fused disconnect switch? Because the load panel EM is also protected, is that being redundant? A few of then staff has said to do away the disconnect switch. However I see the fused disconnect is serving to protect the load as an additional measure because the size of the portable genset and the ckt brk that protects the genset can vary, so I cannot rely on it alone.

The contractor also suggests to use disconnect because over in Japan here they have trouble finding the right fuse, as long as the trip item is faster than the load circuit breaker. Is that workable?

Bill, David, what is your opinion?

 

[alehman]

I've never heard any concern expressed for connecting the neutral on an energized circuit. If that is a concern for you, then yes, 4-pole would be a better choice.

If it's likely that a generator with output breaker larger than the ATS rating will be connected, then overcurrent protection would be appropriate.

I don't understand the last question.

 

[a10jp]

On the last question, I typed too fast, I meant the contractor is more comfortable with circuit breaker than using fuse elements. My comment to them was to make sure the trip time for the external circuit breaker will trip faster than the load panel, but usually I would design using a fused disconnect in this situation.

 

[waross]

I am not familiar with your codes. In North America we would not use a neutral/ground conductor. We would use one conductor for the neutral and a second for grounding. The neutral conductor is normally run with the phase conductors and would be shown running through the transfer switch even though it has no connection in the transfer switch. Typically, there will be an insulated terminal in the ATS where theneutrals from the normal supply, the generator and the out going neutral are connected together but to nothing else.
A properly sized grounding conductor is then run to bond exposed metal surfaces.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[alehman]

It looks like your diagram is substantially correct in that regard, provided the note referring to "To building neutral/ground..." doesn't imply any connections not shown, and the "neutral bonding kit" is an isolated neutral bus without connection to ground.

Also, per the NEC the neutral conductors must be routed in the same raceway with the phase conductors (via the ATS). Not off to the side as you have shown.

 

[waross]

I missed that note, alehman.
The neutral bonding kits that I am familiar with are isolated terminals with a screw (often brass) that may me installed to bond the neutral to the sheet metal enclosure. The screw should NOT be installed. Your one connection between neutral and ground should be existing in the main service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[a10jp]

Hi Bill, Alehman, thanks for the comments. After I changed the 3-pole switch diagram with your sugeestions, I went to discuss with the customer. The comment I received was "it has to be 4-pole switch", for the following reason:

"All new or replacement 3-phase transfer switches shall have a switched neutral (4-pole switch) to prevent backfeeding current through the generator
neutral when the generator is not in operation and to prevent overcharging of exciter batteries by neutral
current."

I recognize the following:

1. The notes is applicable to a standby generator vs a portable generator as in our case.

2. I assume the above quoted statement is referring to "backfeeding current" through the neutral back to the generator when it is not in operation due to a fault in the main distribution system in a solidly connection neutral, 3-pole transfer switch situation. This is my assumption for the following discussion. If I am wrong, please point out.

3. The question is, based on what we have disucssed early on in this forum, in a 3-pole solidly connected neutral scenario, when your generator side of the switch is open (generator circuit is open), and a ground fault exists on the main distribution side, is there any possibilty for the fault to travel through back to the generator, if we all agree there is no N-G bond in the generator. This backfeeding concept contradicts my understanding how the fault should realistically travel back the system ground at the MDP.

4. How do you overcharge the exciter batteries by neutral current through only the 3-pole transfer switch arrangement and that a 4-pole switch can gurarantee the same will not happen? A little confusing how it can actually happen.

5. I did not see anywhere in the reason given above that mentions ground fault sensing as one of the condition for considering 4-pole switch vs 3-pole.


Here the customer is the AHJ, therefore I will not challenge their decision and will select a 4-pole switch (diagram attahced). However, I believe there are some very good discussion here.

 

[waross]

I feel your pain. I particularly resist manual neutral switched transfer switches. I have several times seen expensive damage when a nervous owner closed a cheap transfer switch slowly and the neutral pole closed last. But, what the AHJ says is the law. It is an argument that is not worth winning.
As for back feeding on the neutral:
If the neutral is not grounded at the generator there will not be any back feeding.
If the neutral is grounded at the generator some of the neutral current may flow through the generator neutral.
But, if the physical work is done well this should not be a problem.
In most installations the impedance of the normal neutral ground connection is much lower than the circuit out to the generator and back. As a result most of the neutral current will flow in the normal circuit rather than going out to the generator and back.
If the physical work is poor, there may be a higher impedance with even less current in the generator neutral circuit.
The neutral current in most installations is much less than the phase or line currents. Generally the current that may flow through a generator grounded neutral is a small part of a small current.
It will, however, degrade many types of ground fault protection.
Overcharging of the exciter batteries??? Sounds AutoMagic to me. This may be a case that Harry Potter can solve with witchcraft. I don't generally see any batteries on an exciter so overcharging would certainly be a challenge.
The AHJ may not always be right but he is always the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter