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a10jp (Electrical) (OP)
30 Aug 08 4:04
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?  
waross (Electrical)
30 Aug 08 4:39
I haven't seen a fusible manual transfer switch. I have seen lots of non fusible switches.
If you are using a manual transfer switch, your system is probably simple enough to use a solid neutral and a 3 pole switch. Normally you would use a service entrance fused switch or service entrance breaker to feed the unfused manual transfer switch.  The standby side would be protected by the generator breaker. The grounding electrode's would be connected to the utility neutral in the service entrance switch. The generator neutral would be run to the service entrance switch and connected to the utility neutral, thereby grounding it. The equipment grounding conductor would be run back to ground the generator frame, but not the neutral at the generator.
Four pole switching is usually an expensive adder to an automatic transfer switch and is used where the system is too complex for a single common ground point.
In some instances a 4 pole transfer switch must be used, but it is much simpler and cheaper and more dependable to use 3 pole switching whenever possible.
If you are not connecting a standby generator, what are you connecting?
 

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

Helpful Member!  gepman (Electrical)
30 Aug 08 10:24

I agree with waross in how he describes to connect the neutral to ground at the two different sources.  I don't see why a 3 pole switch would be any more reliable than a 4 pole switch.  I always specify 4 pole switches for a couple of reasons.  One is that a 3 pole switch is not always compatible with the ground fault interruption in some older systems.  Second is that in a few cheap generators I have seen neutral to ground connections which are not easily removed.  I can always NOT use the fourth pole and solidly connect the neutral but it is impossible to add the fourth pole once you have a 3 pole switch.

I think that the reason you won't find any 4 pole fused transfer switches is that the NEC requires, Art. 240.22, (and therefore UL won't list) that if you have overcurrent protection in the neutral it must also interrupt all of the ungrounded phase conductors at the same time.  This is difficult to do with a fuse.  You can see that you would have a serious safety issue if your neutral fuse opened but the phase conductors did not.

You could use 4 pole circuit breakers.  Cutler-Hammer makes 4 pole ATS's out of two 4 pole circuit breakers.
a10jp (Electrical) (OP)
30 Aug 08 12:40
THank you guys.  Thanks gepman I think your explanation makes sense.  I have seen 4 pole ATS setup but not a fusible MTS with 4-pole, specifially one that is listed as service entrance by UL.  That was my original question.  The MFR ha stated even if they can manufacture such a 4 pole fusible switch if will never be listed by UL.  

FOr this application, the MTS will be fed by a portable generator to feed a portion of the facility load(refrigerator load, 225A) for a restaurant.  This answered waross's question.  I was concerned if the emergency load, which is 3ph, 4W) the neutral is not disconnected or switched, you could have a potential of backfeeding of power if the neutral is bonded.  I always like to use 4 pole switch and make sure the switching neutral has a proper location to bond to.  

I noticed they do not make 4 pole fusible transfer switch, I was planning to use a 4-pole fusible disconnect switch which gives me a solid neutral connection (which is listed for service entrance) and then feed to a 4 pole nonfused DT transfer switch.  But before that I wanted to understand why a fused DT transfer switch is not made, and I think it is answered above.   Thanks guys.   
waross (Electrical)
30 Aug 08 13:46
I have seen some expensive damage caused by an early neutral break in cheap transfer switches several times. (Made in China, not UL, third world, no approvals required.) I have seen mechanical failures that left one pole open on many old switches. I have never had an open neutral problem in a switch with a solid neutral.
I agree with gepman that ground fault protection issues may require a 4 pole switch and some generators may have a dificult to remove ground jumper, but I have always been able to solve the problems and use a three pole switch.
Back feeding; You will not back feed power on the neutral if the phase connections are open.
That said, there will probably come a day when I need to go with a 4 pole switch. In the range of 225 amps at a restaurant I doubt that you will have complex grounding issues and most new generators have an easy to remove internal ground connection.
I would use a 3 pole switch.
gepman would use a 4 pole switch.
I have never seen a transfer switch rated for service entrance. A fused switch would have to have a solid neural as gepman points out.
Neither is wrong.

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

sobeys81 (Electrical)
31 Aug 08 10:12
Just a quick comment here ! Please correct me Waross if I am wrong.

As per CEC, If your service entrance is more than 400 A then you need ground fault protection and if your generator is rated for the same amperage and has ground fault protection as well, then Utility neutral will be disconnected during transfer switching and Generator neutral will be connected to the load neutral.

But ultimately, the generator neutral will be connected to the building ground at the generator. I think it is just a proper operation of generator ground fault protection rated with a high amperage.


 
waross (Electrical)
31 Aug 08 14:27
Hello sobeys81;
One caveat before I start to ramble. A solid neutral presents no problems to the system and will not back feed power. The reason for switching the neutral has been to accommodate different system grounding methods. A solid neutral was a safe dependable trouble free installation. As ground fault monitoring and tripping is applied to more and smaller systems, there arise more complications that may be solved by switching the neutral of a standby generator installation.
Over the years I have seen too many devices, switches, breakers, and contactors fail with one pole open to be enthusiastic about any switching of a neutral. During my years in the tropics with no codes or inspections, little training and questionable workmanship I saw the damage that resulted from open neutrals fairly often. With this background you will understand why I resist switching neutrals when I can avoid it.
But, back to your question.
I am a 12 hour drive away from my current CEC code book.
I have just spent some time reviewing the 1998 code. In 1998, the limit was 2000 amps rather than the 400 that you quoted.
And in 1998 ground fault protection for generators only applied to generators operated in parallel with the supply authority.
Under the 1998 code, it may be possible to use a CT on the common system grounding link and use logic to direct the trip signal to either the utility breaker or the generator breaker as required. It may be several days or more until I am reunited with my current CEC.
With the more complex systems and ground fault trip systems coming into use I may someday bow to the inevitable and follow gepman's lead and start using 4 pole transfer switches.

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

a10jp (Electrical) (OP)
1 Sep 08 3:48
Hi waross, I think the problem I faced with in the beginning is that there is no neutral bus in a fused 3-pole doublr throw switch.  So when the load is switched over only the 3-phase is switched, but the neutral is not.  Futher, they will be connecting to a portable genset in the future, which I am not sure how they will be connecting for sure.  I am locating in Japan and grounding is a very foreign subject than what we accustomed to in the US, and it is no joke.  Because there is no neutral bus in the DT switch, where else would they be connecting the ground.  I fear they will be connecting to a separate grounding electrode (common in Japan) and in this case without any direction connection to the service neutral, it will completely violate NEC art. 250.20 & 250.30.  

My intention is to assure that I give them a dedicated place to connect the neutral from the portable generator before I switch.  The sequence of operation I am thinking is, first you disconnect the normal load with a 4-pole non-fused DTS, then you switched to a 3-pole + solid neutral fused safety switch (overcurrent protection is required in the design.  The ground from portable genset will be connecting to the neutral at the safety switch.  Let me know if I going about the wrong way.
waross (Electrical)
1 Sep 08 4:23
You may easily buy a neutral bus and field add it to your 3 pole double throw switch. It may well be a listed option.
Many wholesalers stock neutral bus kits for 3 pole fused and unfused switches separately. You only buy the neutral block if you need it. It is a connection point for the neutrals that is mounted on a small insulator. There are tapped holes in the switch box for mounting. There is also a third tapped hole and a supplied screw that will connect the neutral bus to the switch box if this is to be the neutral grounding point for the system.
I would use a 3 pole switch and install a neutral block to land the neutral on. Cheaper than a 4 pole switch.
gepman would install a 4 pole switch and switch the neutral also. These are different points of view. Neither is wrong.
 

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

davidbeach (Electrical)
1 Sep 08 12:03
For what it's worth, I'm with Bill on this one.  Arrange the system in such a manner that there is one neutral-ground connection, downstream of the transfer switch and go with a 3-pole switch and a solid neutral.  Far less can go wrong that way.  If the neutral is switched, it needs a different contact in the switch so that it opens last and closes first.

It can be hard to avoid 4-pole switches when there are multiple transfer switches in a system, but with only one transfer switch it will (almost) always be better in the long run to have a 3-pole switch and a solid neutral with the neutral-ground bond happening downstream of the switch.
a10jp (Electrical) (OP)
1 Sep 08 20:52
Thanks Dave, Bill.  Thankn you for your invaluable advice.

 
davidbeach (Electrical)
1 Sep 08 22:30
No Dave here, it's David.
alehman (Electrical)
2 Sep 08 0:35
David,
I'm not clear on how locating the N-G bond downstream of the ATS helps. Can you describe the ground fault sensing scheme?
 
davidbeach (Electrical)
2 Sep 08 9:02
alehman, describe what you are seeing (or not seeing) in how the ground sensing works or not.

There's no ground connection to the neutral on the generator, so no ground fault current in the circuit between generator and bond location, all current is on one of the four conductors (ABCN).  Source from the utility transformer is similar although it is generally more difficult to fully isolate neutral from ground at the service transformer.  The NEC deliberately ignores any ground reference created there.
alehman (Electrical)
2 Sep 08 21:16
If the neutral sensing is downstream of the N-G bond it would work properly, not if upstream.

For services, the NEC requires ground fault sensing "for solidly grounded wye services" and "shall operate the service disconnect". 230.95  I guess that's somewhat open to interpretation, but normally it would be at the service disconnect.

For generators, the NEC isn't specific on the relay location (when one is required). Obviously the closer to the generator, the better the coverage.

The grounding electrode must be connected to the neutral somewhere between the service drop and the service equipment neutral bus. For dual-fed services in a common enclosure or grouped together, the connection can be at the tie point. The main equipment bonding jumper must be in the service disconnect enclosure.

I think your suggestion for placing the N-G bond at the ATS would work provided the ATS is "grouped together" with the service disconnect.

Typically I prefer 3-pole switching. In some cases we can put the ATS in the service switchboard and put the neutral sensors on the load bus which solves the problem. For separate ATS's, I've looked in to remote mounting residual neutral sensors, but manufacturers seem to have significant restrictions on the secondary wire length.
 
a10jp (Electrical) (OP)
4 Sep 08 7:12
Hi Alehman, thanks.  For the application we have, we will be using MTS only to backup power for the refrigerator load in the restaurant.  The portable and the hookup are all manual, so becasue of that, I was particularly concerned how to properly handle the neutral connection when the spec specifically called for a switching neutral setup.  The MTS will be located close to the main service and I have specifically asked for the installer to include bonding kit on the 3 pole double throw switch for bonding to the main neutral at the service point.
TxPowerEE (Electrical)
4 Sep 08 18:55
" I have never seen a transfer switch rated for service entrance. "

These are made by several mfg's but of different designs.  CH, ASCO, Russel, Thompson some use breaker pairs some use a breaker in conjunction with a traditional UL listed contactor based ATS.
alehman (Electrical)
4 Sep 08 21:34
a10 - I was well off-topic. You most likely do not have ground fault protection so my comments pertaining to that would not be relevant.

Per the NEC, the equipment bonding jumper must be in the service disconnect enclosure. If you use a service rated MTS, do that and you should be good to go. If not, put the bonding jumper in the service disconnect per code rather than at the MTS. Either way, no N-G connection at the generator.

 
a10jp (Electrical) (OP)
5 Sep 08 0:25
Thanks for your advice Alehman.
RyanJackson (Electrical)
25 Sep 08 21:15

Quote:

It can be hard to avoid 4-pole switches when there are multiple transfer switches in a system, but with only one transfer switch it will (almost) always be better in the long run to have a 3-pole switch and a solid neutral with the neutral-ground bond happening downstream of the switch.

I don't see how this can be done in a compliant manner. If a three pole switch is used, the n-g conection would have to be made upstream of the transfer switch, becasue the transfer switch can't be upstream of the service equipment (230.82). This all changes of course, if the transfer switch is the service equipment, but I find this to rarely be the case. Having said that, I don't see how a n-g connection downstream of the transfer switch would not violate 250.24(A)(5) and 250.6.
davidbeach (Electrical)
25 Sep 08 21:44
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 (Electrical)
25 Sep 08 21:50
I agree completely.
a10jp (Electrical) (OP)
8 Feb 09 22:30
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 (Electrical)
9 Feb 09 0:22
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 (Electrical) (OP)
9 Feb 09 2:05
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 (Electrical)
9 Feb 09 7:12
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 (Electrical)
9 Feb 09 9:07
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 (Electrical)
9 Feb 09 9:19
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 (Electrical) (OP)
13 Feb 09 9:33
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 (Electrical)
13 Feb 09 10:04
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

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