4 pole switching neutral
4 pole switching neutral
(OP)
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?






RE: 4 pole switching neutral
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
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"Why not the best?"
Jimmy Carter
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
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.
RE: 4 pole switching 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
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"Why not the best?"
Jimmy Carter
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
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
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"Why not the best?"
Jimmy Carter
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
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
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"Why not the best?"
Jimmy Carter
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
RE: 4 pole switching neutral
RE: 4 pole switching neutral
I'm not clear on how locating the N-G bond downstream of the ATS helps. Can you describe the ground fault sensing scheme?
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
RE: 4 pole switching neutral
RE: 4 pole switching neutral
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?
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
RE: 4 pole switching neutral
A properly sized grounding conductor is then run to bond exposed metal surfaces.
Bill
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"Why not the best?"
Jimmy Carter
RE: 4 pole switching neutral
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.
RE: 4 pole switching neutral
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
RE: 4 pole switching neutral
"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.
RE: 4 pole switching neutral
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
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"Why not the best?"
Jimmy Carter