Why specify a 4-pole ATS?

[donw]

A 4-pole switch is more expensive and you need a second neutral bond at the generator.

The only reason I can see to specify a 4-pole ATS over a 3-pole would be distance of the generator from the switch. Is that right?

 

[iwire]

The only reason I can see to specify a 4-pole ATS over a 3-pole would be distance of the generator from the switch. Is that right?

I don't see what distance has to do with it at all.

Something that does mater is using a 4 pole switch when the feeder to the ATS has GFP protection.

 

[iwire]

Here is some more info

Ground-Fault Current: Problems and Solutions

 

[donw]

Bob, when I mentioned distance, I was thinking about a ground fault getting back to the genset. It would have to go back the the neutral bond in the service and then back to the genset. I was thinking that if the generator was far away, it might be better to have a 4-pole and bond the generator's neutral, too.

 

[jimmac49]

Ats

Ats

I think you want a "seperately derived system" the neutral conductor must be broken as well. Look at Article 700 of the NEC there are some distintions between legally required systems and optional systems.

Jim

 

[quogueelectric]

I dont know if this will help you I am not an engineer but in lets say 35+ years of service I have noticed that I have only seen the neutrals broken in hospitals. This I felt was particularly dangerous to electricians and the reason I noticed it is because it scared me many moons ago. I was testing an ats switch for something and thought that a feeder was dead. I tested all phases to ground first and saw 0 volts. To my surprise when I tested phase to phase I read 480 and this was a 400 amp ast switch I dont remember the exact details but because the neutral was switched the energised Phase conductors had no ground reference until the switch transfered.

 

[iwire]

I have noticed that I have only seen the neutrals broken in hospitals.

Any transfer switch that is downstream from equipment ground fault protection should be switching the neutral.

If not you can trip the GFP on a the main.

 

[donw]

Jimmac49, I looked at 700, and I do not see any requirement for emergency or legally required standby systems to be separatly derived. It sounds like Bob's reason (GFI protection) would be a good reason. My application will not have GFI (it'll probably be a 1000A 480V service, but I'll avoid using a main breaker.)

 

[MIEngineer]

Here is some more info

Ground-Fault Current: Problems and Solutions

Bob,

It is my understanding that in a 3-pole ATS the generator Neutral is not bonded to Ground as shown in the schematic, there is only a driven gound to tie to the enclosure if necessary. The only bonding is at the service entrance. Am I missing something?

Only when you switch the Neutral do you bond the Neut. & Ground at the genset. Then you have a separately derived system?

I have seen 4-pole ATSs used in data center to completely isolate them from the utility when on generator power.

 

[iwire]

Am I missing something?

Follow the link I provided, the image is an example of how not to do it and is illustrating the problem of multiple bonds with solid neutral switches.

 

[MIEngineer]

Thank you. I should learn to read before jumping to conclusions.

 

[steve66]

I dont know if this will help you I am not an engineer but in lets say 35+ years of service I have noticed that I have only seen the neutrals broken in hospitals. This I felt was particularly dangerous to electricians and the reason I noticed it is because it scared me many moons ago. I was testing an ats switch for something and thought that a feeder was dead. I tested all phases to ground first and saw 0 volts. To my surprise when I tested phase to phase I read 480 and this was a 400 amp ast switch I dont remember the exact details but because the neutral was switched the energised Phase conductors had no ground reference until the switch transfered.

Sounds to me like it was missing a bond at either the generator, or the service.

I think a generator feeding multiple services is another reason to use a 4 pole switch.

Steve

 

[bob]

When you use a 3 pole ATS the bonding jumper at the gen set is removed. The existing electrical systems EGC is brought to the gen set and bonded to the frame. The system neutral is brought to the gen set neutral.
With a 4 pole ATS the gen set neutral and frame are bonded together along with a ground rod. This neutral becomes the new neutral for the switched load.

 

[donw]

This is a little off subject, but with a 3-pole ATS and a non-separately derived system, do you usually bond the genset frame to a grounding electrode system (considering it a separate structure) or is the equipment ground good enough?

 

[quogueelectric]

Any transfer switch that is downstream from equipment ground fault protection should be switching the neutral.

If not you can trip the GFP on a the main.

I concur I just never thought about it not being the engineer. It makes perfect sense now.

 

[azhapp]

Swt'd Neutral

Swt'd Neutral

In our data centers we typically switch the neutrals with 4P ATS. We run the data centers at 120/208v, and have UPS and multiple genset backups. Our prime power overseas is typically also multiple gensets, sometimes questionable, so we need the capability to switch over...completely. The 4P ATS provides the ability to do all the switching for cutover and maintenance without ground loops...but you really MUST follow the Code with your grounding/bonding topology. Retrofits require research into the existing equipment to ensure personnel & eqpt safety.