3 Pole ATS's on 480V older 480V systems

[mshields]

I'm in the process of conducting a survey of ATS's at a hospital. The service is 480Y/277V and 4000A. They do have ground fault protection as you would expect but all of the ATS's are 3 pole. We are looking to replace multiple generators and before I tell them they have to replace all of the ATS's, I'm just looking for a sanity check. As far as I know, it's a no brainer that they are going to need 4 pole switches and to bond the neutral to ground at the generator in order for the ground fault to work properly. The reason I wonder if I'm missing something is that, while these switches are old; like 30 years old, what's changed from then to now. Was ground fault not required before a certain time?

 

[tom baker]

I believe GF protection requirement started in the early 70s. Before then the common distribution system was 480 3 phase.

 

[texie]

Here is an excellent white paper on this subject that I often refer people to (2 parts):

https://mart.cummins.com/imagelibrary/data/assetfiles/0056629.pdf

https://mart.cummins.com/imagelibrary/data/assetfiles/0056582.pdf

 

[ron]

Are the feeders downstream of the generator or the ATS 3 wire? Maybe there are only 3 wire loads?

You will have a hard time convincing them of an issue if they haven't had false trips of the GFP in 30 years.

But you are right, there is something that doesn't add up.

 

[mshields]

Having boned up on the 3 vs 4 pole issue, I wonder if any of you can tell me this. The ground fault that cannot be sensed in scenarios involving 3 pole transfer switches, is the ground fault at the generator; when on the generator, ground fault travels to the neutral via the service ground and then on the neutral through the solid connection and that GF sensor sees a zero summation of current; What if instead of a main breaker on my generator of 1000A or more, I go with individual breakers each feeding individual ATS's, each one under 1000A and therefore not requiring Ground Fault. The sum of those breaker cannot exceed 125% of the FLA but as long as I obey that rule, I won't need GF correct. Would that solve the problem. Again, I'm not strarting from scrath. I've got a boat load of existing 3 pole transfer switches I'm trying NOT to have to change out.