3 vs 4 pole transfer switches

[mshields]

I know that you having a ground fault requirement on the generator side as a result of having any breaker in the EPSS being 1000A or more (assuming 480V). My question is, if you have a relatively large utility service that does require ground fault, but owing to the size of your generator, you do not need it on the generator side, THEN you could go with 3 pole ATS's.

In other words iits all about having ground fault on the generator side only.

Is that accurate?

 

[xptpcrewx]

I know that you having a ground fault requirement on the generator side as a result of having any breaker in the EPSS being 1000A or more (assuming 480V). My question is, if you have a relatively large utility service that does require ground fault, but owing to the size of your generator, you do not need it on the generator side, THEN you could go with 3 pole ATS's.

In other words iits all about having ground fault on the generator side only.

Is that accurate?

You don’t need GFPE for emergency systems. In the case of essential power systems, the life safety branch would therefore not require GFPE.

 

[tjkusa]

I understand you’re question to be to be whether or not a 3P ATS is satisfactory if you have a 1000A+ service with a genset that is smaller than the service with a genset breaker < 1000A. I’m assuming a 277/480V 3 phase power distribution system (I’ve seen consultants confuse the 150V+ to ground GF requirement incorrectly thinking it required GF on large 120/208V systems).
I would recommend a 4P ATS in this situation with genset grounded as a separately dervived system. Check out the article here: https://www.csemag.com/articles/choosing-between-3-pole-and-4-pole-transfer-switches/ where they give a scenario in which “In fact, the ground fault may not be correctly sensed by the system until the ATS returns to utility power. However, it might be seen at the normal-source breaker, causing the breaker to trip, even though the fault is not fed from the normal source.“
I agree with your general approach of 3P ATS for systems where both service and genset are < 1000A but would use 4P where either normal and/or standby system is greater than or equal to 1000A bringing GF systems in to play.
Xptpcrewx, ground fault indication is still required for the emergency or ‘legally required standby system’. Reference 700.31 and 701.26.

 

[paulengr]

I know that you having a ground fault requirement on the generator side as a result of having any breaker in the EPSS being 1000A or more (assuming 480V). My question is, if you have a relatively large utility service that does require ground fault, but owing to the size of your generator, you do not need it on the generator side, THEN you could go with 3 pole ATS's.

In other words iits all about having ground fault on the generator side only.

Is that accurate?

If you use a 3 pole ATS can you lose connection to the system bonding jumper in any event you are considering? Don’t just think of utility loss. If the main distribution panel is quite far away and the generator only protects a portion of the load, say only the server room in a large facility, this is a strong case for separate grounding and bonding with a 4 pole ATS. Think about for instance routine maintenance on any equipment between the service entrance and the ATS. If the genset feeds the whole facility and is located right beside the ATS and service entrance it might not matter.

 

[tjkusa]

If you use a 3 pole ATS can you lose connection to the system bonding jumper in any event you are considering? Don’t just think of utility loss. If the main distribution panel is quite far away and the generator only protects a portion of the load, say only the server room in a large facility, this is a strong case for separate grounding and bonding with a 4 pole ATS. Think about for instance routine maintenance on any equipment between the service entrance and the ATS. If the genset feeds the whole facility and is located right beside the ATS and service entrance it might not matter.

Good points on maintenance, they also added the requirement for signage at the main bonding jumper location the the MBJ serves an additional power system in an attempt to address this. Never seen this signage in the field however..

 

[xptpcrewx]

Xptpcrewx, ground fault indication is still required for the emergency or ‘legally required standby system’. Reference 700.31 and 701.26.

True but alarm/indication only function isn’t GFPE.