SERVICE GROUNDING ON A MAIN TIE MAIN BOARD UTILIZING SERVICE ENTRANCE TRANSFER SWITCHES ON BOTH SIDES.

[PE (always learning)]

Hey everyone,

I had a question about a design I'm currently in the middle of where they want to utilize (2) 3000 amp service entrance rated transfer switches on either side of a main tie main board. Currently, I show the neutral ground bond and service ground at the tie breaker location, but with the service entrance rated transfer switches do I need to rethink my service entrance grounding locations? It is my understanding that the service ground should only be in one spot and the tie makes the most sense to me. Please let me know what you think and reference the one-line attached for more information.

 

[jcbabb]

I'd put a GEC and N/G bond at each ATS.

 

[ATSman]

All of the double-ended subs I've seen, and as you said, the N-G is bonded at only one point: at the tie location. In the event of a GF this is the logical place to monitor the GF current and open the required source. I don't know why that would change with the addition of the 2 ATSs.

 

[__dan]

I was curious to see more replies. I am sure there are differences of opinion about this.

Two factors I see that weigh more than the others. At the point where the main or system bonding jumper is installed, in a fault, it is collecting fault current from the EGC facility grounding system and routing it onto the system neutral for the final path back to the source, the transformer or generator winding.

I see the ATS is a four pole, switching the neutral. I do not see any scenario where the fault current path can be switched or go through a switch. For this reason the bonding jumper and EGC GEC point would have to be line side of the ATS, so the fault current path avoids going through the neutral switch pole.

That makes the other source at the ATS, the future generator, also separately derived, and would need its own system bonding jumper, again line side and not routing fault current through the switched neutral. Conceivably the ATS could have two main or system bonding jumpers, both line side and isolated from each other by the neutral switch. The generator could also bond back at itself also, but not both.

The ATS's being the first disconnect and remote from the distribution gear, I do not see any scenario to bond downstream.

Another factor. The utility transformer likely also has its own frame bond to the system neutral but normally it does not count as the service point or the main bonding jumper. Typically unavoidable, this is a second present N to G bond outside your control.

This creates a parallel path for neutral and fault current to take both paths, making them noisy. The key factor is to be aware of this and mitigate it, by making the parallel path and exposed equipment at a minimum, meaning closest to the transformer and its other N to G bond, and not extending or exposing more of the equipment to this effect and hazard. moving the main system bonding jumper futher downstream to the much larger more remote distribution gear only increases this unwanted condition.

With the four pole ATS's and if you do not switch the fault current or run its path through the neutral switch, you could have four main or system bonding jumpers, two in each ATS, line side only at the switch.. The ATS I am sure has a moveable field installed busbar jumper, so I am sure it has been installed in every possible config. Which is why I am sure there are different opinions about this.

Once I became comfortable with a proposed scenario, I would shop it around with the gear and the generator manufacturers for their take on it.

 

[hillbilly1]

I would think you would need a four pole tie breaker, and the bond at each service rated transferswitch. I am assuming this tie is a backup if either source fails. I think you will either nullify or reduce the ground fault protection at either end, or have nuisance trips because of the utility bonding at both transformers paralleling the neutral currents.

 

[JoeStillman]

A lot of main-tie-main gear has aux contacts that are used to control the ground fault protection. This is so you can trip the right breaker, depending on the positions of the mains and tie. If you throw 4-pole ATS's and separately derived generators in the mix, you will have to totally re-think the ground fault protection.

I think 3-pole ATS's and non-separately derived generators are the way to go. Stick with one bonding jumper.

 

[__dan]

A lot of main-tie-main gear has aux contacts that are used to control the ground fault protection. This is so you can trip the right breaker, depending on the positions of the mains and tie. If you throw 4-pole ATS's and separately derived generators in the mix, you will have to totally re-think the ground fault protection.

I think 3-pole ATS's and non-separately derived generators are the way to go. Stick with one bonding jumper.

Agree. I did not want to suggest more of a redesign but if it were me, on the first pass that's what I would be looking for.

With three pole ATS's, the only way I could see it working is the main dual source bus, the entire length, would have to be considered 3 pole 3 wire with no neutral connected loads.

3 p 3 W can be done except the lighting, depends on how much 277 lighting there is. If it is only a small fraction of the total load which happens commonly, lighting would come off a 480 delta to 480 Y transformer so there is only all delta loads at the dual source main distribution bus and everything can be L to L only or 3 P 3 W.

There will still be additional N to G bonds at the service transformer and parallel paths for neutral current there onto the grounding paths, but you can clean that up by having only line to line connected loads at that dual source bus, making 3 pole ATS's and one main bonding jumper possible.

When I look at something like this it is the most common thing to find done wrong.

 

[PD1972]

I think the main bonding jumper needs to be at the service entrance ATSs to meet 250.24(B). Not sure if you have a choice here as the MTM switchboard wouldn't be service equipment based on your one-line. The MTM switchboards I have seen with 3P tie breakers, a common MBJ, and modified differential ground fault relays for the MTM breakers have all been service entrance applications. The transfer switch scheme throws a wrinkle in that design.