Main bonding jumper in paralleling switchgear with utility and generator stand-by

[EI4]

Hello all. We are working on a paralleling switchgear enclosure that houses two service disconnect switches (circuit breakers) as well as two optional stand-by generator circuit breakers with a tie circuit breaker on the common bus between the service CB group and the generator CB group to act as the transfer mechanism. The tie circuit breaker is 3 pole so the system is not separately derived. Since the system is not separately derived, our recommendation is that the main bonding jumper must be provided only in the paralleling switchgear enclosure (in the service disconnect sections) and not also locally at the generators (generator neutral to case). Now, the paralleling switchgear manufacturer has noted that with this approach you will lose the neutral-to-ground connection for the generators if/when the neutral disconnect link in the service disconnect sections is removed for maintenance/service. We agree with them that this will happen, and it is because the neutral disconnect link must be located on the load side of the main bonding jumper connection per UL 891. The manufacturer has asked us to confirm we are okay with that kind of a contingency scenario.

So the question is: is this thing a big deal? To me, it would seem like it is not. It would seem like if one disconnects the neutral to do some service/maintenance AND if one de-energizes the paralleling switchgear to do such service/maintenance, they should make sure the generators are not turned on too, and how much of a deal it would be to have the generators ungrounded for the duration of maintenance. If, on the other hand, the maintenance involves removing the neutral disconnect link AND testing while energized, then I am not sure what exactly to think of it. But it would still seem like the emphasis at that point should be on the fact that BOTH the services and the generators are commonly ungrounded (in the sense of "no connection of equipment ground") for the duration the testing is taking place, not that the generators are somehow uniquely handicapped/posing a problem to the system because of the lack of the neutral-ground connections for them. What are your thoughts on the matter? Given our 3-pole tie breaker that does not switch the neutral, it definitely seems like it is a no-go to provide local bonding at the generators AND in the paralleling switchgear (because the service neutrals must be grounded somehow too) just as a way to address this potentially non-existent problem, and we definitely cannot eliminate the neutral disconnect links (which would be a violation of, among other things, NEC 230.75), and also, per UL 891, we cannot connect the MBJs to the load side of the NDLs.

Any help would be highly appreciated.

 

[waross]

You are overthinking it.
That said, I applaud your diligence.
Separately derived systems may share a common neutral.
All grounded systems on the continent may be argued as having a common neutral by way of the ground connection.
My preferred connection for standby sets is a common neutral for everything and a single bonding jumper to ground.
If you cannot use a common neutral, you must use a four pole transfer switch.
In over 50 years of field work I cannot remember ever having to remove the bonding jumper.
I have seen the damage when the neutral pole was late closing on a switched neutral.
Hopefully you will have a main neutral bus to which all neutrals may be connected.
From that,one jumper to the ground bus.
If each service entrance switch has a main neutral bus, install a suitable jumper between the two buses and ground only one of them.
By the way, I haven't installed a standby generator since last Thursday.
There may be an exception in the case of very long conductors between a generator and the transfer switch.
In a case that the impedance of the neutral conductor from the transfer switch is so high as to limit over-currents below the instantaneous trip settings of the protection, I would have a long serious talk with the AHJ asking for an exception to allow a neutral ground at the generator.
My justification would be the high impedance and current limiting of the neutral conductor.
My second justification would be that as a separately derived source, there is a parallel to the utility grounded neutral at the pole mounted transformer as well, as the consumer's ground in the service entrance.
Failing an exemption, I would be forced to use a four pole transfer switch.
It hasn't happened to me yet, but it is a possibility.
By long distance I am visualizing over about 1/4 mile, but the final determination would be the current limiting of the neutral conductor impedance with a short circuit to ground.
Note that a common utility connection is a ground on the neutral at the transformer and a second ground at the service entrance.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Mr. EI4 (Electrical)(OP)9 Jul 23 15:08
1. I take it that the system is a LV three-phase with neural solidly grounded system. The different power sources are with their neutrals solidly grounded separately and individually.
2. The concern by the switchgear manufacturer called for attention is NOT without reasons.
3. I noticed there are various complex GF protection schemes by using three pole breaker with solid link neutral.
4. I strongly suggest the use of four pole breaker and four pole transfer switch. By this all four pole (three phases and the neutral) are switched in one operation, which eliminates the concern by the manufacturer and potential GF protection problems which may not be apparent at this stage.
5. Four pole breakers and transfer switches are available by all breaker manufactures, with very small extra cost and physical size.
Che Kuan Yau (Singapore)

 

[EI4]

Yes, four pole tie circuit breaker (which is what accomplishes the transfer mechanism here) (and maybe all other breakers too) may still be an option. We will try to give more consideration to it.

 

[waross]

Yes, four pole tie circuit breaker (which is what accomplishes the transfer mechanism here) (and maybe all other breakers too) may still be an option. We will try to give more consideration to it.

Now one of us is confused.
Are your two services fed from different transformers?
Can you supply a one line diagram?
It is difficult to see how you can transfer with only a tie breaker.

3. I noticed there are various complex GF protection schemes by using three pole breaker with solid link neutral.

One or two properly located rogowski coils will solve those issues.
I suggest a common neutral bus with one jumper from the bus to ground.
Use rogowski coils to differentiate ground faults between the two services.
Alternately: a common ground bus for each service.
plus
A second intermediate bus.
A jumper from each service neutral to the intermediate bus.
Ground detection CTs to be placed on these jumpers.
One jumper from the intermediate bus to ground.

If one service fails, that tie breaker should be opened before transferring to the generator.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!