MBJ in paralleling switchgear with utility and generator stand-by

[DJ7]

Hello all. We have a system with two large 480Y/277V utility services feeding a large facility (two services are acceptable for our application per NEC 230.2 (C)). In addition, there are two generators to provide stand-by power (optional stand-by). There is a paralleling switchgear enclosure that houses the service disconnect circuit breaker for each service, a 3-pole tie circuit breaker section, and the circuit breaker for each generator (and the paralleling controls, ground fault detection, etc.). So the configuration is main-tie-main. And broadly, the intended operation is that the tie circuit breaker is normally open, and closes when neither of the services is available to turn on the generator(s). Since the tie circuit breaker is not 4-pole, the neutral is not switched, and per the NEC rules to prevent objectionable currents, we believe that the neutral must be bonded to the equipment grounding system only in the paralleling switchgear*, and not also at the generators. Everyone is kinda okay with this but the paralleling switchgear manufacturer has brought up the following point:

Per some of the standards that they use like UL 1581/891, and also to meet NEC 230.75, they will provide a neutral disconnect link in each of the service disconnect sections. And according to the manufacturer, UL 1581/891 requires them to provide the main bonding jumper connection on the line side of the neutral disconnect link**. Because the MBJ is on the line side of the NDL, when the neutral disconnect link is removed for the purpose of service and maintenance tests, etc., the generator neutrals will not have a connection to the equipment grounding system, if they install per our recommendation which is to not bond the generator neutrals to the generator cases (local grounding). The manufacturer has asked us to confirm we are okay with that kind of a 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) 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, 230.75), and also, per the manufacturer, we cannot connect the MBJs to the load side of the NDLs**.

Any help would be highly appreciated.

*The plan is to provide main bonding jumper in two places, one in each of the service disconnect sections, per 250.24 (C), without claiming 250.24 (C) Exception No. 1. Not 100% sure if this is the right approach. Maybe only one MBJ has to be provided in the paralleling switchgear (Exception No. 1 has to be claimed)? This is one of the other questions I have about this system.

**I do not have access to UL 1581/891 so I cannot personally verify this. But I have assumed it to be correct.

 

[don_resqcapt19]

Hello all. We have a system with two large 480Y/277V utility services feeding a large facility (two services are acceptable for our application per NEC 230.2 (C)). In addition, there are two generators to provide stand-by power (optional stand-by). There is a paralleling switchgear enclosure that houses the service disconnect circuit breaker for each service, a 3-pole tie circuit breaker section, and the circuit breaker for each generator (and the paralleling controls, ground fault detection, etc.). So the configuration is main-tie-main. And broadly, the intended operation is that the tie circuit breaker is normally open, and closes when neither of the services is available to turn on the generator(s). Since the tie circuit breaker is not 4-pole, the neutral is not switched, and per the NEC rules to prevent objectionable currents, we believe that the neutral must be bonded to the equipment grounding system only in the paralleling switchgear*, and not also at the generators. Everyone is kinda okay with this but the paralleling switchgear manufacturer has brought up the following point:

Per some of the standards that they use like UL 1581/891, and also to meet NEC 230.75, they will provide a neutral disconnect link in each of the service disconnect sections. And according to the manufacturer, UL 1581/891 requires them to provide the main bonding jumper connection on the line side of the neutral disconnect link**. Because the MBJ is on the line side of the NDL, when the neutral disconnect link is removed for the purpose of service and maintenance tests, etc., the generator neutrals will not have a connection to the equipment grounding system, if they install per our recommendation which is to not bond the generator neutrals to the generator cases (local grounding). The manufacturer has asked us to confirm we are okay with that kind of a 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) 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, 230.75), and also, per the manufacturer, we cannot connect the MBJs to the load side of the NDLs**.

Any help would be highly appreciated.

*The plan is to provide main bonding jumper in two places, one in each of the service disconnect sections, per 250.24 (C), without claiming 250.24 (C) Exception No. 1. Not 100% sure if this is the right approach. Maybe only one MBJ has to be provided in the paralleling switchgear (Exception No. 1 has to be claimed)? This is one of the other questions I have about this system.

**I do not have access to UL 1581/891 so I cannot personally verify this. But I have assumed it to be correct.

Actually you do, but just like the free access to the NEC, it is not very user friendly...in fact it is exactly the same user interface.
You just have to register with an e-mail address and then you can select digital view and read the document.

 

[DJ7]

Actually you do, but just like the free access to the NEC, it is not very user friendly...in fact it is exactly the same user interface.
You just have to register with an e-mail address and then you can select digital view and read the document.

I never even thought of this before. Thank you very much!

 

[__dan]

You say the two services are main tie main and those are three pole. It seems you say that's existing. It's possible that gear would have one or two main bonding jumpers, very close to each other in the same gear. Two utility service neutrals to one site GEC and EGC system in one piece of gear.

I am assuming or guessing the generator paralleling gear is an entirely separate and possibly remote located gear package. And possibly you indicate this is new and not there yet. It's not clear the present state.

I don't see it said if the generator ATS is is 4 pole or 3. Do the generators parallel to a common bus, the utility never parallels in your mtm gear.

One thing I can say for sure is that if you plan a manually removable additional main bonding jumper in the paralleling gear, or at any location on the system, what will happen is at any future time there will be no one onsite who know if the jumper should be in or out. I don't see how you can plan a system that requires servicing that requires making sure the mjb comes out and goes back in for a service procedure.

That makes no sense and I'm sure it will not make sense in the future. No one will touch it unless, after the fact something bad happens, and then the jumper gets blamed. That's the only time the jumper would get looked at again, only if they have to.

I could try to read the OP a second time to see if it reads differently.

Any large dual source bus, it is likely more than one mjb will happen either by planning or otherwise, along with the problems that may occur.

First thing I would do is to survey and find the location of all of the existing mjb's, at the main tie main service and at the generator gear, and Amp clamp them for circulating current on the main bonding jumper(s). I would expect to find something large there already. It would help me to know what problems are there already before I touch it.

Then look at the load side. If you can assure all of the loads are line to line connected with no, zero, line to neutral connected loads, that's the clean way to do it and mitigation of circulating noise current on the facility grounding system is accounted for by that arrangement. If you have line to neutral connected loads at the dual source bus equipment, shared neutral current on the grounding paths is likely a problem.

Throw in GFI trip on the feeder breakers at the dual source bus and the scenario really needs a good look by someone who really knows what they're doing. That's my point that guy is not going to be there in the future when you're planning it's necessary for some reason to move the removable MJB link. The manufacturer may provide that but its installation position would become permanent.

 

[__dan]

If the generators closes to the bus with three pole breakers, the generators are non separately derived and do not have or get a main bonding jumper.

As far as the manually removable neutral link, I do not know under what testing conditions that link would ever be removed. The link needs to be in and I don't see why you would ever want to remove it for a testing procedure.

 

[Joethemechanic]

We have a system with two large 480Y/277V utility services feeding a large facility (two services are acceptable for our application per NEC 230.2

Round House in Philadelphia? One 600kw powered by a 16v71 Detroit Diesel, and One 900kw powered by a 16v92 Detroit Diesel? I think both units were made by Spectrum

 

[DJ7]

You say the two services are main tie main and those are three pole. It seems you say that's existing. It's possible that gear would have one or two main bonding jumpers, very close to each other in the same gear. Two utility service neutrals to one site GEC and EGC system in one piece of gear.

I am assuming or guessing the generator paralleling gear is an entirely separate and possibly remote located gear package. And possibly you indicate this is new and not there yet. It's not clear the present state.

I don't see it said if the generator ATS is is 4 pole or 3. Do the generators parallel to a common bus, the utility never parallels in your mtm gear.

One thing I can say for sure is that if you plan a manually removable additional main bonding jumper in the paralleling gear, or at any location on the system, what will happen is at any future time there will be no one onsite who know if the jumper should be in or out. I don't see how you can plan a system that requires servicing that requires making sure the mjb comes out and goes back in for a service procedure.

That makes no sense and I'm sure it will not make sense in the future. No one will touch it unless, after the fact something bad happens, and then the jumper gets blamed. That's the only time the jumper would get looked at again, only if they have to.

I could try to read the OP a second time to see if it reads differently.

Any large dual source bus, it is likely more than one mjb will happen either by planning or otherwise, along with the problems that may occur.

First thing I would do is to survey and find the location of all of the existing mjb's, at the main tie main service and at the generator gear, and Amp clamp them for circulating current on the main bonding jumper(s). I would expect to find something large there already. It would help me to know what problems are there already before I touch it.

Then look at the load side. If you can assure all of the loads are line to line connected with no, zero, line to neutral connected loads, that's the clean way to do it and mitigation of circulating noise current on the facility grounding system is accounted for by that arrangement. If you have line to neutral connected loads at the dual source bus equipment, shared neutral current on the grounding paths is likely a problem.

Throw in GFI trip on the feeder breakers at the dual source bus and the scenario really needs a good look by someone who really knows what they're doing. That's my point that guy is not going to be there in the future when you're planning it's necessary for some reason to move the removable MJB link. The manufacturer may provide that but its installation position would become permanent.

Sorry, the entire thing will be new. The main-tie-main configuration here means "the two utility services are on this side of the tie circuit breaker, and the two generators are on the other side of it". The utility service disconnects (circuit breakers), the tie circuit breakers, and the generator circuit breakers will be in a single enclosure called the "paralleling switchgear". We are trying to finalize the details of the grounding and bonding design/submittal review.

There is no transfer switch in the routine sense. The transfer between the utility source(s) and the generator(s) will be accomplished by the tie circuit breaker. That tie breaker is 3 pole and not 4 pole, so the neutral is not switched. This makes the system NOT separately derived.

 

[__dan]

Design review stage, even if it is your design, I believe it would be a vendor supplied package. The gear manufacturer's engineers, their reps, are going to have the best grasp of what you want to do and how to do it. Multiple service sources with multiple main bonding jumpers is an issue for noise and circulating current on the facility grounding paths, and the problems that come along with that.

Starting at square one, the utility dual source feeds. The utility may offer dual primary feed or dual transformer on the same primary feed. In either case they normally never parallel so that's typically main tie main with open transition switching. The tie never closes unless one of the mains is open, and they are interlocked to prevent paralleling.

If you also have a tie breaker to connect the generators, it may look like 'generators tie main tie main'. A one line diagram would help.

True utility dual primary feed, the utility has statistical outage surveys showing A is still up when B is down, and the reverse. True dual primary is extremely reliable and can shave significant downtime off for the facility. Single primary feed with dual transformers is not the same animal. If you're looking for critical reliability and uptime, I would ask the utility about true dual primary if available and just know the difference between the two.

Probably, utility feeds never parallel, so there's a tie breaker between the utility mains. If you open both mains, the generator(s) can close to the bus without the tie (maybe). It's not clear if the generators close to the source bus or the load side bus.

The entire gear package can be 3 phase 3 wire plus ground, provided you do not have any line to neutral connected loads. That's one of the clean ways to do it with 3 pole breakers and solidly connected neutrals to all of the sources. They all use one main bonding jumper system in the gear.

Lighting load would be a factor. If it is relatively small compared to the total load, lighting could be taken off the dual source bus as 3 phase 3 wire no neutral and run to separate 480 delta to 480 Y lighting transformers. There are advantages to this including reduced arc flash at the load panelboards. If the lighting is huge or most of the load, I would not recommend a huge isolating transformer. Then the main gear would be intended 3 phase 4 wire with neutral and placement of the main bonding jumper system would have to be fussier.

 

[jim dungar]

Now is not the time to go cheap.
If all of your main breakers and tie breaker are all in a single price of equipment you will require a complete outage in order to perform routine maintenance inside of the enclosure. Think of using separate enclosures with at least a Main-Tie-Tie-Main arrangement.

 

[DJ7]

Design review stage, even if it is your design, I believe it would be a vendor supplied package. The gear manufacturer's engineers, their reps, are going to have the best grasp of what you want to do and how to do it. Multiple service sources with multiple main bonding jumpers is an issue for noise and circulating current on the facility grounding paths, and the problems that come along with that.

Starting at square one, the utility dual source feeds. The utility may offer dual primary feed or dual transformer on the same primary feed. In either case they normally never parallel so that's typically main tie main with open transition switching. The tie never closes unless one of the mains is open, and they are interlocked to prevent paralleling.

If you also have a tie breaker to connect the generators, it may look like 'generators tie main tie main'. A one line diagram would help.

True utility dual primary feed, the utility has statistical outage surveys showing A is still up when B is down, and the reverse. True dual primary is extremely reliable and can shave significant downtime off for the facility. Single primary feed with dual transformers is not the same animal. If you're looking for critical reliability and uptime, I would ask the utility about true dual primary if available and just know the difference between the two.

Probably, utility feeds never parallel, so there's a tie breaker between the utility mains. If you open both mains, the generator(s) can close to the bus without the tie (maybe). It's not clear if the generators close to the source bus or the load side bus.

The entire gear package can be 3 phase 3 wire plus ground, provided you do not have any line to neutral connected loads. That's one of the clean ways to do it with 3 pole breakers and solidly connected neutrals to all of the sources. They all use one main bonding jumper system in the gear.

Lighting load would be a factor. If it is relatively small compared to the total load, lighting could be taken off the dual source bus as 3 phase 3 wire no neutral and run to separate 480 delta to 480 Y lighting transformers. There are advantages to this including reduced arc flash at the load panelboards. If the lighting is huge or most of the load, I would not recommend a huge isolating transformer. Then the main gear would be intended 3 phase 4 wire with neutral and placement of the main bonding jumper system would have to be fussier.

The utility sources are existing and their configuration is dual transformers on the same primary feed. The circuit breakers associated with the services are interlocked with each other so that both do not operate at the same time (no paralleling). They both come and feed a main bus. On one end of this bus, the tie breaker is connected, and on the other end of the tie breaker, a second main bus is connected. This second bus is fed from the generators via their circuit breakers.

 

[DJ7]

Now is not the time to go cheap.
If all of your main breakers and tie breaker are all in a single price of equipment you will require a complete outage in order to perform routine maintenance inside of the enclosure. Think of using separate enclosures with at least a Main-Tie-Tie-Main arrangement.

It is a fairly advanced stage of the project so the big stuff like two enclosures vs one cannot be rethought at the moment. All will be in one enclosure, and the most important consideration at the time is the proper grounding and bonding for the system as already designed. If routine maintenance requires complete outage, then so be it.

 

[Joethemechanic]

If routine maintenance requires complete outage, then so be it.

I'd really double check on that

A very similar system to yours that I worked on that was very critical infrastructure and had 2 services. each from a different substation, not to mention 2 large generators, and a huge battery UPS system

Well on that system, complete outages were strictly forbidden for any reason, not even for 1 minute. Loss of any of any of the layers of redundancy required approval from the big brass, and preparations had to be made

 

[DJ7]

I'd really double check on that

A very similar system to yours that I worked on that was very critical infrastructure and had 2 services. each from a different substation, not to mention 2 large generators, and a huge battery UPS system

Well on that system, complete outages were strictly forbidden for any reason, not even for 1 minute. Loss of any of any of the layers of redundancy required approval from the big brass, and preparations had to be made

I do not personally know much about service/maintenance at all. With how much frequency would you say is routine maintenance undertaken?

 

[Joethemechanic]

I do not personally know much about service/maintenance at all. With how much frequency would you say is routine maintenance undertaken?

It's been close to 20 years ago, but I seem to remember every 6 months. And it was like pulling teeth to schedule.

 

[jim dungar]

I do not personally know much about service/maintenance at all. With how much frequency would you say is routine maintenance undertaken?

At least yearly for some components like breakers, up to 5 years for stuff like insulators and racking mechanisms.

 

[Joethemechanic]

That 900kw unit was donated by the Polec Family after the lawsuit.

The Eddie Polec murder case centers on the murder of a 16-year-old Edward William Polec.[1] that took place on the front steps of his church in the Philadelphia Fox Chase neighborhood on the night of Friday, November 11, 1994.

911 dropped the ball big time that day

Eddie Polec murder case - Wikipedia

en.wikipedia.org

 

[MiamiValleyelec]

Hello all. We have a system with two large 480Y/277V utility services feeding a large facility (two services are acceptable for our application per NEC 230.2 (C)). In addition, there are two generators to provide stand-by power (optional stand-by). There is a paralleling switchgear enclosure that houses the service disconnect circuit breaker for each service, a 3-pole tie circuit breaker section, and the circuit breaker for each generator (and the paralleling controls, ground fault detection, etc.). So the configuration is main-tie-main. And broadly, the intended operation is that the tie circuit breaker is normally open, and closes when neither of the services is available to turn on the generator(s). Since the tie circuit breaker is not 4-pole, the neutral is not switched, and per the NEC rules to prevent objectionable currents, we believe that the neutral must be bonded to the equipment grounding system only in the paralleling switchgear*, and not also at the generators. Everyone is kinda okay with this but the paralleling switchgear manufacturer has brought up the following point:

Per some of the standards that they use like UL 1581/891, and also to meet NEC 230.75, they will provide a neutral disconnect link in each of the service disconnect sections. And according to the manufacturer, UL 1581/891 requires them to provide the main bonding jumper connection on the line side of the neutral disconnect link**. Because the MBJ is on the line side of the NDL, when the neutral disconnect link is removed for the purpose of service and maintenance tests, etc., the generator neutrals will not have a connection to the equipment grounding system, if they install per our recommendation which is to not bond the generator neutrals to the generator cases (local grounding). The manufacturer has asked us to confirm we are okay with that kind of a 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) 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, 230.75), and also, per the manufacturer, we cannot connect the MBJs to the load side of the NDLs**.

Any help would be highly appreciated.

*The plan is to provide main bonding jumper in two places, one in each of the service disconnect sections, per 250.24 (C), without claiming 250.24 (C) Exception No. 1. Not 100% sure if this is the right approach. Maybe only one MBJ has to be provided in the paralleling switchgear (Exception No. 1 has to be claimed)? This is one of the other questions I have about this system.

**I do not have access to UL 1581/891 so I cannot personally verify this. But I have assumed it to be correct.

Why run neutrals at all? 3 hots and a neutral from x-former to service main, bond there. 3 hots and a ground from load side of main to ats, no neutral from the generator. X-formers where you need 277