Triple MBJ service with secondary tie breaker double ended with generator tap.

[Iconclass]

Prime example of GE engineers and the local AHJ not understanding the basics of Kirchoff's current law when it come to the NEC on grounding.. I have a double ended service with a three pole secondary tie breaker. The service also includes a generator tap on one end of the tie point on the load side. The secondary tie breaker is a three pole unit. The system has a continuous solid neutral bus between the two service points which are at different ends of the service. There are two Main bonding jumpers installed at both ends of the service. There exist a generator tap which is on the load side of the service via bus duct which also has a main bonding jumper installed. With the three main bonding jumpers installed there is a division of current according to Kirchoff which is a violation of 250.6 (A) (B) which clearly states no objectionable can flow. The way GE set up the service is that current can flow on the main bonding jumpers in the system. This is a 480 three phase four system at 4,000 amps on each end of the tie. The MBJ is not rated for 4,000 amp as the neutral bus is but is paralleling the load through it via triple MBJ's. This is a hospital installation.

 

[brian john]

And if you have GFPE affect the operation

 

[__dan]

If there are any service supplied line to neutral connected loads, I would expect some neutral current on one of the stray and unintended paths, the grounding paths. Stray unintended neutral current from the multiple source multiple n to g bonds, on the grounding paths, I would say is objectionable.

The way to do it cleanly is to have no neutral connected loads on the dual or triple source bus, only line to line loads. No neutral current, neutral connected loads at the service, would be the mitigating strategy. Essentially the dual source bus is three phase three wire plus ground. The upstream source(s) are certainly Y, with a neutral bus also in that gear, but nothing should be connected to the neutral at the dual source bus.

All of the 277 Volt lighting would be an obvious target. Put that load downstream of a service connected 480 delta to 480 Y transformer, to create a new and clean neutral to connect 277 V lighting loads to (also greatly reducing AFC flash hazard).

Now if you have rectifier front end loads (large UPS, drives), which you will, that front end is internally Y connected and grounded, putting what would or could be neutral currents on the ground paths. That can create large measurable circulating current on the ground path. But since the manufacturer of that load determined that the grounding path and not the neutral path is correct for that current, I do see and know of ways to both accommodate and mitigate large circulating noisy current on the grounding paths. I might be tempted to agree with the device manufacturer if they say connected to ground is proper, a neutral at the line side is not necessary, so it may be hard to call that (large) current "objectionable".