renosteinke, I work in an industry where power interruption in not acceptable. We have redundant utility feeders, redundant generators and redundant UPS systems, most of which are designed for make-before-break switchover, with few exceptions. The equipment has to be designed to parallel and always includes circuitry to ensure phase-matching and open the breakers if paralleling is lost. It's especially important on UPS systems where the UPS, Static Switch and Maint Bypass switch are fed from 2 or 3 separate sources.
It certainly puts a burden on the contractor to test properly and do it right the first time, especially in situations when we're unable to schedule an outage or test offline and have to test on the operational building. But, that's why we're paid the big bucks!
As this has changed from 2002, I'll transcribe:
NEC 2005, 250.30(A)(1) Exception 1: For separately derived systems that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.
That's a mouthful, but if you break it down, it pretty clearly authorizes bonding the neutrals together. It's stated elsewhere in 250 that the neutral bus bar is considered a bonding jumper.
Same clause, with extraneous verbage removed for clarity:
For separately derived systems that are dual fed in a common enclosure, a single system bonding jumper to the grounded circuit conductors from each power source shall be permitted.
[This message has been edited by SteveFehr (edited 09-28-2006).]
