250.97 Bonding

[shortcircuit2]

With the increasing use of transformer-less inverters and ungrounded array wiring systems, 250.97 does't seem to apply to the high voltage metalic raceway connections at juction boxes, etc as outlined in 250.97 for grounded systems.

Is there a concern here for a need to properly bond these high voltage ungrounded systems? With voltages up to 600 volts residential and some systems coming out soon at up to 1000 volts, proper grounding is necessary for GFDI operation. This may be equally important for Arc Fault protection?

 

[grasfulls]

ungrounded system

ungrounded system

With the increasing use of transformer-less inverters and ungrounded array wiring systems, 250.97 does't seem to apply to the high voltage metalic raceway connections at juction boxes, etc as outlined in 250.97 for grounded systems.

Is there a concern here for a need to properly bond these high voltage ungrounded systems? With voltages up to 600 volts residential and some systems coming out soon at up to 1000 volts, proper grounding is necessary for GFDI operation. This may be equally important for Arc Fault protection?

If they are not grounded then there is no potential to ground, yes/no? So I would have to agree, 250.97 does not cover this as it opens with "For circuits of over 250 volts to ground". If there is no potential, then what would bonding metal raceways, etc, accomplish? 250.96 references "parts that are to serve as equipment grounding conductors," so that does not come into play either.

There is a nice article here, it has a lot of references to NEC 690 you may find of interest:
http://www.nmsu.edu/~tdi/pdf-resources/IAEI September-October 2010.pdf

 

[shortcircuit2]

If they are not grounded then there is no potential to ground, yes/no? So I would have to agree, 250.97 does not cover this as it opens with "For circuits of over 250 volts to ground". If there is no potential, then what would bonding metal raceways, etc, accomplish? 250.96 references "parts that are to serve as equipment grounding conductors," so that does not come into play either.

IMO...I think that bonding may be equally important on the ungrounded system of over 250 volts also. I read that the GFDI device in a transformer-less inverter is set at 300-milliamperes. If there where a high-impedance ground-fault path created by a poor grounding connecting there may be a shock or fire hazard if the GFDI doesn't see it, and proper bonding as per 250.97 would add an extra level of defense.