On Mon, 07 Jan 2008 16:10:27 -0500, Spehro Pefhany
wrote:

Hi, guys,

Question for those knowlegable about electrician and code type stuff..

I have a situation where there we are wiring what is, in effect, a
transfer switch which will connect external 120VAC/60Hz power to two
sets of loads (sort of like the 240VAC/center tapped situation in a
home) OR a pair of 2kW inverters.

When external power is supplied by a line cord, the neutral and ground
are effectively tied together, however once the line cord is
disconnected the ground (chassis etc.) and the inverter neutrals are
tied to each other, but floating with respect to the ground.

Are there safety or "code" (not sure any code really applies to this
situation when it's unplugged and far away from any power lines)
issues with letting neutral and ground float wrt each other in an
inverter power situation from an electrical wiring point of view?

Safety concern: Almost sounds like you are running two separate
120V inverters in series to get 120/240V 1Ph center-tapped power.

I sincerely doubt the inverters are going to like that arrangement
at ALL, since there isn't any mechanism to lock them in frequency sync
with each other while off grid, or to handle parallel load sharing.

It might work if all the loads are 120V connected between the
separate hot lines and the grounded line. For switching over two
separate sets of 120V equipment that are wired on a "three-wire
circuit" for ease of cabling. And both sides of an average inverter
are floating in regards to safety ground - unless they have their
Neutral tied to the internal chassis ground, and you wire the chassis
ground to an earth ground reference point.

But any 240V connected loads phase-to-phase are going to see a wild
ride on the voltage as the inverters heterodyne against each other, at
least till the "Magic Smoke" from one or both inverters puts an end to
the experiment.

The power source needs to be designed for the load being driven.

-- Bruce --