On Mon, 07 Jan 2008 21:40:05 -0800, the renowned Bruce L. Bergman
wrote:

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?

Hi, Bruce, thanks for your comments--

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

Nah, there are two inverters because one will not handle the multi-kW
loads (there are a number of loads on each inverter). The inverters
are not sychronized so the AC voltage between the two "lines" might be
anyhere from zero to 2 * 120VAC at any given time. It's a bit of
complexity that's not completely germane to the issue-- I mention it
ONLY because it affects the total voltages that might show up, and
thus safety. The neutrals are tied together because the intention was
to switch (transfer) only the "hot" lines, but I'm thinking now that's
not a good idea.

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.

There are four similar parallel loads on each. No current flows
between the neutrals when operating on inverter power.

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.

Correct. But if we tie the neutral to the chassis at the inverter then
we must switch the neutral from the inverter AND from the external
power, right? (transfer the connection) Otherwise we have two
'grounds' when external power is supplied.

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.

Sure, it's not a problem in this case.

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

-- Bruce --