[Nut-upsuser] UPS wiring question (electrical code)

[Greg Troxel]

In the US, our National Electrical Code requires (being a little fuzzy
as this is just setup to ask my real question):

  grounding conductor (earthing conductor in UK, "green wire" in US)
  bonded from the main ground system (more or less at the entrance
  panel) to all exposed metal including case and ground bus of any
  subpanels

  a single bond from neutral ("grounded conductor", white in US) to
  ground at the service entrance (SE).  (I think in the UK this bond is on
  the power company side and the SE has N and G separate.)


When you have a wall outlet, it has neutral/hot/ground, and neutral and
ground are connected way back at the SE.  The UPS metal case is bonded
to ground.  So if hot touches it, the breaker trips.

Now, you plug a device, with a metal case, into the UPS.  It has
hot/neutral/ground with case bonded to ground.   Surely ground output
from the UPS is bonded to ground input.

When the UPS is operating in bypass and not inverting, then input hot
and neutral are just connected to output hot and neutral.  So it's, for
the moment, just an outlet strip.

Then, power fails.  Input hot/neutral/ground are still connected back to
the SE, but let's say SE hot has been opened up because a distribution
fuse blew.

The UPS starts the inverter and produces power on the output
hot/neutral.

My question, finally is:

  When on inverter, is the output neutral still bonded to the input
  neutral?  Generally?  Required?

  Are there systems where the output neutral is not connected to the
  input neutral, and the UPS connects output neutral to output ground,
  more or less making a "separately derived system"?  Is this ever
  permitted by code?  Do any UPS units do this?