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

[Jonathan A. Kollasch]

On Wed, Nov 13, 2024 at 06:29:50PM -0500, Greg Troxel via Nut-upsuser wrote:
> 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?
> 

Just took a continuity meter to a North American-sold APC BR1500 with
no battery pack installed and with AC input and outputs disconnected:

Neutral on the battery-backed outputs was not connected to the input
neutral, but was connected to the surge-only outlets.  Input ground was
not connected to either the battery-backed or surge-only output neutral.
Input ground and output ground are connected.

Admittedly that's not the condition you wanted tested, but it does at
least indicate that there's a relay on the neutral path between input
and output.

I know that at least some brands or models will have [do not unplug
input power to test battery backup] warnings on their cords or in their
documentation.  But that might merely be because the input ground becomes
disconnected from any actual earthing.

Also, I'm not sure how much electric code can say about the internal
workings of an appliance; I'd think that'd more be the domain of something
like UL.

	Jonathan Kollasch