Inspector thinks grid tie inverter is separately derived and wants SPD installed

[ryangittens]

I thought we were past the issue of whether or not a grid tie transformerless inverter is a separately derived system.

An inspector is asking us to install a separate SPD per NEC 285.24 (C) for separately derived systems on a residential gird tied solaredge inverter.

Is it still up for debate that grid tie inverters are not separately derived? Any helpful resource to help convince the inspector other than the obvious NEC definition of a separately derived system?

 

[ryangittens]

I thought we were past the issue of whether or not a grid tie transformerless inverter is a separately derived system.

An inspector is asking us to install a separate SPD per NEC 285.24 (C) for separately derived systems on a residential gird tied solaredge inverter.

Is it still up for debate that grid tie inverters are not separately derived? Any helpful resource to help convince the inspector other than the obvious NEC definition of a separately derived system?

Update: the inspector conceded.

 

[wwhitney]

So, does 285.24(C) really require an SPD, or just tell you how to install one if you want it?

And at first reading without further thought, it looks like the PV panels themselves meet the definition of SDS. Obviously the AC side of the grid tie inverter is not an SDS.

Cheers, Wayne

 

[jaggedben]

An SDS, as far as I know, must be one of two things. It's either a system derived from an isolation transformer, or a system with no connection whatsoever to other systems, i.e. an offgrid system. A PV system that uses a non-isolated inverter is no more of an SDS than is a load that uses a non-isolated switched power supply. Some PV systems (older and larger ones, mostly) do have isolation transformers, and may be subject to requirements for SDSs in so much as 690 does not supersede. But those PV systems are not being installed much in residential these days.

And wayne is right, 285 (now 242 in the 2020 as I found out) does not require an SPD, just regulates its installation. So it's good yhat the inspector conceded, as they were wrong on multiple counts.

 

[wwhitney]

The 2017 definition:

Separately Derived System. An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections

So perhaps I misunderstand how a non-isolated inverter works--do the fast switching components that turn DC into AC constitute a "direct connection"?

Cheers, Wayne

 

[jaggedben]

...

So perhaps I misunderstand how a non-isolated inverter works--do the fast switching components that turn DC into AC constitute a "direct connection"?

Cheers, Wayne

Yes. Makes no more difference than a manual disconnect in the circuit.