Backup inverter Neutral-ground convention?

The code is simple. Neutral is tied to safety earth at only one location or ground loops are formed and by convention this is at the source of power. Another rule is that safety earth should never be switched. So, a source of power is a distribution transformer or a generator (read inverter) All other electrical connections are consumers. Consumers use distributer connections for phase, neutral and safety earth. Consumers are not allowed to create their own earth.

In the case of multiple sources and a common load, there can only be one earth reference and neutral of each source is connected to that singular earth reference at each energy source. Ideally, all sources should be as close as possible to the earth reference. Steve

So, it would seem to follow inescapably that the neutrals must be switched where there are two alternative sources of power, even if they are isolated from each other, yes?

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.

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Hi, Bruce, thanks for your comments--

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.

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

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.

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

You might enjoy reading the manual for Trace SW series inverters.

Pg 43, A-3, and B-1. I have a pair of 4024s connected to a conventional panel. Mixed 120V and

240V loads.

Wayne

Yes, absolutely. With single phase loads, neutral is active and I must point out again, do not connect neutral to the inverter chassis. The inverter internally will connect safety earth to the inverter chassis. Do not tie multple inverters together unless they can be synchronized and they must be designed to do that. If you intend to power a common load with two inverters, they cannot do this together. It is one or the other. Steve

Why not tie neutral to the inverter chassis ? That's effectively the same thing as having ground at the source. In this case a literal ground will be located (hopefully) well away from the inverters, so chassis is as close as we got.

Best regards, Spehro Pefhany

The relevant parts of the NEC are those that deal with "Separately Derived Systems." One common example of a separately derived system is a control system with a 480V/120V transformer that powers sensors, pilot devices, contactors, etc. Usual practice is to bond one side of the secondary of the control xformer to the equipment frame. That conductor becomes a neutral.

I don't have a clear enough picture of your equipment in all its possible configurations to tell whether the NEC would consider it a separately derived system. Is it portable? Is there an earth ground other than thru the line cord?

Not really. It's in a moving vehicle.

Best regards, Spehro Pefhany

The NEC does have a section on RVs, which even if not directly applicable, may give some guidance.

Based on my understanding of the situation, I'd create a neutral by bonding the common connection between the inverters to the vehicle's chassis, and switch the neutral along with the hots when selecting inverter or utility power.

I might reconsider if the inverter batteries can be charged by utility power, and there isn't isolation somewhere between the utility input to the charger and the inverters' outputs.

Inverters are running from a 300A generator on the vehicle power system, and they provide input-output isolation.

Thanks, Ned, and everyone else who offered suggestions.

Best regards, Spehro Pefhany