Inverter grounding – when there is no EHU connection to the sockets

[Ray-Vic]

When stand alone operation, the DC N conductor has no bearing on AC. If AC output is double insulated, you don’t have a N AC, you have to make one. But if you do, you have to undo before hook up.

Ok. So linking the DC N to the AC out N ie tying a leg of the AC out to ground (albeit accepted there technically isn't one initially) wouldn't 'create' an N AC? I assume installing a 'mains present' relay would do the second job (n/c contact opening when mains incoming is present). I can get me head around most but not all of this, so am trying to really understand as much as possible before potentially killing us!

 

[Raul]

Ok. So linking the DC N to the AC out N ie tying a leg of the AC out to ground (albeit accepted there technically isn't one initially) wouldn't 'create' an N AC? I assume installing a 'mains present' relay would do the second job (n/c contact opening when mains incoming is present). I can get me head around most but not all of this, so am trying to really understand as much as possible before potentially killing us!

Don’t confuse negative DC ground with AC protective earth PE. In a isolated inverter AC out is 2wires, neutral does not exist, don’t bond anything to the chassis, unless you inverter provides a thirs wire yellow/green for PE.
Read the inverter manual and try to understand what PE is for, and if your inverter is a safe appliance to be grounded to the chassis.

 

[autorouter]

I'm considering putting a Renogy inverter in my motorhome so been reading posts like this with interest. Given that the Renogy inverter uses passthrough for the mains, could you not divert the incoming site supply to all outlets and satisfy both safety concerns by tying the DC N conductor to AC output N when on stand-alone DC in supply, and then having a 'mains present' relay which cuts this connection when on site supply?

That's exactly what Victron do inside their Multiplus inverter/chargers. A relay connects one of the isolated AC wires to the chassis/protective earth when there is no hookup supply, so that it becomes a neutral wire. When hookup is connected, the relay switches off, and the earth is provided by the hookup post.

Note, as Raul says, the DC negative is irrelevant to this. In fact by coincidence the DC negative is also connected to the chassis, but that's nothing to do with the AC side of things. I think you're confusing things by using 'N' for both DC negative and AC neutral.

 

[Ray-Vic]

Don’t confuse negative DC ground with AC protective earth PE. In a isolated inverter AC out is 2wires, neutral does not exist, don’t bond anything to the chassis, unless you inverter provides a thirs wire yellow/green for PE.
Read the inverter manual and try to understand what PE is for, and if your inverter is a safe appliance to be grounded to the chassis.

No I'm not confusing N and protective earth, I understand they are different things. Putting it another way, when on DC power, the AC outlet from my inverter would be 'floating', with neither conductor tied to ground, so I would measure 115v (or so) between each and ground. Now if I took one of those conductors (the theoretical neutral out on the socket (blue wire)) and linked a wire between that and the DC N on the inverter, would that 'create' a N on the AC side in as far as off grid use goes, then cutting that connection when on site power?

I'm trying to establish whether I would be able to take the AC inlet from site power directly through the inverter on passthrough to power all sockets when on hook up, and then continue that when on DC power by then linking as above, or should the output then just be left floating with the noted safety concerns. I suspect an RCBO could then be installed on the inverter output?

Confused.com!

 

[Ray-Vic]

That's exactly what Victron do inside their Multiplus inverter/chargers. A relay connects one of the isolated AC wires to the chassis/protective earth when there is no hookup supply, so that it becomes a neutral wire. When hookup is connected, the relay switches off, and the earth is provided by the hookup post.

Note, as Raul says, the DC negative is irrelevant to this. In fact by coincidence the DC negative is also connected to the chassis, but that's nothing to do with the AC side of things. I think you're confusing things by using 'N' for both DC negative and AC neutral.

Ah ok thanks, that's great. So in reality it's actually doing the same thing to install a relay to break the link, but unless the inverter is designed to cope with such scenarios it probably isn't best to do it externally?

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[oldiesontour]

Just a couple of sort of related points ..when we and cl site hook up earth HAD to be from a stake ,not from PME TAG from supplier.
2. Looks like Arc sensitive breakers will be coming in to new regs despite astronomic cost(compared to RCD/MCB.and so several sparkeys have told me unwanted/needed tripping.
From my experience earth stakes have been last resort in new build for years ( PME incoming supplied tag preferred)

 

[Ray-Vic]

As an additional question from the above, other than the fridge being on when running on the inverter powering all the sockets (easily remedied), are there any other pitfalls to doing this?

Edit: Just realised myself it will also connect the mains charger, so that would need to be disconnected also.

 

[Raul]

Ah ok thanks, that's great. So in reality it's actually doing the same thing to install a relay to break the link, but unless the inverter is designed to cope with such scenarios it probably isn't best to do it externally?

It has to be done by a relay, to allow you to operate in both modes: ehu-relay open, and stand alone relay closed/ bonded. Unless you remove the bond manually, each time you connect to ehu; but, that’s until you forget once.
If you have ehu and the bond, that becomes very dangerous, as you will link the ehu neutral to your chassis, very bad. To the chassis should only be bonded the PE.
Hence, if folk don’t understand AC earthing it’s hard to explain, and best for them to seek professional help.

 

[Ray-Vic]

It has to be done by a relay, to allow you to operate in both modes: ehu-relay open, and stand alone relay closed/ bonded. Unless you remove the bond manually, each time you connect to ehu; but, that’s until you forget once.
If you have ehu and the bond, that becomes very dangerous, as you will link the ehu neutral to your chassis, very bad. To the chassis is only bonded the PE.
Hence, if folk don’t understand AC earthing it’s hard to explain, and best for them to seek professional help.

Perfect, thanks. Yes, this is what I was eluding to. Having a relay which operates when on ehu with a n/c contact which ties the DC - to AC N. When on ehu this relay operates to open the connection (and also power the fridge and charger).