110V A/C Inverter in a 28VDC chassis ground system.

[Invoke]

Hello-
I have an application where I am using a 110VAC inverter in a vehicle that employs a 28VDC electrical system that is grounded to the chassis. The inverter is wired to a GFCI in the vehicle and the old inverter had the ground wire tied to the neutral wire. Is this normal? It seems to make sense since the chassis of a vehicle can both be considered the ground and the return for most applications. Since it is a vehicle there is no true earthing but how is this typically handled for vehicles?

 

[GregLocock]

I prefer to have the inverter hard wired (both neg and positive) to the battery with a separate fuse and switch, and then run the earting strap from the inverter to the body or frame /if/ I need to.

My primary concern is preventing ground loops in instrumentation. Therefore I run with and without the earth starap and see which generates the least hum.

Safety wise I'd have thought any modern inverter was already double insulated.

Cheers

Greg Locock


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[Comcokid]

To a large extent, the use of GFI on a inverter in a vehicle does not make a lot of sense. Yes, I know they do it.

A low input voltage DC/AC inverter has to step-up the input DC to create the AC output. Almost always this will be done using a switch-mode circuit driving a transformer. The transformer provides isolation from the incoming DC to the AC-output circuit stages. The only 'connection' between the input DC and the AC is only parasitic winding capacitance, and possibly a small capacitor for return of noise currents. The current that can flow through these capacitances are far smaller than the aproximate 2ma minimum to trip a GFI.

Now, the GFI does begin to make sense if the vehicle also has provisions for external AC hookup, like a RF or cabin-boat.

Because a vehicle does not normally have the equivalent of a neutral and earth connection like your house (unless it has external AC provisions) you would probably install a GFI more like you would in a old house with 2-wire AC without a ground - that is you connect the ground to neutral. The GFI will not be able to trip on hot-to-ground faults, but can still trip on unbalanced hot/neutral faults. But unbalance hot/neutral faults are harder to get in a vehicle unless you are trying to poke the fingers on each hand into two differnt outlets.

 

[Invoke]

Thanks for the informative replies. In response to comcokid, the vehicle has no external A/C provisions. It is just the inverter --> GFCI --> Junction box with 2 three-prong sockets. The inverter is an outback and I think part of my confusion is that I don't know exactly how it is wired up inside. I am guessing it looks like a house box where the ground is tied to neutral right before it is earthed. In the inverter I assume this is happening right at the bottom node on the transformer secondary winding.

Greg, I am assuming you are earthing the primary (DC) side of the inverter. Is this true?

 

[GregLocock]

via the negative lead to the battery, from there there is an earth strap to the to the body, of course.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?