Earth question again

[howardclark]

I installed an inverter (1500W), shore power, and a changeover switch which supplies a small circuit breaker box allowing two circuits one of which runs a domestic fridge and the other gives me a double socket.
I have a socket tester to ensure the shore power is connected the correct way around.
Now when I test with shore power the Earth is present, no problem.
The inverter has a live and neutral but no earth. If I test the sockets with the inverter on no earth shows. Just to see what happens I ran a wire from the circuit breaker earth busbar to the negative side of the supply battery. Still no earth! I'm not happy letting 240V through with no earth and presumably no protection from the circuit breaker, what have I done wrong please?

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[Ivy]

Don't rely solely on the socket tester, check the wiring physically. There may be something in the inverter that causes the apparent fault, check with the manufacturer.
Make sure you have a R.C.D earth leakage circuit breaker installed to cover both supply sources, these compare the amount of electricity going in through the live wire with the amount coming out of the neutral wire, any difference ie. A short to earth or someone holding a live terminal, and it will trip out. I would not use 240V on a boat without one. Don’t rely on the marina having one either, it may have been bypassed and you won’t know.

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[tr7v8]

If I am reading your post right the inverter hasn't got any earth connection.
In which case you need to wire the earth pin on its socket(s) back to the mains earth(s) AND the battery negative. Inverters generally have totally floating outputs because they have an isolation transformer. In real terms the Isolating ransformer will do what the RCD does.

<hr width=100% size=1>Jim

Draco 2500

 

[bedouin]

You could argue that not having an earth is safer than having one - without it the inverter floats and you can touch the live without getting a shock.

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[M_Russell]

You don't really need an earth when running from the inverter as the outputs are floating. The only reason you can kill yourself touching live on mains power is because the suppliers deliberatly reference mains to ground. That's not the case with the isolated output from your inverter so you could happily hold the live wire all day without ill effect (though holding both live and neutral will have the usual life threatening effect).

An RCD wont work (i.e trip when you press the test button) when running from the inverter because of the lack of earth reference.

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[howardclark]

Now I'm confused; I have run both shore and inverter through an RCD via the selector switch. Are you saying that the configuration of the convertor means an RCD is not required?

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[bedouin]

Yes - for most invertors an RCD is useless. An RCD works by detecting small differences in the current flowing in the live and neutral wires - the assumption being that that is flowing to earth.

That only works if neutral is connected to earth at some remote location - which is not the case with invertors.

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[howardclark]

Thanks for all the advice - I'm now reassured my set up is safe

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