A shocking inverter.

AndrewB

AndrewB

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Can anyone explain this?

Some of our onboard 240v sockets have copper-coloured metal facias to match the varnished sapele finish of our joinery.

No problems at all if I am connected to shore power. But when I use my inverter - a 1kW Moore Power unit - we get nasty shocks off the metal facia. I've double checked that the wiring to the sockets is correct with no shorts.

Why is this? Must we install all-plastic sockets?
 
Yeah! I was thinking about earths too but presumably the inverter isn't earthed to anything so there's little point in its output having an earth connection??? Could the earth line to the sockets have become connected to live or neutral somewhere, perhaps in the inverter?
 
DANGER - DANGER - DANGER!

YOU HAVE A LETHAL WIRING PROBLEM. DO NOT USE YOUR INVERTOR until it is sorted!

MAINS VOLTAGE IS LETHAL

Your mains wiring carries a third wire for earth, which is grounded when plugged into the shore supply, and ensures that if any fault develops allowing the live feed into contact with metal casings - or in your case the metal switch plates - it will be safely discharged to ground, and supply trips will blow without risk to users.

When you use your invertor, the earth wire is not grounded. You are getting shocks now because of stray induction currents in the earth wiring. That's not the problem: if anything goes faulty and ANY PART that should be earthed becomes live - then EVERYTHING connected to the earth circuit - including all your nice metal switch plates - will be carrying the full mains voltage. You will only find out when you touch any one of them, and it kills you.

You MUST have some means of grounding the 'earth' circuit of your mains installation BEFORE you use the invertor again.

This may not be as simple as it sounds and you need advice.

There are other problems too with using a large Invertor which is not grounded - the voltage across the supply terminals may be 240volts, but again induction problems can raise the potential of the whole thing a great deal higher, so that while you have 240 v AC across the output, there may be possibly a much higher voltage developing between the supply and ground potential. This may be another reason for the shocks you are getting.

As long as the shore supply is properly grounded, it is of course absoluteley safe to continue to use the mains installation.

If it proves impossible to provide a proper ground for the Invertor, then you can only safely use appliances that are 'double insulated', or have fully insulated (i.e. plastic) casings, and have no earth wire connection (like for example power tools which only have twin conductor cable fitted) so are not dependent on earthing for theiur safety if a fault develops.

Remember - EVERYTHING connected to the green earth wire of your mains installation COULD BECOME LIVE, when the Invertor is running until this sis sorted.
 
Re: DANGER - DANGER - DANGER!

Many thanks for your advice, I will take this very seriously.

The inverter is only used when away from shore power, and a shore earth. I will check (as Thistle suggests) whether the earth line of the power circuit becomes live when the inverter is in use, but it ought to be isolated (could the RCB cause a problem?)

I have had a number of shocks already, they are unpleasant, but clearly not lethal!
 
Re: DANGER - DANGER - DANGER!

In mains istallations the neutral conductor of the source is connect to earth, bringing it down to the same "potential" as the surroundings i.e. your boat. With the inverter there is no hot or neutral until one of the leads gets grounded, which then would take all those "floating" voltages down to ground. That is just the principle. Get a certfied electrician to sort that out for you and/or follow the instructions of the inverter manufacturer.

My inverter also showed 80 volts from any of the leads to ground as well as the inverter casing even though nothing internally is connected.
 
Re: DANGER - DANGER - DANGER!

Any proper marine inverter will have 3 pin output sockets where the earth pin is grounded to the casing. It is essential that the casing is grounded to a proper grounding plate on the hull. Many small inverters are sold without an obvious ground connection. These should only ever be used with double insulated equipment until a ground wire is provided.
 
Re: DANGER - DANGER - DANGER!

Quite right Boatmike - these things can be lethal, and there is often little or no warning of how unsafe things can be if they are not properly earthed - particularly in the marine environment where earthing may not be so simple, and the risks higher because of the damp or wet conditions.

Andrews' shocks from his lightswitches may just be uncomfortable in normal dry conditions - but in a storm at sea with a lot of water and spray about they could well be lethal.
 
Re: DANGER - DANGER - DANGER!

[ QUOTE ]
Many thanks for your advice, I will take this very seriously.

The inverter is only used when away from shore power, and a shore earth. I will check (as Thistle suggests) whether the earth line of the power circuit becomes live when the inverter is in use, but it ought to be isolated

[/ QUOTE ]

No - dont! There is too much risk involved to be messing about unless you REALLY know what you are doing.

You already know the earth is live. Testing it will only tell you that you get shocks from it - and you already know that!

Unless you KNOW how to sort it safely (and - with utmost respect - you would not be asking us if you have the specialist skills needed) - you need someone who can look at and test your installation to tell you what is specifically wrong and how to sort it.

Electricity at mains voltage kills instantly and without warning.
 
I agree with all that has been said by others.

We have a 2kw inverter which feeds the ring main (metal socket plates again for aesthetic reasons!) This is however bonded to a sintered copper earth shoe by a length of 6sqmm cable as is the ring main equipotential bonding loop and all the other earths on the boat. We have a "source selector" two pole changeover switch which selects either shore power or inverter. However the earth circuit is fixed. Incidentally, to stop forming a battery between us, other boats and the pontoon, a galvanic isolator is in the line of the shorepower earth.

You certainly do seem to have a problem. First suspect must be the internals of the inverter itself but if you have any sockets with neon indicator lights, these will come a close second as the often unprotected thin wiring of these and their associated resistors can easily be displaced and short to the metal casing of the socket plate. Another source of a short could be water ingress to a switch on a socket plate allowing a track to the plate through a trail of corrosion derrived oxide - this would explain why you don't appear to be getting the full whack of a 240V shock.

On a marine 240v installation much attention must be given to the waterproofing of the backs of the socket boxes with waterproof boxes and cable glands being used. Ideally, hardware to IP68 should be used and both the shorepower and the inverter power sources should have an RCD (RCCB) in circuit as well as a competent earth.The whole of your system needs checking over and a proper earth fitted before it is ever used again.

Steve Cronin
 
And I agree with all that you and Old Harry have said too..... How nice. Lets start a new trend!
I think Harry is quite right when he says get a marine electrician on the job too. Mistakes in this area can be more than just shocking... You can get killed.....
 
My first suspicion is that (assuming that the invertor is electrically sound) that maybe your AC earth and neutral are not bonded together on the boat when the invertor is in line. The rule is, in case it has not been followed, and if it has not been followed it may be what is causing your problem -

The AC Earth and Neutral must be bonded together at the source of the power and there only. This means that when one is on shore power the earth and neutral on board the boat are not bonded together because they are bonded on shore. When not on shore power but on the invertor (or generator) then the neutral must be bonded to the earth on the boat. The AC earth on the boat must always be bonded to the sea (in the case of your metal boat, obviously to the hull does that).

This means that if an invertor is installed then when it is switched into circuit to supply the boat's AC the same switch action must bond the earth and the neutral together on the boat (and obviously also isolate the shore power from the boat's AC distribution), and when it is switched out of circuit to shore power then the same switch action must break that bonding between the neutral and the earth.

Some invertors do this automatically - I don't know about yours.

I know from many previous threads that this rule is often not understood or followed by amateurs.

Also, ignore all the advice that you need not earth the boat's earth conductor to the sea (has also been said in many threads, including another current one) - is just plain bad amateur advice, especially on a metal boat, and a professionally built boat would never be built like that.

Our own boat has metal facias and I have seen them on other boats I have been involved with (all wired correctly /forums/images/graemlins/smile.gif), and have never had a problem with them - so it is possible to use them.

John
 
Could be, Ships Cat, then again could be a number of other things wrong too. All of them life threatening.

Andrew has electrical power present on his boat where it should not be, and is in considerable danger because of it. For safetys sake he needs a competent electrician to find out why.

But I agree, correctly wired metal fittings are not only not a problem - they are probably safer if damp gets where it should not!
 
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