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Graham and Jo

Inverter Case Earth terminal

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We have 240 volts on the boat which runs a charger for the battery and can be used for mains appliances. It is all connected properly with a small distribution panel and a galvanic isolator. 

 

We also have three small inverters on the boat fur use when away from home. Two of them are used to charge phones etc and one to run the cooker ignition circuit. One is a pure sine wave Inverter and this is the only one with an earth terminal on its case. It is actually the lowest rated at 150W.

 

Is it necessary to connect this terminal to the hull?

 

Cheers Graham

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11 hours ago, Graham and Jo said:

We have 240 volts on the boat which runs a charger for the battery and can be used for mains appliances. It is all connected properly with a small distribution panel and a galvanic isolator. 

 

We also have three small inverters on the boat fur use when away from home. Two of them are used to charge phones etc and one to run the cooker ignition circuit. One is a pure sine wave Inverter and this is the only one with an earth terminal on its case. It is actually the lowest rated at 150W.

 

Is it necessary to connect this terminal to the hull?

 

Cheers Graham

 

Personally I would earth it.

 

150 watts is circa 650mA of current. 30mA across the heart is enough to kill.

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11 hours ago, Graham and Jo said:

We have 240 volts on the boat which runs a charger for the battery and can be used for mains appliances. It is all connected properly with a small distribution panel and a galvanic isolator. 

 

We also have three small inverters on the boat fur use when away from home. Two of them are used to charge phones etc and one to run the cooker ignition circuit. One is a pure sine wave Inverter and this is the only one with an earth terminal on its case. It is actually the lowest rated at 150W.

 

Is it necessary to connect this terminal to the hull?

 

Cheers Graham

Personally I wouldn’t bother. Mr Hound is of course theoretically correct, but I think in practice the risk is far less than many other risks we routinely tolerate. And earthing the case only really helps if there is an RCD in the circuit. Presumably there isn’t, on the output of this inverter?

 

Scenario 1) case not earthed: For some inexplicable reason, touch inverter live whilst also holding the case. Outcome: nothing happens.

 

Scenario 2) case is earthed. For some inexplicable reason, touch inverter live whilst also holding the case. Outcome: up to 650 mA of current flows through you hands and across your chest. Bye bye time! (This presumes the inverter output isn’t fully floating).

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There isn't an RCD on the output side. Presumably the outputs from the other inverters are just floating as they have no reference zero volts. Is this right or do they use the 12volt side as a reference? Of course the 12 volt side is floating compared to the hull. 

 

I have become very interested in the boat electrics since the faults after the last BCN challenge. I now have a complete list of all the wires in the engine loom by colour! I also found out that 5 wires in the multi pin connector between the engine and the control panel don't anything!

 

That is a bit of a diversion from my original question.

 

There is another thought I had in this subject. I have heard that the switched mode power supplies on phones etc can interfer with the galvanic isolator so I haven't connected them to the shore power sockets but then I began to worry about the inverter with the earth wire. 

 

Cheers Graham

 

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7 minutes ago, Graham and Jo said:

Of course the 12 volt side is floating compared to the hull. 

 

Are you sure? Unless you have fully insulated return starter motor and alternator(s) your 12V system will be grounded to the hull.

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Just now, David Mack said:

Are you sure? Unless you have fully insulated return starter motor and alternator(s) your 12V system will be grounded to the hull.

Ah good point, hadn't thought of that! 

 

Cheers Graham

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2 hours ago, David Mack said:

Are you sure? Unless you have fully insulated return starter motor and alternator(s) your 12V system will be grounded to the hull.

And if it isn’t then it should be. 

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My take on the question for what its worth.

 

It this inverter is the one for the cooker i would connect the earth  unless the cooker is double insulated which I doubt and would also ideally fit an RCD for it.

 

It seems the other inverters are stand alone units not ever connected to the boat's own mains circuit so connecting the earth to the hull would have no effect even if a metal case of a power tool say became live. The way to protect against that would be to fit a "garden type" plug in RCD between inverter and whatever is plugged into it. Anyway, today most small stuff seem to be double insulated.

 

I am with Nick on this.

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6 hours ago, Graham and Jo said:

There isn't an RCD on the output side. Presumably the outputs from the other inverters are just floating as they have no reference zero volts. Is this right or do they use the 12volt side as a reference? Of course the 12 volt side is floating compared to the hull. 

 

I have become very interested in the boat electrics since the faults after the last BCN challenge. I now have a complete list of all the wires in the engine loom by colour! I also found out that 5 wires in the multi pin connector between the engine and the control panel don't anything!

 

That is a bit of a diversion from my original question.

 

There is another thought I had in this subject. I have heard that the switched mode power supplies on phones etc can interfer with the galvanic isolator so I haven't connected them to the shore power sockets but then I began to worry about the inverter with the earth wire. 

 

Cheers Graham

 

Ah yes the old “switch mode power supplies send spikes down the earth lead and upset GIs” one!

 

Might have been true in the early days of SMPS but not any more. A typical phone charger these days has a plastic Earth pin so it’s not even connected to earth.

Edited by nicknorman

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20 hours ago, Graham and Jo said:

We have 240 volts on the boat which runs a charger for the battery and can be used for mains appliances. It is all connected properly with a small distribution panel and a galvanic isolator. 

 

We also have three small inverters on the boat fur use when away from home. Two of them are used to charge phones etc and one to run the cooker ignition circuit. One is a pure sine wave Inverter and this is the only one with an earth terminal on its case. It is actually the lowest rated at 150W.

 

Is it necessary to connect this terminal to the hull?

 

Cheers Graham

 

Please try to ensure these small inverters are kept at least 1.8 metres (6 feet) from each other. 

 

The reason for this is they are un-synchronised mains supplies, hence there can be 415 volts between any two, depending where each inverters output is in its sinusoidal waveform. A 415 volts shock will almost certainly kill.  The 1.8 metres separation means that you can't physically reach any two inverters at the same time (unless you have arms like an orang utan 😁

Edited by cuthound
Missing worm

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5 hours ago, Tony Brooks said:

My take on the question for what its worth.

 

It this inverter is the one for the cooker i would connect the earth  unless the cooker is double insulated which I doubt and would also ideally fit an RCD for it.

 

It seems the other inverters are stand alone units not ever connected to the boat's own mains circuit so connecting the earth to the hull would have no effect even if a metal case of a power tool say became live. The way to protect against that would be to fit a "garden type" plug in RCD between inverter and whatever is plugged into it. Anyway, today most small stuff seem to be double insulated.

 

I am with Nick on this.

The cooker one was fitted by Calcutt when they put the cooker in. It doesn't have an earth connection. There is no ring main in the boat just one socket for the battery charger and one socket for any portable appliance we might fit. This is in the back cabin.

 

36 minutes ago, cuthound said:

 

Please try to ensure these small inverters are kept at least 1.8 metres (6 feet) from each other. 

 

The reason for this is they are un-synchronised mains supplies, hence there can be 415 volts between any two, depending where each inverters output is in its sinusoidal waveform. A 415 volts shock will almost certainly kill.  The 1.8 metres separation means that you can't physically reach any two inverters at the same time (unless you have arms like an orang utan 😁

The closest two are 12 feet apart.

 

1 hour ago, nicknorman said:

Ah yes the old “switch mode power supplies send spikes down the earth lead and upset GIs” one!

 

Might have been true in the early days of SMPS but not any more. A typical phone charger these days has a plastic Earth pin so it’s not even connected to earth.

Thanks for that I didn't know anything about this so thought I had better ask. I am young and I will learn (this is only partially true!)

 

Cheers Graham

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