A/C Generator Neutral and Ground joined - how

[fuss]

I wish to change my generator from a floating system to one where the yellow/green wire and the blue wire are joined at the generator.
I want to do this so the rcd trips and also for consistency... at the moment with the floating system it doesnt trip.

My inverter automatically joins neutral and ground when in use and so the rcd works fine.
When its not working it disconnects neutral and ground.

The generator has no such functionality so I wonder how other people have done this.

Its not that unsafe having a floating system for the inverter and generator as they are producing their power onboard and so the electrons are not trying to get back off the boat

But it would save me some time (apart from the fact that I cant come up with a good idea at the moment) if someone had already a good solution for this.

 

[bristoljim]

If your generator has a terminal box connect in there - otherwise anywhere convenient BEFORE the DOUBLE POLE change over switch you should have to select geny/inverter/mains.
Older inverters did not have the neutral to earth connection as manufactered,fortunately they modern ones do.

Cheers Jim

 

[cimo]

rcd operation is independent of earth, so it should trip whether your system is "neutralised" or not.
("neutralised" is the term given to systems which have earth and neutral connected).

The neutralised point must be upstream of the rcd as bristoljim says.
It's possible that the rcd will not trip if the neutralised point is downstream (defeating the purpose of the rcd, and potientialy dangerous).

I'd advise you double check your ac installation. ensure you have one single neutralised point between the source and rcd. (getting a registered sparkie to cast a critical eye is always worthwhile, if he brings his meter, all the better).

 

[VicMallows]

rcd operation is independent of earth, so it should trip whether your system is "neutralised" or not.
("neutralised" is the term given to systems which have earth and neutral connected).

If the source is truly isolated from ground, how would the RCD ever trip?....there will never be an imbalance in the current in the two wires.

Equally, you would never get a shock by touching either one of the conductors. (You would if you touch each wire at the same time ....but the RCD would not know and would not trip!)

Of course 'truly isolated from ground' in the case of a generator is quite difficult to achieve, so this approach is not recommended.

 

[cimo]

If the source is truly isolated from ground, how would the RCD ever trip?....there will never be an imbalance in the current in the two wires.

Equally, you would never get a shock by touching either one of the conductors. (You would if you touch each wire at the same time ....but the RCD would not know and would not trip!)

Of course 'truly isolated from ground' in the case of a generator is quite difficult to achieve, so this approach is not recommended.

fair comment Vic. that's confusing on a second read. A path to earth under fault & the rcd should do it's job. If no path exists under fault, the required imbalance can't be certain & there's no guarentee the rcd will trip - until such time a path to earth is presented. This "earth" needn't be the actual "earth at source" was my poorly made point. As you say, hard to achieve true isolation.

Of course, an RCD isn't able to detect phase to neutral faults.

I believe we're all advocating a neutralised source. The important point is it's location in the installation. (I'm wondering now if the OP may already know this!).

 

[nigelmercier]

Why change a very safe isolated supply (in effect the same as an isolating transformer) to one with a lethal terminal?

 

[VicS]

Why change a very safe isolated supply (in effect the same as an isolating transformer) to one with a lethal terminal?

I think you will find that the recommended practice even with an isolating transformer is to create a neutral on board by earthing one side of the output to the boats internal earth and ( except where isolated) DC negative.

No connection to the pontoon supply earth of course.

 

[nigelmercier]

I think you will find that the recommended practice even with an isolating transformer is to create a neutral on board by earthing one side of the output to the boats internal earth ...

If so, that is crazy. The whole point of an isolating transformer is to make it safer, like a bathroom shaving socket.

 

[fuss]

I'd advise you double check your ac installation. ensure you have one single neutralised point between the source and rcd. (getting a registered sparkie to cast a critical eye is always worthwhile, if he brings his meter, all the better).

Thanks very much for all the replies.
I included the text above as it makes the point that I am trying to solve. How can I connect the blue wire and yellow/green at the generator only when it is active and disconnect it when the generator is inactive.... otherwise ....

When back on shore power there would be two connected points, which is not ok.

It would be good if someone had a solution to this as I can't be the first person with this problem.

 

[david_bagshaw]

Thanks very much for all the replies.
I included the text above as it makes the point that I am trying to solve. How can I connect the blue wire and yellow/green at the generator only when it is active and disconnect it when the generator is inactive.... otherwise ....

When back on shore power there would be two connected points, which is not ok.

It would be good if someone had a solution to this as I can't be the first person with this problem.

normally there is a double pole change over switch between generator and shore power.

or have i missed the point

 

[fuss]

normally there is a double pole change over switch between generator and shore power.

or have i missed the point

Yes thats true but this switch normally only does the live and neutrals a point that is normally near the switchboard. (brown and blue)
It does not connect the ground (yellow/green) to the neutral (blue) at the source, which in this case is the generator.

 

[crisjones]

However if things are wired correctly the neutral and live at the double pole changeover should be wired directly to the generator AND ONLY to the generator, there should be no other connections to the live and neutral on the downstream (generator) side of the DPCO switch. If this is the case then disconnecting the DPCO switch will break the connection between generator neutral and the rest of the boat, hence it does not matter if the earth remains connected. In fact it is best that there is never any switch in the earth circuit, that way it can never be accidentaly disconnected.

The best solution is to have a DPCO switch on the generator feed and another one on the shore power feed, the two DPCO switches should then have a mechanical or electrical interlock so that only one switch can be on. With this arrangement all earth connections can remain continuous at all times, you just switch the live and neutrals between shore and generator but you can never have them both connected at the same time.

There are different ways of acheiving this and if you are unsure how to do it then you really should consult an electrician.

 

[fuss]

However if things are wired correctly the neutral and live at the double pole changeover should be wired directly to the generator AND ONLY to the generator, there should be no other connections to the live and neutral on the downstream (generator) side of the DPCO switch. If this is the case then disconnecting the DPCO switch will break the connection between generator neutral and the rest of the boat, hence it does not matter if the earth remains connected. In fact it is best that there is never any switch in the earth circuit, that way it can never be accidentaly disconnected.

Yes, you are right, thanks.
It does not matter if the earth remains connected as the 3 way switch does break the earth neutral connection as the neutral does not go anywhere unless the 3 way switch is selecting the generator.....so I will connect the ground and neutral permanently at the generator and that will solve it.

Makes me now wonder though...why does the inverter do this through a complex relay system rather than just join them permanently with a jumper wire like an victron isolating transformer has.

 

[mjf107]

If so, that is crazy. The whole point of an isolating transformer is to make it safer, like a bathroom shaving socket.

Agreed if your only using one appliance per source as your example The problem occurs if you have separate faults on both legs of the isolating transformer or alternator output, as you can then get full voltage across these points So you reference one side to earth in effect creating "first fault", its essential to then use RCD protection. Metal boats need special precautions, equipotential bonding and the equivalent of double insulation of mains input If your considering using an alternator on a metal boat these points would equally apply

Michael