Generator polarity?

[Tim Good]

When I got my boat I notices when I fire up the Genset, the polarity check lights don't come on. I asked a marine electrician and he tested it from the socket and confirmed it was the wrong way round. He said it isn't hugely critical short term but I should switch it over.

Is this just a case of swapping the output cables around or something more complicated?

 

[VicS]

When I got my boat I notices when I fire up the Genset, the polarity check lights don't come on. I asked a marine electrician and he tested it from the socket and confirmed it was the wrong way round. He said it isn't hugely critical short term but I should switch it over.

Is this just a case of swapping the output cables around or something more complicated?

More info needed

This/these are the polarity lights on your boat's shorepower/ AC mains system ?

You are connecting the genset aia the shorepower connection or via a permanent connection and a transfer switch ?


Did the electrician find that the polarity was reversed at the generator output or that it was correct there and was incorrect in the boat's system.

is the output from the generator actually polarised?

Is the output from the generator an earthed centre tapped system ?

 

[Tim Good]

This/these are the polarity lights on your boat's shorepower/ AC mains system ?

Yes and also tested at the 13amp socket with a tester.

You are connecting the genset aia the shorepower connection or via a permanent connection and a transfer switch ?

Permanent connection. My 240v switch goes left for shore power and right for genset.

Did the electrician find that the polarity was reversed at the generator output or that it was correct there and was incorrect in the boat's system.

Good question. I think he said they put them the wrong way round on the genset but he was in a hurry and doing me a favour so needs to be confirmed.

Is the output from the generator actually polarised?

Not sure... What do I need to do to check that?

Is the output from the generator an earthed centre tapped system ?[/QUOTE]

Not sure... What do I need to do to check that?

 

[rogerthebodger]

I have 2 neons on my electrical panel red connected neutral to earth, green line to earth. correction is red off green on. If reversed phase (polarity) red would be on green off if non on this indicated no earth.

If your generator is a simple single phase (not center tap) you have the neutral line reversed and all you need to do is swap the feed wires at your selector switch.

The neutral is the neutral because it is connected to the earth line at the generator.

This has been discussed at great length several in the past so a search of the forum will show up all the discussions.

 

[superheat6k]

Essentially the output from your generator will be AC so doesn't have a 'polarity' - this is changing 50 times per second. However, it is convention to have one as 'Live' the other as 'Neutral'. In my view it is a misnomer to describe this as a 'Polarity reversal', but I am sure others will be along shortly to disagree with me on this point.

The neutral should be connected to the Protective Earth at its source at the generator, tying neutral at its source to the earth or 0 volts, the other output conductor becomes the Live, i.e. 240v.

If where you connect into the ship's AC system these are reversed then you have a danger of inadvertently applying 240v 'Live' along the boat's neutral line. The issue here is that should any item on the system develop an earth fault it will create a direct short circuit from the 240v live back to the neutral via its earthing pont. Even worse if the earth is not very good throughout the boat it is possible for partially earthed metal parts to become live if a piece of equipment becomes faulty, e.g. the guard rail wires can become connected directly to the 240v Live.

The idea of the neons mentioned is to show a reversed shore supply 'Live' & 'Neutral'. As the source neutral is normally connected at the power source to earth it is at the same voltage as earth, so by connecting a neon between the shore supply neutral & earth it will stay off unless 240v 'Live' is connected which will cause the Neon to light up.

NB the source neutral will not be otherwise connected to your ships earth when the generator is switched off, and its neutral is disconnected, so it is perfectly feasible to have the 'Live from shore power on the neutral line, just not very safe.

If you need to add a Neon to provide this simple function DO NOT use a 240v LED for this purpose if you have a Galvanic Isolator fitted. If you do the LED provides a low voltage leakage path straight past the GI to earth at the shoreside neutral earth connection. Please do not ask me how I know this !

Getting your head round this can be quite difficult, so I would suggest reading Nigel Calder's Boatowners Mechanical & Electrical Manual, and if in any doubt get a marine experienced electrical to check the boat over.

 

[VicS]

Essentially the output from your generator will be AC so doesn't have a 'polarity' - this is changing 50 times per second. However, it is convention to have one as 'Live' the other as 'Neutral'. In my view it is a misnomer to describe this as a 'Polarity reversal', but I am sure others will be along shortly to disagree with me on this point.

.

You are right of course but the term "polarity" is widely if not universally used to distinguish between live and neutral in an AC system . The term is even used in the IEE wiring regulations!

I cannot think of another word that could be used in its place when talking about AC systems.

When reverse polarity indication lamps are fitted they should be connected to the incoming "earth" conductor on the shoreside of the galvanic isolator so that their current does not flow through the GI.

This diagram shows how they should be connected.
Note they will not show the polarity of a generator connected via a transfer switch!