Reversed polarity when using generator

Thepipdoc

Thepipdoc

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Just collected my boat (Sea Ray 270) from a marina where it had previously been for the previous 10 years of its life. In the marina it was hooked up to a shore power supply and there were no problems with any of the electrics.
I've since moved the boat to a swinging mooring and I'm using a Honda EU20i generator to power the heavy electrical items i.e. microwave by plugging in the generator to the boat using a shortened shore power lead. I've noticed that when everything is switched off the reversed polarity indicator light is NOT lit but as soon as I turn on the power to the microwave the reversed polarity light is lit. All the sockets appear to work fine though I haven't tested them with a proper tester. Someone suggested that it may be because the generator isn't grounded. Is this likely? Any suggestions? Is it potentially damaging?
Thanks in advance
 
Simple enough question: does the gennie have a UK 3 pin square socket outlet or is it a European one? A lot of Euro systems (including our Waeco inverter) don't care about reversed polarity. It may be a simple as reversing live and neutral in a special lead to go from the gennie to the shorepower inlet.

By way of example, our inverter has a mains pass through facility, so the inverter drops out when shorepower is present. For some reason, however, it internally reverses the polarity on what it passes through. When I fitted it, it tested fine right up until the point when I reconnected shorepower, at which point it blew the pontoon. So the incoming shorepower now has it's polarity reversed at the point at which it connects to the inverter. The inverter reverses it again internally, so what is fed to the distribution panel is the right way round. Without shorepower, the inverter produces AC with the correct polarity, so everything is OK.

Weird, but worth a look.
 
I suspect that the genset has no neutral to ground bonding.
Without this bond in place on mine the rev polarity neon flickers not as bright as when it is reversed but still noticeable.
I think that its OK to add the bond in the plug but check with Honda.

Also without this link no on board RCD will work.
 
I can't run an RCD on the output of my inverter, either. I fitted one, but had to remove it as it trips instantly. Why would that be?My understanding is that it will trip if it senses a difference in the current in the live and netural lines, which implies a leakage to earth. If there's an equal current in live and neutral, the absence of an earth shouldn't matter, should it?
 
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the absence of an earth shouldn't matter, should it?

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Yes it does. If there is no earth or neutral to earth connection at the generator or inverter the leakage current has no way to get back to the neutral on the generator/inverter.
 
[ QUOTE ]
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the absence of an earth shouldn't matter, should it?

[/ QUOTE ]

Yes it does. If there is no earth or neutral to earth connection at the generator or inverter the leakage current has no way to get back to the neutral on the generator/inverter.

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In which case I'm not getting electrocuted, so it shouldn't trip, no? If the current in live equals the current in neutral, then everything is hunky dory. If I stick my finger onto a live conductor, unless there's a return path from me to neutral on the inverter/gennie I can't get a belt from it. If there is such a path, then there will be current imbalance and the thing trips.

My understanding was that the UK concept of earth-neutral bonding was to ensure that a metal chassis (say your toaster or washing machine) couldn't go live in the event that there was a live-chassis short. Without the neutral-earth bonding you could have got a belt by connecting th elive chassis to ground via your body. This would protect you from leaning on the tap (grounded) while touching the faulty toaster, for example.

In theory, at least, this bonding is unnecessary if the appliance is protected by an RCD, isn't it?
 
If there is no neutral/earth bonding then where does the difference in the current go.

By Kirchurffs law the sum of currents at any point in a circuit must be equal.

The same current at the live terminal must equal the current at the neutral terminal.

If there is a current leakage to ground/earth at the appliance the only way it can get back to the neutral terminal is if the earth and neutral is connected at the supply i.e generator/inverter. Same also applied hen an isolating transformer is used
 
Quite. But in a domestic environment, 'earth' and 'neutral' are connected at the power station or sub-station, and they are also connected to 'ground'. So you can have 10A flowing through live, 9A flowing through neutral and 1A flowing from live through me to the tap to ground and hence back to 'earth/neutral' back at the power station. The imbalance in current sensed by my consumer unit is what trips it.

My problem is the other way round. If the current in the live conductor and the current in the neutral conductor are equal, but no connection between neutral and earth my RCD on the inverter output trips. Why? Would connecting the inverter neutral to 'earth' fix this? What purpose does 'earth' serve on a boat no longer connected to shorepower?

Incidentally, I am aware that were I to stick a finger on each conductor the RCD wouldn't trip as I would be just another load, as far it was concerned, so what use would the RCD be at sea anyway?
 
I would suspect that the inverter is a modifed sine wave one. Its quite common for this to happen.
As for the absence of earth there has to be a return path somewhere even if there is no earth wire.

Another thought is that there may be a neutral to ground short after the RCD this will also cause the RCD to trip as some of the return current goes back down the earth.
 
OK, so the incoming shorepower goes to the main RCD. This doesn't trip, so there can't be a neutral-ground short in the boat wiring. It then goes to the inverter (which is modified sine wave), and thence to the distribution panel. If I put an RCD bwetween the inverter and the panel, it trips continuously regardless of whether shorepower is connected. I'm mystified.
 
As I said above some MSW inverters case RCD to trip because of their waveform being fairly unrelated to sine wave.
it is so obtuse that you can take two RCDs from the same manufacturer and one will work and one wont!
What make is the inverter?
 
Ah, that (sort of) makes sense. The inverter is a Waeco 2000W MSW jobbie. I'm not unduly bothered because we're protected by the main RCD while on shorepower and the inverter is only used infrequently.

Presumably, if we're off shorepower then in the absence of an on-board neutral-earth bond the earth is 'floating' and if the toaster goes AWOL and the 'earthed' casing goes live then a) nothing will happen and b) if I touch the case, as I'm not part of a return path to the inverter neutral I won't complete a circuit and won't get a belt.
 
Depends on how the inverter is wired if the neutral of the inverter is bonded inside the inverter you will get a shock. However if its floating you may not. Sorry to be obtuse but I don't know just how that one is wired.
 
The idea behind UK earthing is to achieve an equipotential "enclosure" where everything has the same potential to earth which is why everything is bonded together and back to neutral, the so-called PME earthing. On land this is fine until Paddy on a road gang digs up a mains cable by mistake and cuts only the neutral. Now that the earth is no longer a separate return there is nowhere for the current to go and the neutral downstream of the break becomes live, along with every metal appliance in your home, hence the need for an RCD and separate earth stake to trip the power off. Otherwise your eyes light up, your hair crackles and it hurts!

Quite how any of the above relates to your problem is something else of course!
 
Thanks to all contributions - I'm happy to confirm that the problem has been solved by running a link wire between the earth and neutral in the 3 pin plug that’s plugged into the genny. Problem instantly solved.
As an aside, can I just ask if this reversed polarity issue that I had would have flattened an engine battery? The reason I ask is that for some strange reason the starboard side engine battery was completely flat directly after running the genny. when I attempted to start the starboard engine it was as dead as a dodo! It was probably something else but I was just wondering what it could be?
 
Do I take it that the incoming shore power is connected to the output of the inverter and then to a single RCD. If so this could be your problem as the supplied must be totally seperate.

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The third panel down is my mains supply selection panel. It has seperate RCD for each input and a selector switch on the left to select which input supply (shore/inverter/generator) goes to the distribution panel below.
 
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