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Reverse Polarity ....

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Evening all, to all you marine electrical 'experts' out there I wonder if you can help with this .....

I've been reading a lot about potential reverse polarity issues when connecting to shore power so thought i'd get myself a little gadget to test my 240 volt circuit on Karizma.

Before I came down to the boat I tried testing my home circuit and all was good two red lights, ?Ma sorry cant remember what it read (RCD on the meter), 240V (L-N on the meter) and zero (N-E on the meter).

Plugged it into the boat (whilst connected to shore power in my home boat yard) and got: 36mA (RCD), 234V (L-N on meter), and 03V (N-E on the meter)  - this fluctuated between 3 and 6 volts.

So I'm assuming I shouldn't really be seeing  3-6 volts on the Neutral to Earth? - can anybody give me a clue what to go looking for?



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what you have to understand is the different types of load, resistive and reactive (coils and capacitance's) these shift the phase very slightly and this imbalance can cause a neutral current/ voltage. if you have a RCD or other protective device fitted it will trip the circuits if this current exceeds 30mA.

on the sort of cables we use at work (mains electric feeders) a neutral cable can also act as the earth - depending how it is wired, so imbalances in the loads between the phases can cause a current in the neutral cable.

for domestic wiring an imbalance on the neutral/ earth - up to 2v is normal, and up to 5v can be accepted, but anything higher might indicate the cable is being overloaded.


A rule-of-thumb used by many in the industry is that Neutral to ground voltage of 2V or less at the receptacle is okay, while a few volts or more indicates overloading; 5V is seen as the upper limit.

so your 3-6v is almost within tolerance, and assuming you are running from a shore power outlet, it is possible that somewhere along the path, there is a less than perfect connection, but whether thats on the boat or ashore, is anyones guess.

it could just be an indication of distance from the power source- which would seem plausible given the rcd showing current at 36mA, this to me would indicate distance from the rcd trip coil.

it could all indicate for instance that the shore power cable had been neatly coiled and was introducing an imbalance (this is why coiled extensions have a lower power rating than uncoiled ones)

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15 minutes ago, grendel said:

it is possible that somewhere along the path, there is a less than perfect connection, but whether thats on the boat or ashore, is anyones guess.

I quite agree. It occurred to me after I posted, that the problem could well be on the bank, not on the boat.

All the same, I have known battery chargers to leak quite often and this is usually because they have been installed in a little locker with no ventilation, where they overheat.

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I don't think you have reverse polarity. You probably have induced voltage on the N conductor which is quite common. Anything under 50volts is deemed safe, though you wouldn't want that much induced voltage.

If I have misread this, microwave ovens can cause reverse polarity in installations, but I've no idea how or why 😁

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Things to point out re. Earth Leakage

If a mains powered appliance is double insulated (also known as a Class II appliance and indicated by an image of two black squares, one inside the other, on the label) then there is no earth wire in the plug or device. So unless there is a major fault there cannot be any leakage to earth.

For mains powered equipment that do have an earth wire (also known as a Class I appliance), you can find a lot of modern item with electronic inside can have an amount of 'natural' leakage to earth due to the design of internal power supplies among other things. Whilst this should be well below the levels to cause an Earth Leakage Circuit Breaker to trip (ie 30mA), if you have a number of items connected like this, added up they can cause 'nuisance' tripping that could mask a genuine fault on the circuit.

If in doubt, always consult a qualified electrician.

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On 12/02/2022 at 12:05, annv said:

Hi You dont say what electrical equipment you have are the earth points connected to the engine earth when was the boat wired, as Vaughan says disconnect each item as you check meter to isolate which which could be the cause. John

Hi John, from what I've worked out at the moment, the boat has a galvanic isolator and it looks as though all the 240v earth cables have been connected correctly to the Shore GRD (rather than the boat ground), so for now I'm assuming all is good.

More investigation required as I'm now just curious (rather than concerned).

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  • 2 weeks later...

A little update for those that might be interested.......

Been out on the river this weekend and moored up at both Loddon and Somerleyton and used the BA shore power whilst there.

Decided to check the voltage with the meter and got Zero Neutral to Earth value :default_eusa_dance:so looks as though there's definitely no issues with the boat :default_beerchug:


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