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cisco said..Thank you.
There is one item I believe every boat should have if it takes 240 volt power on board from an external source such as a marina.
What I am referring to is the round grey object in the lower right of the pic with the red, green and orange lights.
It can save your life and prevent you from frying your boat's electrics. It is a polarity and earthing indicator. Plug it into your extension lead with the power on and it will tell you if it is safe. The writing on it tells the story. You should be able to get one in any electrical supply store for $10 to $20.
Cisco, what you say is not really correct. Polarity makes no difference with AC power and will not fry your Electrical equipment. If the active and earth were transposed in the lead, yes you could have a lethal situation, though this is highly unlikely unless the lead in newly made and untested. We have rules for 240Vac wiring in boats and if your boat is wired correctly it will protect against crossed polarity and earth faults. The boat should have earth leakage protection fitted (the same as a caravan) and the supply you plug into should also have earth leakage.
The best protection you can have if you are unsure is plug your boat into shore power via a portable RCD (safety switch) . if your LED plug gives you peace of mind, that's good as you can not be too careful with electricity.
As an Electrician, my advise would be.
1. Don't use pre-made cheap Electrical leads to connect to shore power as I have seen too many fail. Have an electrician make you a good lead out of 1.5mm flexible cable.
2. Have your leads tested and tagged every 12 months.
3. Make sure you are protected by an RCD (safety switch) either from the boat or from the supply you are plugged into. If unsure use a portable one.
4. Never do your own 240Vac Electrical work on your boat. Only use an electricial contractor who is competent in boat electrics as there are a lot of rules a domestic electrician may not be aware of.