A simple electrical polarity indicator.
We have all heard horror stories about dodgy shorepower installations abroad, and following the recent electrocutions on a pontoon in Cyprus, I decided to install a simple indicator.
It consists of three neon lights (not LED) which cost a few pence each from Maplins, and which take a matter of minutes to install. They are connected together in a triangular circuit between the three poles of the incoming shore supply. The amber light is connected between live and neutral. This indicates that power is present, and should normally be on. Green is connected between earth and live, and again should normally be on, indicating that there is an earth connection. Red is connected between the earth and neutral, and this should never come on. If it does, it means that either the polarity is reversed (in which case switches on sockets will not actually isolate an appliance from the live, although they will stop it from operating) or that there is no earth at all (in which case the green will also be lit).
The following truth table shows possible light combinations and the meaning and implications
Green and amber
Normal, correct connection
Use with normal precautions
Red and amber
Live and neutral reversed
Dangerous installation. Switches will not isolate equipment correctly, although appliances will function. Risk of shocks if equipment malfunctions.
Green, red, amber
No earth, polarity may or may not be correct, earth pin could be live
Extremely dangerous installation. Serious risk of shocks. Equipment may function, but metal casings could be live!
Neutral disconnected, circuits live
Hazardous. Switches will isolate, equipment will not operate, but may be live.
Reversed polarity, live pin disconnected, but appliances may be live
Very dangerous installation. Switches will not isolate equipment correctly, equipment will not operate, risk of shocks if equipment malfunctions.
The more sharp-eyed among you may be wondering what the control switch above the socket is for. A few years ago a friend of mine had his Mercruiser outdrive very badly damaged by corrosion. He had always been meticulous in replacing the anodes every season, whether they needed it or not. The casing was actually ruined, and a replacement part (excluding labour) cost him several thousand pounds. I was in the same marina, and had noticed that my anodes had disappeared in a matter of a few weeks. I investigated with a test meter, and identified an earth fault in the supply coming in to the marina. Essentially, everyone in the marina with shorepower was getting their anodes burnt away. To give them their due, the marina sorted the problem pretty quickly once it was drawn to their attention, but it made me think seriously. I considered a galvanic isolator, but these were rather more than I was prepared to pay, and in the particular circumstances I described, the earth fault was so serious that the galvanic protector would probably have offered little protection.
My solution to the problem was to install my own earth on the boat, with the earth circuit permanently connected to the underwater metalwork. At the same time, I installed a switch (above the socket in the picture) which disconnects the shore earth. I carried out the appropriate earth tests (as prescribed in the wiring regulations – being qualified as an electrician helps here!) and discovered that my earth was significantly better than the shorepower one. When we are afloat with shorepower in seawater, the switch is put to the off position to protect the anodes. Ashore, or when using the generator, or shorepower in freshwater (or in water of low salinity, for example in a tidal river where there is significant freshwater input) we turn the switch on and use the shore earth.
I have to emphasise that my solution to the earthing problem does not comply with the wiring regulations, as it requires intelligent input and use by a “competent person” as defined in the regulations, because forgetting to switch it on again when required (or not knowing when to do so) can result in a hazardous installation. However, if you are competent and have suitable test equipment to check the earth continuity, it is a very effective way of ensuring that your anodes do not protect other boats in the marina.