What is the reverse polarity warning light for on my shore power panel?

On the AC panel  on some boats is a warning light for reverse polarity. This is an issue when plugging into European marina shore power. In continental Europe the mains voltage wiring and switching is different to the UK.  The live and neutral are reversible and a continental plug can be inserted either way round into the socket. Because of this all European appliances use double pole switching. That means that when an appliance is switched off at the wall socket both the live and the neutral wires are isolated.

UK appliances use single pole switching with only the live fused and switched at the plug and socket. This means that if the supply is reversed the socket switch offers no protection as the appliance would still be live with 240 volts potential between the neutral and earth. If the Reverse Polarity light is illuminated the shore power plug should be rewired with live and neutral reversed. Some boats that regularly travel to the continent carry a reversing adaptor which is a short lead with plugs and sockets wired in reverse order.

Most of the time a polarity reversal will not cause a problem and appliances with function perfectly well. It is advisable though to unplug an appliance to isolate it when not in use.