Question: Recently I was onboard my cruiser and cleaning out my lazarette where my on-board water heater is also located. The boat was in the water and I was plugged into shore power.
As I was reaching down into the lazarette to pull out some of the excess gear I was cleaning up, my arm rubbed across the case of the water heater. Much to my surprise I experienced a really jolting electric shock! I’m OK, but I sure don’t understand why this happened. I always thought that if there was a fault with any of my shore-power supplied equipment, a breaker would trip.
I did notice that where the power cable goes into the side of the heater there was a green wire just hanging loose on the side of the heater case. What’s up here?
Answer: Glad to hear you are OK. Things could easily have gone another way.The photo I've included might help to explain what is wrong. It's a cutaway of a typical marine water heater that I took at a boat show.
Inside you can see some coiled piping. This coil is where engine cooling water flows through the heater. When you're not plugged into shore power, but are running your engine, the heater uses that fairly hot coolant water to heat your potable water. Inside that coiled pipe, you can see the electric heating element that heats the water when you are plugged in.
Several things have apparently gone wrong on your boat. First, the heating element inside your heater tank has developed a breakdown in the electrical insulation on that heater coil. It has developed what we refer to in the trade as a “short to ground." In your case, the green wire you see hanging loose is in fact the grounding conductor for the power feed to your water heater. If it were connected to the metal case of your heater it would conduct this shock current back to the source of power for the heater and would most probably trip the circuit breaker supplying the heater.
So, to fix all of this you need to make sure to replace the electric heating element in your heater and get that green wire connected to the case of your water heater.