Why Is My Water Heater Giving Me Shocks?

A broken heater element and a disconnected ground wire point the way to a dangerous situation.

1st May 2014.
By Ed Sherman

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.

This cut-away view of a water heater shows the coiled piping that circulates engine coolant to heat water when the engine is running. Insde the coil is the electric heating element that works when shore-power is being used.

This cut-away view of a water heater shows the coiled piping that circulates engine coolant to heat water when the engine is running. Insde the coil is the electric heating element that works when shore-power is being used.

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.


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About the author:

Ed Sherman

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Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.
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http://www.EdsBoatTips.com

One thought on “Why Is My Water Heater Giving Me Shocks?

  1. Hi Ed. Glad everything’s okay. My husband loves your site because he loves boats, too. Anyway, water heaters does that at times even in our boat. It’s a good thing my husband knows how to fix stuffs like that before any of us get ‘shocks’ like that. Lol. Anyway, great post! :)

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