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Posted by on Feb 21, 2011 in Boat Maintenance, Safety and Seamanship, UK, US |

Shore Power Electrical Safety

Why does a shore power cord melt before the dock box circuit breaker trips?

Question: Recently I went down to check my boat, which is stored in the water over the winter. I have it plugged into shore power and have a small electric space heater connected to a timer in the cabin. I run the heater about 30 minutes every morning to help keep things from freezing when I’m away from the boat, and keep it running when I’m on board.

bad-shore-cord1

The melted plug end

During my last visit I noticed a strange odor coming from the dock box and at the shore power inlet on my boat. After shutting the power off to the boat at the dock box I disconnected my shore power cord and found some really bad melting at both ends of the cord and at the inlet on the boat and the dock box. How come the circuit breaker on the dock box didn’t trip before both ends of my shore cord and the inlet receptacle on my boat burned and melted?

bad-shore-cord2

The melted receptacle

Answer: This problem is all too common and as I’ve explained before these all important connections should be carefully checked for any signs of overheating rather frequently, especially when running high-current draw appliances like electric space heaters.
The reason the breaker doesn’t trip with this type of fault is explained by a very simple mathematical equation known as OHM’S Law. The equation states that if voltage is a constant and electrical resistance (due to a loose or corroded connection) increases, then amperage must go down. Excess amperage is what trips circuit breakers. In the case of a low quality electrical connection, the amperage is decreased to a point where the circuit breaker, which is rated at 30 amps in this case, will never see the 30 amps or more that would force it to trip. Electrical current continues to flow and excess heat is generated, which causes the extreme melting seen in the photos you sent along.

The newest type of shore power plugs and sockets are available from Smart Plug. Now’s a good time to upgrade your current setup!

Ed Sherman

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