neutral to ground link_3223.jpg
My Ask Ed section has been getting a workout this week! All good because it means people are finally thinking about their boats and less about shoveling snow or how to get away from it. Tom wrote in overnight with a really good question that needs sharing. Here’s his query:
I’m finding whatever AC amperage is being used by the load I switch on shows up in the ground wire of my shore power cord; any ideas on how to find the source of this leak? Inverter on or off and taken out of the system makes no difference. This is a 240 50A supply. The dock master isolated the leak to my boat by switching off the surrounding boats.
My visual inspection didn’t reveal anything obvious.
This is a very common problem unfortunately and usually occurs because a land based and trained electrician has done some work on the boat. The issue that Tom is describing is caused by what is known as a false ground. On a boat these can be found in several locations, one of the common ones is at the back of a 120/240 volt appliance such as the photo of an electric range shows below: (clothes dryers can also be culprits)
!(http://www.edsboattips.com/images/stories/neutral to ground link.jpg)
The photo above was scanned from one of the chapters in my book Advanced Marine Electrics and Electronics Troubleshooting. You can get your very own copy by clicking on the Amazon photo in the left column of the site here.
In Tom’s case I believe that the problem is actually located behind his electrical panel on board vs. at an appliance. Somewhere back there he has a buss bar with a bunch of green colored wires going to it. He also has another buss bar with a bunch of white colored wires going to it. Somebody along the way has connected these two buss bars electrically. They must be isolated from one another! Tom’s boat is lethal. Anytime he plugs in at the dock and turns on an AC appliance on board, some of the AC current is actually leaking out of the bottom of the boat into the water. The fault current is splitting between the AC neutral and the gounding conductor, which is also part of the boat’s bonding system. This means that underwater metals like sea cocks and the like are connected to this same system. (See yeaterdays post) If a swimmer gets near the boat when this appliance is turned on, they will swim through a voltage gradient in the water and could quite easily be electrocuted.
Tom, get this attended to immediately! The ABYC Standards are quite clear on this matter. The ONLY place where the AC neutral (white) and ground (green) are connected together on board is at sources of AC power on the boat, such as inverters and generators. In the case of inverters they are only linked when the inverter is actually producing AC power.