Wired and Wonderful

Mike Montanaro of Cornelius, North Carolina, asks:

“Don Casey’s thorough and very informative article on the proper way to install shorepower on a boat (Sail, January) clarified a lot of points for me. But he says that if you touch both wires in a 12-volt DC circuit it is unlikely to cause injury. This assumes your body is the only current draw. What would happen if there was an appliance at the end of that circuit and you touched the wires while the appliance was running? Would the current draw go into the test dummy? If so, how much 12-volt current can be harmful? What would happen if multiple batteries are wired in series and the system is 24, or even 48 volts? It’s still DC, but increasing the volts increases the amp draw. I’m curious about this because someday someone, maybe even me, might see just one battery and not know there are three more behind it.

Casey also says to install the AC isolator as close to the inlet fitting as possible. I would have thought it best to put it as close to the ground plate as possible— to keep current from getting into the system. If it’s installed near the inlet fitting, hasn’t the current reached the isolator through the system, and therefore already had a chance to do its dirty deed?”

Don Casey replies:

For almost everyone, body resistance is sufficient to keep the current potential at 12 volts below harmful levels. Quadruple the voltage and you quadruple the potential current. Because we are talking about DC, or direct current, the risk to your heart rhythm remains low, although at 50 volts one can experience a painful burn.

Whether there is an energized appliance on the circuit makes no difference. The current passing through your body is determined entirely by the voltage and the resistance your body offers between your two points of contact.

Finally, locating the isolator as close as possible to the inlet fitting minimizes the potential for stray current to find its way aboard, or into the water beyond the isolator—meaning between the isolator and the shoreside connection. Stray currents do not flow to the isolator where they are stopped. If the current is not flowing through the isolator, it is not flowing anywhere in the grounding wire.

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