The Case for Cut-Off Switches

March, 07 2002

Commonly, shipboard devices like windlasses and inverters are installed with fuse protection in their DC supply lines as is appropriate. What is overlooked many times in these installations, however, is that there may be situations in which the fuse has not blown because the device is not drawing an abnormally large current and yet the device is causing damage that can only be stopped by interrupting the supply current.

A Blue Sea Systems' staffer reports:

"On a Nordic 53 powerboat, we were steaming up the Straits of Juan de Fuca in patchy fog with large ships passing nearby when we suddenly noticed the AC Voltmeter in the pilothouse fluctuating wildly. Almost simultaneously, the engine room bilge pump lights came on. We knew we had a very unusual problem going on in the engine room and ran down to the engine room deck and through a stateroom that contained the small access door to the engine room. Peering inside we could see substantial flooding and then spotted a stream of hot seawater from the starboard engine exhaust cooling water system spraying directly into the Trace 3000 Watt inverter. The inverter was continuing to produce AC power but was shorting the AC current into the exhaust water stream, potentially energizing the engines and the extensive engine room metalwork with AC current. This is an extremely hazardous situation in a flooded engine room.

To add to the problem there was no switch in the inverter DC supply line, only a fuse that would not blow because the inverter, as dangerous as it now was, consumed only its normal current. Because shutting down the ship's entire DC system would be a threat to safety in the shipping channel, we were forced to climb behind the running engine, in a flooded bilge with live AC flowing around us and disable the fuseblock."

This dangerous procedure could have been eliminated with a simple disconnect switch located near the engine room door, or better still, a remote disconnect solenoid with a control in the pilothouse.

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