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  Locating Switches, Disconnects, and Over Current Protection

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Author Topic:   Locating Switches, Disconnects, and Over Current Protection
jimh posted 05-20-2006 11:26 AM ET (US)   Profile for jimh   Send Email to jimh  
In any electrical distribution system, it is generally recommended and usually required that an over-current protection device like a fuse or circuit breaker be located in the branch of the circuit which is coming from the voltage source and not in the brach of the circuit which is the return to the voltage source. In marine applications this means that the circuit protection should be placed in the 12-volt POSITIVE lead.

In some certain applications, typically involving radio transmitters, a fuse is also often placed in the NEGATIVE 12-volt lead. This is to provide additional protection to the device itself in the even of a failure of other return circuits to the battery negative. Here is an example of how the protection could be important:

The chassis of a radio transmitter is usually at 12-volt battery negative potential. In addition, the transmitter output transmission line is usually also at chassis potential. And the antenna is usually an unbalanced antenna which may be grounded to the vessel itself. (Here we are talking about a vessel made from aluminum or steel, i.e., a good electrical conductor.) In addition, the battery negative lead is usually bonded to the vessel itself.

If there were a failure of the 12-volt negative distribution main wiring, it is possible for the antenna, its ground, the transmission line shield, and the radio chassis to form a new path for return currents from other devices to flow back to the battery. If the negative lead of the radio is fused between the radio itself and the power distribution, this fuse will protect that path from over current.

In literally all wiring, it is common practice to place the control switch or disconnect in the voltage source (postivie) lead and to not switch or fuse the return (negative) lead, except as noted above. This is especially important in marine applications. If a disconnect is placed in the negative lead, this will leave the device with a positive voltage applied to it. Because of the much higher risk of galvanic corrosion in a marine environment (where there are often dissimilar metals immersed in water), having a positive voltage applied to an electrical device will increase the risk of galvanic corrosion.

jimh posted 05-20-2006 11:28 AM ET (US)     Profile for jimh  Send Email to jimh     
For advice on wiring a bilge pump following these precepts, see:

Cockpit Sump Pump
http://continuouswave.com/whaler/reference/bilgePump.html

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