kashurst
Well-known member
I have had a look at this and as per PaulRainbow, I don't understand the point of a negative isolation switch. To add I have a degree in electrical and electronic engineering and have designed a fair few things. I cannot find a valid reason. Some websites stated "stray" galvanic corrosion currents. Which again seems to be just BS. To get a current you need a circuit. To stop a circuit you only need to break the current path at one point.
I have had boats with one and most without. I even had a Princess 37 (1980s) that had two sets of fuses, one for the +ve and one for the -ve. I suspect this negative isolation thing has just crept in as a "belt and braces" thing that doesn't actually do anything really. Yes there is an arguement for a live feed from say a bilge pump shorting to another -ve wire but all these things should/do have their own fuses and you can put a really big current through a very small piece of copper wire. A tin/lead fuse even 50amps or more will almost certainly go pop before a copper wire fails. And again it is all pretty unlikely stuff and when the boat is being used the isolators are all on anyway!
I wonder if someone did it to stop a problem they didn't really understand, whatever the "problem" was, it stopped and the idea caught on. In early fibreglass boats the manufacturers used to electrically connect all the metal skin fittings to the anodes. Now they generally don't. There is an arguement for doing it if the skin fitting and valve contain different metals, but these days they I believe are the same metals with a nylon ball.
I have had boats with one and most without. I even had a Princess 37 (1980s) that had two sets of fuses, one for the +ve and one for the -ve. I suspect this negative isolation thing has just crept in as a "belt and braces" thing that doesn't actually do anything really. Yes there is an arguement for a live feed from say a bilge pump shorting to another -ve wire but all these things should/do have their own fuses and you can put a really big current through a very small piece of copper wire. A tin/lead fuse even 50amps or more will almost certainly go pop before a copper wire fails. And again it is all pretty unlikely stuff and when the boat is being used the isolators are all on anyway!
I wonder if someone did it to stop a problem they didn't really understand, whatever the "problem" was, it stopped and the idea caught on. In early fibreglass boats the manufacturers used to electrically connect all the metal skin fittings to the anodes. Now they generally don't. There is an arguement for doing it if the skin fitting and valve contain different metals, but these days they I believe are the same metals with a nylon ball.