Negative switch

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.
 
kashurst said:
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.
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You told us you had a degree in Electrical Cars🤷🏼‍♂️🤔😳
 
Could it be to cut off not all of a set of circuits but just some key ones, leaving e.g. bilge pumps, certain lights, and fridge still running ? A selective shut off in fact ?
 
sarabande said:
Could it be to cut off not all of a set of circuits but just some key ones, leaving e.g. bilge pumps, certain lights, and fridge still running ? A selective shut off in fact ?
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You can do that with the positives, route everything through the isolator except for "always on" circuits, which have their own busbar, direct to the batteries (fused of course).
 
Early 2000s Bavarias (and probably earlier) sailing boats all had a single isolator in the -ve located under the chart table. No individual +ve isolators. Later changed to the now normal individual bank isolators and no -ve.

I think post#41 has nailed it - lot of ignorance, particularly about electrolytic and galvanic action leading to some strange ideas about how to wire DC systems and bonding..
 
I was just wondering......if my boat was designed and built with the switch...and Paul (as he has said) utilise it for another purpose and re lables it, if there was a disaster...would the insurance pay up ?
 
billskip said:
I was just wondering......if my boat was designed and built with the switch...and Paul (as he has said) utilise it for another purpose and re lables it, if there was a disaster...would the insurance pay up ?
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Good question. Although I don't understand the point of a negative isolator switch, I personally would not remove it, if the manufacturer had fitted one.
It does nothing either way and the insurance company can't subsequently complain.
 
kashurst said:
Good question. Although I don't understand the point of a negative isolator switch, I personally would not remove it, if the manufacturer had fitted one.
It does nothing either way and the insurance company can't subsequently complain.
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Maybe it was designed in for a purpose un imaginable to some, but the possibility of a future addition of some device or other, who knows?
Bottom line AFAIK is whether my insurance will pay up if I'm faced with a multi million pound claim for an environment clean up, marina fire or whatever...

As Paul has suggested in the past he is a professional and its his job, if someone took his advice on here and it went tits up, could he be sued?
 
billskip said:
I was just wondering......if my boat was designed and built with the switch...and Paul (as he has said) utilise it for another purpose and re lables it, if there was a disaster...would the insurance pay up ?
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Unfortunately, the defence of "I don’t understand what the negative isolation switch does, so I removed it" is not likely to work.
 
noelex said:
Unfortunately, the defence of "I don’t understand what the negative isolation switch does, so I removed it" is not likely to work.
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If i'm going to repurpose a single pole switch I know exactly what that switch does. It's not required by any regs and is not fitted by the vast majority of builders, who build boats using the same engines, drive trains, electrical components etc etc and don't fit a single pole negative isolator.

Repurposing it simply means connecting the negatives direct, that's the same as turning it on and the same as all of the other boats not fitted with single pole negative isolators.

Your misinformation doesn't help anyone. You've clearly read some stuff on a few websites and it totally went over your head, but now you think you know all about it. Wrong.

While you are educating us all, can you explain how fitting a negative isolator, as required by the ISO, helps prevent stray current corrosion ? Try and answer without using Google, if you can.
 
PaulRainbow said:
Repurposing it simply means connecting the negatives direct, that's the same as turning it on and the same as all of the other boats not fitted with single pole negative isolators.
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This is like saying I will remove all liferafts, automatic fire extinguishers, storm sails etc etc because this makes the boat the same as the vast majority of pleasure vessels that do not have this equipment installed. While true, it is not sensible to remove or disable systems that add to the safety and functionality of the vessel.

Boatbuilders do not fit these devices for fun. You obviously cannot understand their purpose, but it would be much better if you do not disable equipment that has been installed by the boatbuilder (and advise others to the same)
PaulRainbow said:
While you are educating us all, can you explain how fitting a negative isolator, as required by the ISO, helps prevent stray current corrosion ? Try and answer without using Google, if you can.
Click to expand...
Negative isolation helps prevent DC stray current corrosion by reducing the chance of multiple unintended ground connections to seawater.
 
noelex said:
Negative isolation helps prevent DC stray current corrosion by reducing the chance of multiple unintended ground connections to seawater.
Click to expand...

How does fitting an isolator remove or prevent "multiple unintended ground connections to seawater".

The only condition in the regs that require the fitting of a negative isolator is for 2 wire isolated systems. That's when there is no connection to the water. If there is a connection to the water, there is no requirement to fit a negative isolator.

You're just demonstrating your complete lack of understanding of the subject matter. Might be time to stop digging.
 
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