Negative switch

[jakew009]

I think the " simultaneous" and single isolation switches are for different wiring layouts ?

Well it’s weird because they refer to an ‘earthed’ system vs a fully isolated system, whatever that means in the context of a fibreglass boat.

 

[bedouin]

I think the " simultaneous" and single isolation switches are for different wiring layouts ?

I think fully insulated means that it doesn't have a ground so there is no distinction in safety terms between +ve and -ve in which case dual isolation is sensible.

As with others I am struggling to think of any circumstances in which isolating an earthed negative helps

 

[bedouin]

Well it’s weird because they refer to an ‘earthed’ system vs a fully isolated system, whatever that means in the context of a fibreglass boat.

Most boats do have an earthed system - often bonded to the anode.

 

[jakew009]

I think fully insulated means that it doesn't have a ground so there is no distinction in safety terms between +ve and -ve in which case dual isolation is sensible.

As with others I am struggling to think of any circumstances in which isolating an earthed negative helps

Yeah I can see a potential safety advantage of dual pole isolation on a system with a floating ground, but I’m bamboozled how that is going to result in a “reduced risk of fire and stray current corrosion”.

 

[PaulRainbow]

Definition of an isolated system, according to the ISO :

fully insulated two-wire DC system
system in which both positive and negative poles remain isolated from the ground (earth), e.g. not
connected to the water through a metallic hull, the propulsion system or earthed through the AC
protective conductor.

 

[PaulRainbow]

What is the purpose of this negative switch in a Princess V42?

Has this boat ever been coded ?

 

[PaulRainbow]

Commercial vessels in Australia, including yachts offered for charter, require a DC negative cut off switch.

They do here, but that's required for coding (Lord knows why), it isn't part of the ISO that builders have to comply with, except under exceptional circumstances and those circumstances require a double pole isolator.

Without a negative cut off switch, all electrical devices have their negative supply permanently connected even if the circuit breaker or switch is turned off. The concern is if a high power circuit (with corresponding large cabling) creates a short circuit with a smaller diameter negative wire. The large fuse in the high power circuit is not always enough to protect the smaller diameter negative wire, which can overheat and cause a fire. Fortunately, this is not a common scenario, but isolating the negative supply has a benefit in reducing the fire risk and a larger benefit in reducing the chance of stray current corrosion.

Electrical problems cause a high percentage of boat fires, but the exact cause is not often made public. I am not sure I could even post links to fires that have been caused by more serious deficiencies such as a lack of fusing, even though we are agreed fuses are essential.

Many boats manage perfectly well without the means to isolate the negative supply, but having this option reduces the chance of stray current corrosion and fire. It is not something I would remove when the boatbuilder has already installed such a system.

 

[PaulRainbow]

Without a negative cut off switch, all electrical devices have their negative supply permanently connected even if the circuit breaker or switch is turned off. The concern is if a high power circuit (with corresponding large cabling) creates a short circuit with a smaller diameter negative wire. The large fuse in the high power circuit is not always enough to protect the smaller diameter negative wire, which can overheat and cause a fire.

As i said previously, this is nonsense.

If the positive conductor is isolated, a short between a positive and negative conductor cannot overheat, there is no circuit.

If a short occurred whilst the owner was away, when he returns to the boat and turns the positive isolator on there is a circuit and who knows what will happen.

If the boat had both positive and negative isolators, there is (as above) no circuit when the isolators are off and a short cannot cause anything to overheat.

If a short occurred whilst the owner was away, when he returns to the boat and turns the isolators on there is a circuit and who knows what will happen. Exactly the same as if there was only a positive isolator.

In short, your scenario above is exactly the same, no matter what you do or do not isolate.

 

[jakew009]

Definition of an isolated system, according to the ISO :

fully insulated two-wire DC system
system in which both positive and negative poles remain isolated from the ground (earth), e.g. not
connected to the water through a metallic hull, the propulsion system or earthed through the AC
protective conductor.

So pretty much every boat with an engine would be an earthed system then (unless I guess the propshaft was fully isolated from the engine and there was no chance of any bilge water etc).

 

[PaulRainbow]

Many boats have "essential" circuits (such as bilge pumps, battery charger) left permanently on. These are not disconnected when the main battery switch is off.

The positive wire from these circuits can form a short circuit with the negative wire of any other circuit on the boat (because none of the negative wires are turned-off even if the battery switch and circuit breakers are in the off position. A negative isolation switch removes much of this risk by disconnecting the negative supply for all the non essential circuits.

If the positive wire from an "always on" circuit, such as the auto bilge pump short circuited to a negative conductor, the fuse in the positive wire would blow. Having a negative isolator makes no difference, again.

 

[PaulRainbow]

So pretty much every boat with an engine would be an earthed system then (unless I guess the propshaft was fully isolated from the engine and there was no chance of any bilge water etc).

I've worked on lots of boats with negative isolators, can't think offhand, of any that were not French production boats. But, non of those had double pole isolators, so one might assume they were not 2 wire isolated systems. The only boats i have ever worked on that have had double pole switches have been coded commercial vessels.

 

[noelex]

As i said previously, this is nonsense.

If the positive conductor is isolated, a short between a positive and negative conductor cannot overheat, there is no circuit.

You are assuming that all positive conductors are isolated when the battery positive switch is turned off. This is not always the case. Devices such as bilge pumps and battery chargers are often wired such they remain active when the battery positive switch is turned off.

If there is no negative isolation switch, all the negative wires for every system on the boat remain connected back to the battery even if the main positive battery switch and all the circuit breakers are turned off.

In a fault condition, such as chafed wiring, a short circuit can develop between one of the active positive conductors and any of the many boat’s negative wires.

For example, if the positive wire to the battery charger and any other negative wire become chafed and make contact, a short circuit is created. The only fuse protecting this scenario is the fuse on the battery charger circuit. If this is say a 50A charger, the fuse may be 70A (depending on wire size). This is unlikely to be adequate to protect the smaller gauge wire used in circuits such as those designed for lighting even though this thinner negative wire is creating part of the short circuit. This thinner negative wire may overheat and melt well before the fuse designed to protect larger gauge wire blows.

The above fault conditions are rare so the fitting of negative isolation switches is not essential, but you should not remove or disable a switch that has been fitted by the boatbuilder, especially without understanding the consequences of this change. A negative isolation switch adds some reduction in risk of fire and a more signifiant reduction in the risk of stray current corrosion. Leave it in place, the boatbuilder has gone to the cost and trouble of installing such a switch for good reasons.

 

[PaulRainbow]

You are assuming that all positive conductors are isolated when the battery positive switch is turned off. This is not always the case. Devices such as bilge pumps and battery chargers are often wired such they remain active when the battery positive switch is turned off.

If there is no negative isolation switch, all the negative wires for every system on the boat remain connected back to the battery even if the main positive battery switch and all the circuit breakers are turned off.

In a fault condition, such as chafed wiring, a short circuit can develop between one of the active positive conductors and any of the many boat’s negative wires.

For example, if the positive wire to the battery charger and any other negative wire become chafed and make contact, a short circuit is created. The only fuse protecting this scenario is the fuse on the battery charger circuit. If this is say a 50A charger, the fuse may be 70A (depending on wire size). This is unlikely to be adequate to protect the smaller gauge wire used in circuits such as those designed for lighting even though this thinner negative wire is creating part of the short circuit. This thinner negative wire may overheat and melt well before the fuse designed to protect larger gauge wire blows.

The above fault conditions are rare so the fitting of negative isolation switches is not essential, but you should not remove or disable a switch that has been fitted by the boatbuilder, especially without understanding the consequences of this change. A negative isolation switch adds some reduction in risk of fire and a more signifiant reduction in the risk of stray current corrosion. Leave it in place. It is there for a good reason.

Why do you keep posting this garbage ?

You're just dreaming up the post unlikely scenarios. Why doesn't the ISO state that double pole isolators must be used in all cases ? Reason is, it's a nonsense scenario.

You may as well argue that all negative wires should be fused, because your scenario is far more likely to occur when all circuits are on. I say far more likely, it's so unlikely in real life you stand more chance of winning the lottery.

 

[noelex]

You're just dreaming up the post unlikely scenarios.

I agree that the reduction in fire risk is not great, but combined with the reduction in the risk of stray current corrosion (which you seem to be ignoring) it is a good reason not to remove or disable this switch when it has been fitted by the boatbuilder.

Boatbuilders and regulation authorities are not stupid. They fit these devices for a reason, even if you cannot understand their purpose. The authorities typically mandate negative isolation switches on commercial vessels where the added cost is justified by the higher usage of the vessel. Recreational vessels typically can conform to a lower level of safety and functionality in a multitude of areas, but to advocate removing these protections when they have already been fitted is bad advice.

 

[LittleSister]

. . .

Boatbuilders . . . are not stupid. . . .

There are quite a few boat owners who might quibble with that!

 

[noelex]

You may as well argue that all negative wires should be fused, because your scenario is far more likely to occur when all circuits are on. I say far more likely, it's so unlikely in real life you stand more chance of winning the lottery.

As you suggest, double pole isolation of all circuits is superior to simply installing a single negative isolation switch.

It is fitted to some yachts and commercial vessels mainily because it offers superior stray current corrosion protection. The negative of all circuits is disconnected anytime the circuit is not in use rather than only when the boat is laid up as is typically the case with a single negative isolation switch. The drawback is that it is very expensive and complex to install. The expense is not justified on most vessels.

 

[noelex]

There are quite a few boat owners who might quibble with that!

Perhaps I should have said boatbuilders are very careful with money.

We can all agree on that .

 

[PaulRainbow]

I agree that the reduction in fire risk is not great, but combined with the reduction in the risk of stray current corrosion (which you seem to be ignoring) it is a good reason not to remove or disable this switch when it has been fitted by the boatbuilder.

Boatbuilders and regulation authorities are not stupid. They fit these devices for a reason, even if you cannot understand their purpose.

More rubbish.

Boatbuilders do not fit double pole isolators and there are no regulations for them to do so, except in exceptional circumstances. Current regulations (here in the UK and in many other countries) dictate that AC protective Earth must be connected to Earth (the water) so the vast majority of boatbuilders also connect DC negative to the same grounding point, so no need to fit double pole isolators.

Why can you not grasp this ?

Jeanneau and Benneteau fit single pole switches to some of their boats, but the onboard systems of these boats are no different to those on many other production boats that don't have the switches. They do not have to fit them and they wouldn't comply with the ISO anyway.

 

[Bouba]

As an aside…if you have battery cutoff switches like this…

The screw under the center cap, can come loose and you might think that you have turned them off only to find out later you haven’t

 

[noelex]

The screw under the center cap, can come loose and you might think that you have turned them off only to find out later you haven’t

Perhaps that is the only reason why the boatbuilder has gone to the trouble and cost of fitting a negative isolation switch. At least one of the battery switches should work.