Negative switch
- Thread starter Pentaracer
- Start date
simonfraser
Well-known member
common on french build boats, this has been asked on here before, don't think there is a good reason for it
i have repurposed mine and changed the label
i have repurposed mine and changed the label
PaulRainbow
Well-known member
It isolates the DC negative. Never seen the point myself. I often convert it to an emergency start switch and re-label it.
noelex
Well-known member
It depends on how the rest of the boat is wired, but isolating the negative supply can reduce the chance of stray current corrosion problems and fire. I would not change the manufacturer’s installation. .
It is not required by the ABYC, but is is compulsory for some other marine standards.
It is not required by the ABYC, but is is compulsory for some other marine standards.
Last edited:
PaulRainbow
Well-known member
Can you post some details of the marine standards that require boats to be built with DC negative switches please ?
Could you also post some links to the boats that caught fire because they didn't have a negative isolator ?
noelex
Well-known member
Sorry for the links. I have been removed them.
Links are sometimes inserted by scammers leading to nefarious websites so you are correct to be wary of them. The links I removed were benign leading to sailing information only, but they have been deleted.
Links are sometimes inserted by scammers leading to nefarious websites so you are correct to be wary of them. The links I removed were benign leading to sailing information only, but they have been deleted.
sarabande
Well-known member
There are a number of links in websearch for negative DC isolation.
DC negative isolation switches at DuckDuckGo
The IET pages are about low voltage in a variety of installations, not just marine, but the Victron pages are more specific. It's more about good practice and practical care of wiring and components than excess of zeal from insurance companies.
DC negative isolation switches at DuckDuckGo
The IET pages are about low voltage in a variety of installations, not just marine, but the Victron pages are more specific. It's more about good practice and practical care of wiring and components than excess of zeal from insurance companies.
noelex
Well-known member
Commercial vessels in Australia, including yachts offered for charter, require a DC negative cut off switch.
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.
jakew009
Well-known member
I don't see how an isolator on the negative side is going to help in any of the situations you describe. The only way of protecting a big positive wire shorting out with a small negative wire would be to fuse the negative side of the circuit as well.
sarabande
Well-known member
But if the negative side is isolated through the switch, how can the positive complete the circuit ? Or am I answering a different question ?
jakew009
Well-known member
The point is once you have isolated one side of the battery, the other side is essentially inert. Isolating both sides adds nothing.
noelex
Well-known member
Precisely. If the negative circuit is isolated then there is no possibility of a short circuit. However, the negative isolation switch will only provide this protection when the switch is in the off position (usually when leaving the boat, or if a fire is detected). Thus the reduction in fire risk, while useful is only small. The reduction in the risk of stray current corrosion from isolating the negative supply is arguably more significant.
Many boats forgo the added layer of protection gained with a negative isolation switch and never catch on fire or suffer stray current corrosion. This is not an essential component. I would not recommend the hassle of fitting one if this is not way the boatbuilder wired your vessel, but I would also not remove or disable this device.
noelex
Well-known member
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.
PaulRainbow
Well-known member
What a load of nonsense. You previously said the positive had to be a heavy wire. If the main positive isolator is off and there is, for instance, a bilge pump permanently live, it will be fused accordingly and if it were to come into contact with a negative the fuse would blow.
The only way to get a short between a heavy positive cable and a light negative is if both positive and negative sides of the circuit is connected, that turning the positive isolator off prevents this from happening, adding a negative switch adds nothing in this scenario.
noelex
Well-known member
Paul, why do you think the boat manufacturers go to the cost and trouble of fitting these switches if there is no benefit?
If you are disconnecting or bypassing these switches, especially in a professional capacity, you need a detailed understanding of the benefits and drawbacks of this action.
jakew009
Well-known member
Presumably because there is some daft regulation in some part of the world and it is easier for the boat manufacturers to comply than argue why said regulation is stupid.
noelex
Well-known member
That begs the question "If there is absolutely no benefit then why did the regulatory body introduce the requirement".
The truth is that there is a benefit; namely a reduced risk of fire and stray current corrosion.
It is perfectly valid to suggest that the reduction of these problems (especially the risk of fire) is only slight and that the extra cost and complexity of installing negative isolation is not justified, but it is important not to start removing or disabling these switches without understanding the pros and cons.
VicS
Well-known member
It was a requirement of ISO 10133 for a negative isolation switch to be fitted, in addition to a positive isolation switch in fully isolated 2 wire systems.
I do not know the reasoning behind that requirement and I do not know if it was carried forward into the new combined standard for DC and AC systems
I pointed out sometime ago that VP wiring schematics showed both switches for engines with isolated electrical systems but the observation was rubbished by P Rainbow
.
jakew009
Well-known member
None of the manufacturers we’re talking about even comply with that anyway because it’s not “simultaneously switched”. That would require a dual pole isolator not two single pole isolators.
sarabande
Well-known member
I think the " simultaneous" and single isolation switches are for different wiring layouts ?
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