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Old 01 December 2015, 09:48   #1
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Isolator switches - again.

There's been quite a bit of discussion on here about the need for and merits of an isolator switch. My stated experience has been the common red key type are prone to failure. On at least one occasion I had one took out a Regulator/Rectifier when it momentarily broke the circuit with the engine running. After fixing that I only use the isolator switch on any boat I've owned to isolate everything except the engine. This was to protect against any repeat failures, or someone accidentally knocking the switch off.

I was out recently on a whale watching expedition with a friend who's bit of a landlubber. After several hours out we started back via Cape Clear island for a break. After I'd made the boat fast to the pier I called back over my shoulder to switch off the engine. Moments later, engine still running, I looked at the blank face with palms upturned below. Yup! He switched off the isolator switch. I was damn glad I'd changed the wiring to have the engine power bypass that switch or I'd be down another Regulator/Rectifier plus running back home with the battery running down all the way.

If you have one of these fitted, it's a five minute job to take the engine positive - the heavy lead - off the downstream terminal and put it on the battery in terminal leaving the positive power leads for all other services on the downstream switched terminal. Once again, I recommend it! 🔧
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Old 01 December 2015, 18:33   #2
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I've had the big rotary switches fail a couple of times as well. No major consequences but it makes you wonder if you're making things safer or less reliable. The Coastline didn't have one fitted and that's how I left it.
It lives in the driveway though, permanently connected to a regulated "step" charger that holds the batteries at 13.8 volts once they're fully charged.
That can push 20 amps so even if something's been left on I'm not coming back to a flat battery.
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Old 03 December 2015, 05:22   #3
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I have an FIA one on the car & that's wired as a cutoff switch with a resistor to protect the alternator:
www.REEDX.net
Whether that would be a better option?
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Old 03 December 2015, 16:55   #4
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I've fitted a Blue Sea battery isolator and it is pretty robust. No issues (touch wood), and highly unlikely to inadvertantly switch it off. When I wired it up I put the fixed VHF straight to the battery and bypassed the isolator. It has an inline fuse.

Worst case scenario with any isolator failing, then carry tools onboard so you can bypass isolator and pass 12v cable straight to battery and secure with jubilee clips.

That fails, then fire up the auxilliary.
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Old 04 December 2015, 08:36   #5
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Just remember that the other reason for having these is fire. I have witnessed a shorted wire lighting up before [admttedly on a car], and getting the battery isolated quickly is critical. It is probably less likely that the primary feed would be affected, but anything that runs off from there could be a problem if it can't be isolated
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Old 05 December 2015, 05:57   #6
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Originally Posted by georgec View Post
Just remember that the other reason for having these is fire. I have witnessed a shorted wire lighting up before [admttedly on a car], and getting the battery isolated quickly is critical. It is probably less likely that the primary feed would be affected, but anything that runs off from there could be a problem if it can't be isolated
Would be interesting to to see what happens to an isolator switch when a short lights up the main feed.....500-600 amps? Obviously way beyond it's operating range and way beyond the point you could rely on it. Pair of heavy duty snips
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Old 05 December 2015, 07:50   #7
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I've fitted quick connect battery clamps on some boats but I wouldn't fancy trying to pull them in a full short situation! I also agree that any isolator switch wouldn't necessarily function reliably in a full short situation, it'd probably just weld the contacts. There are generally a lot less chances for a full short on the main battery cables on a Rib than a metal bodied car.

That protected switch with the shunt resistor seems like a good idea if a bit complicated with the flip/flopping circuits. Maybe a big connected across the contacts (to only allow charging current toward the battery when off) on a standard isolator might do the job automatically.
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Old 09 December 2015, 16:33   #8
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Quote:
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Would be interesting to to see what happens to an isolator switch when a short lights up the main feed.....500-600 amps? Obviously way beyond it's operating range and way beyond the point you could rely on it. Pair of heavy duty snips
I think you will find proper isolator switches can easily handle this. Diesel engines in many motorboats pull 300 to 400A for the starter motor and the switches are usually rated up to about 750A IIRC.

A decent switch in a sensible position should not get accidentally turned off. Everything bar the bilge pump should be fed via an isolator IMV.
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Old 10 December 2015, 12:33   #9
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I think you will find proper isolator switches can easily handle this. Diesel engines in many motorboats pull 300 to 400A for the starter motor and the switches are usually rated up to about 750A IIRC.

A decent switch in a sensible position should not get accidentally turned off. Everything bar the bilge pump should be fed via an isolator IMV.
For petrol engines "a good starter will normally draw 130 to 150 amps when cranking a four cylinder engine, 175 when cranking a V6, and as much as 200 to 225 amps when cranking a large V8" so do you fit a switch rated appropriately to the engine fitted or one that might handle a straight short? We know what the boat builders will do......the cheapest they can get away with.

You have to be wary of how you interpret the manufacturers rating figures.
Many switches I've seen quote "intermittent" figures in the order of 500-1000 amps but that's only for 5 seconds and relates to flowing current. They're not rated to "switch" or "interrupt" that current. (that's left to the starter solenoid)
Also, having done a quick bit of research and it looks like my 500-600amp educated quess at the short circuit current of a 12V lead acid battery was a fair bit short of the mark. The actual figure appears to be over 1000amps --http://www.battcon.com/papersfinal2003/korinekpaperfinal2003.pdf

I'm not advocating pulling out switches or standing on a soap box on this, it's just food for thought. Most "straight shorts" happen when we're working on something and the battery should have been disconnected before we started.
It could happen in normal use but I imagine (with proper maintenance) fairly rarely and I'm still not convinced the switch is going to work. Everything, with the exception of the starter motor, is fused anyway so really it is only the starter and it's feed that constitutes a risk factor.
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