DC emergency isolation switch?
- Thread starter Jalepeno
- Start date
As posted it is common on Beneteau and some other EU boats. I have never understood its use on a fiberglass boat. On an aluminum or steel boat its use will keep the engine isolated when not in use from any current, probably the best way to solve possible corrosion issues in that case.
Of course I have a battery selection/disconnect switch on the positive side as everyone has. I’m asking about an emergency disconnect switch on the negative side of the batteries. Do you ask why? Quite a few years ago I was fishing on a friend’s powerboat when the positive cable from the starter battery to the battery switch chafed though the insulation and shorted to the engine exhaust riser. Fortunately it was the starter battery and not the larger capacity house bank but it still did a lot of damage before we were able to disconnect the cable from battery post. He had wind nuts on the battery posts but it still destroyed the battery, the cable, a wiring harness and melted a hole in the aluminium exhaust riser on his single engine in the one minute it took us to disconnect it. The battery switch on the positive side can not protect against this kind of problem.
His starting battery ideally should have had a battery switch in closer proximity to the battery and if he did not this can be plain unsafe as you found out. Without proper chafe protection, support, fuse and or a conduit a long run before the switch is just un-wise and can be quite unsafe.
All banks should have a switch in the positive feed and ideally the run from the batteries to the switch should be a short as possible.. I am a very strong believer in this even with fuses. This is where ours is in relation to the bank..
Also all banks on sailboats, with small aux engines, can ideally be FUSED as close to the battery as possible. It is part of the ABYC standards that ALL house banks are fused but there is no excuse not to fuse a starting bank.
As a member of this YC I was pretty pissed that we did not install fuses on all our club boats prior to this electrical fire.. This cost the club a pretty good deductible and almost the lives of four junior sailors who had been on the boat seconds before it erupted into flames at our dock. This fire could have been 100% prevented with a battery fuse. Our entire fleet now has fuses on the batteries.
Would not have mattered where a switch was with this fire, there was no getting near it....
It most certainly can and does if it is wired correctly & safely. His simply was not. Battery switches are isolation switches intended to ISOLATE the battery. If his did not it was just not installed correctly and the wire run to the switch was installed in an unsafe manner. Sadly many boats are wired this way and this is why the ABYC has the fuse rule. Our 2005 Catalina had the battery cables running from one side of the boat to the other before getting to the battery isolation switch BUT Catalina installed a fuse within 7" of the bank. I installed a second one, on the start bank, BEFORE taking delivery. If the battery cable travels a long way to the switch the bank should definitely get a fuse to protect against shorts.. A fuse is automatic. If you need to dive into a flaming cabin to get at a battery switch, not as safe..
Another point is that the ABYC prefers to see all battery switches in the positive conductors. The DC neg/ground system is also used for AC ground and disconnecting it from earth can compromise the AC ground protection so care would need to be taken if you also have an AC system..
Can it be done? Sure, but I would add fuses well before doing anything else as they are far safer and multiples faster than any switch in an event like you described above...... Short to ground....POP! You can't even smell the smoke that fast..
Your best "isolation" for the event described is nothing less than a FUSE...
His positive cable run was about 8 ft and the problem occurred in the cable between the battery and the switch so the positive switch did nothing but a switch on the negative side would have enabled an emergency disconnect. As it was we had to disconnect the wing nut on the battery post with a multi-tool very quickly.
How large fuses do you recommend? In theory the house bank fuse could be sized at 40 amps to handle the alternator's full charge current of 35 amps. But in reality both the house bank and the starter battery need to be large enough to handle the starting current for my one cylinder Yanmar YSM-12. I will need to find the rated and maximum starting currents. Also what about the higher starting current if the battery voltage is low? My battery cables to the switch are about 3 ft long, AWG 4 gauge rated at 160 amps max.
How large fuses do you recommend? In theory the house bank fuse could be sized at 40 amps to handle the alternator's full charge current of 35 amps. But in reality both the house bank and the starter battery need to be large enough to handle the starting current for my one cylinder Yanmar YSM-12. I will need to find the rated and maximum starting currents. Also what about the higher starting current if the battery voltage is low? My battery cables to the switch are about 3 ft long, AWG 4 gauge rated at 160 amps max.
This thread will explain how to size....
http://forums.sbo.sailboatowners.com/showthread.php?t=130775
Thanks Maine Sail, that is an excellent article. I must have missed it last summer.
I was already thinking that the Blue Seas Terminal Fuse Block is the way to go and obviously the fuses must be "slow blow" to stand the momentary surge but I'm still not sure what size of fuse. I went down to the boat earlier today and my LinkLITE reports about 100 +/- 50 amps during cranking. The digital display is not instant update so it could be a bit more. Would a 200-amp fuse be about right for this application (3 ft long AWG 4 cables 60 amp rating) or would the 175 or the 150 be better choices?
I was already thinking that the Blue Seas Terminal Fuse Block is the way to go and obviously the fuses must be "slow blow" to stand the momentary surge but I'm still not sure what size of fuse. I went down to the boat earlier today and my LinkLITE reports about 100 +/- 50 amps during cranking. The digital display is not instant update so it could be a bit more. Would a 200-amp fuse be about right for this application (3 ft long AWG 4 cables 60 amp rating) or would the 175 or the 150 be better choices?
Jalepeno
You should use an ANL fuse. They blow at about 160% of their rating after 500 seconds, so no nuisance blows on starting circuits. I wouldn't size a fuse larger than the wire's ampacity.
Here is the ANL chart from Blue Seas:
You should use an ANL fuse. They blow at about 160% of their rating after 500 seconds, so no nuisance blows on starting circuits. I wouldn't size a fuse larger than the wire's ampacity.
Here is the ANL chart from Blue Seas:
