dsw
Member
Just wondering should the earth (neg) from the battery to the
Starter motor have an isolation switch the same as the live on
My steel vessel ?
Starter motor have an isolation switch the same as the live on
My steel vessel ?
battery switch ?
- Thread starter dsw
- Start date
jfm
Well-known member
Normally, I mean excluding anything unusual about your boat, it would be totally pointless. They will be connected thru the hull anyway
rafiki_
Well-known member
I agree with JFM, I can't see the point of additional isolation, assuming the pos side has an isolation switch.
PaulGooch
Active member
I also agree, despite the fact that Jeanneau fitted one to my boat.
Ripster
Active member
Same here - cant really see the need, but our Jeanneau has one too.
mil1194
Well-known member
An extra one hidden away can only make things more difficult for a thief IMHO.
ExcaliburII
New member
Don't do it !
I would say definitely not, this sounds quite dodgy to me on any boat - steel or otherwise - unless the switches are mechanically linked so they cannot possibly be operated separately.
Suppose you wire an automatic bilge pump direct to the battery so it will operate when the master switch(es) are off. This is quite normal. Float switch or whatever in the bilge and a 15A fuse in the positive feed to the bilge pump circuit to keep everything safe in the event of a fault. OK so far.
Now suppose you or your crew come on board one day, switch on the positive master switch but not the negative one, and try to start the engine. No heavy negative path is available straight to the battery, so the 600A or so from the starter motor will flow through the engine block and to the earth busbar of the wiring system via the bonding cable connected to the engine. Then, the only route back to the battery is via your auto bilge pump earth, which is only rated at 15A, not 600. No fuse in the negative cable so, bingo! Lots of burning plasticky smell, smoke and melted wiring if you're lucky. Big orange flames if you're not.
Unless I'm missing something obvious and the wiring system is very carefully designed to avoid this scenario, fitting an independent isolator in the negative cable sounds like a recipe for disaster to me.
I would say definitely not, this sounds quite dodgy to me on any boat - steel or otherwise - unless the switches are mechanically linked so they cannot possibly be operated separately.
Suppose you wire an automatic bilge pump direct to the battery so it will operate when the master switch(es) are off. This is quite normal. Float switch or whatever in the bilge and a 15A fuse in the positive feed to the bilge pump circuit to keep everything safe in the event of a fault. OK so far.
Now suppose you or your crew come on board one day, switch on the positive master switch but not the negative one, and try to start the engine. No heavy negative path is available straight to the battery, so the 600A or so from the starter motor will flow through the engine block and to the earth busbar of the wiring system via the bonding cable connected to the engine. Then, the only route back to the battery is via your auto bilge pump earth, which is only rated at 15A, not 600. No fuse in the negative cable so, bingo! Lots of burning plasticky smell, smoke and melted wiring if you're lucky. Big orange flames if you're not.
Unless I'm missing something obvious and the wiring system is very carefully designed to avoid this scenario, fitting an independent isolator in the negative cable sounds like a recipe for disaster to me.
PaulGooch
Active member
I would say definitely not, this sounds quite dodgy to me on any boat - steel or otherwise - unless the switches are mechanically linked so they cannot possibly be operated separately.
Suppose you wire an automatic bilge pump direct to the battery so it will operate when the master switch(es) are off. This is quite normal. Float switch or whatever in the bilge and a 15A fuse in the positive feed to the bilge pump circuit to keep everything safe in the event of a fault. OK so far.
Now suppose you or your crew come on board one day, switch on the positive master switch but not the negative one, and try to start the engine. No heavy negative path is available straight to the battery, so the 600A or so from the starter motor will flow through the engine block and to the earth busbar of the wiring system via the bonding cable connected to the engine. Then, the only route back to the battery is via your auto bilge pump earth, which is only rated at 15A, not 600. No fuse in the negative cable so, bingo! Lots of burning plasticky smell, smoke and melted wiring if you're lucky. Big orange flames if you're not.
Unless I'm missing something obvious and the wiring system is very carefully designed to avoid this scenario, fitting an independent isolator in the negative cable sounds like a recipe for disaster to me.
Wouldn't happen on a GRP boat, the neg supply isn't grounded to the hull. If the neg isolator was off, nothing at all will work. Ask me how i know this ?
OK, as you asked. Couple of trips ago we arrive at the boat and SWMBO says "oh, we didn't turn the isolator off" (we being "her" as it's her job). No matter. Switch something on, no go. Switch something else on, still no go, seems the house battery has gone flat. This was one of the weeks where we left the shorepower off, we do that every few weeks to make sure the batteries hold up without it. Damn, looks like we might have an iffy house battery. Turn the switch to the "combine" setting, which parallels the batteries, so everything will run off the engine battery for now, still nothing. Engine battery flat as a pancake too. Very odd.
Until i notice the mistake she made at the end of the last trip. Instead of turning the positive isolator off (which isolates both circuits) she had accidentally turned the neg off. Only real problem would be that the auto bilge pumps would not work, unless they had the negs wired direct to the batteries, as well as the positives.
Still agree that there is no need to isolate the neg though.
dsw
Member
i wonder no more and leave things as they are.
