Why put the battery isolator in the -ve line?

[wooslehunter]

Can anyone give a logical reason why the isolation switch should be in the -ve or +ve battery lead? Seems to me that as long as it's close to the battery to provide safety isolation it shouldn't matter.

Dave

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[andyball]

there was a thread on this in scuttlebut a few weeks ago. I actually favour -'ve, esp with metal boats, but opinions vary...as always.

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[snowleopard]

the short answer is that almost all boats operate on a negative earth system. that means that if you switch the negative line then all your electrical equipment remains at +12v with respect to earth and for example a loose wire touching the engine block would cause a short circuit (sparks, fire etc.)

if the switch is in the positive line, everything is at earth voltage and a short to earth has no effect (in fact the line between battery positive and switch remains at +12 so that is kept as short as possible).

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[sailorman]

i switch both now.
originally the neg only was switched & i added a change-over switch, the main reason was i had a "dedicated" engine battery & the only way it was accessable would have been a "jump lead". now the whole system is more versitile

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[andyball]

<font size=1>...a loose wire touching the engine block would cause a short circuit ...</font size=1>

I must be missing something, so I'll ask : if the battery switch (in the -ve cable) is off....how can current flow from the battery unless the loose wire touches the battery terminal or some other point before the switch?.

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[sailorman]

i think "off is in fact" off.
i added the change-over switch purely for the convenience of accessing the engine battery
be off now then bi

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[VicMallows]

For the life of me I can't work this one out also. Maybe a carry-over from car manuals which all seem to recommend you disconnect the -ve when carrying out work on the car. Could it be you might drop a spanner across +ve and chassis and cause a big explosion?. Well, and in light of another thread, disconnect both, lift the battery out, store it in a nice dry place and not on a concrete floor, and while at it charge it with a high-tech charger (ensuring adequate ventilation).

Sorry! ... Vic

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[pvb]

It doesn\'t matter....

You can isolate the batteries equally well by disconnecting either the +ve or the -ve lead. For the best onboard safety, you should have a fuse as close as possible to the battery bank.

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[machurley22]

Re: Why not put the battery...

Why "store it in a nice dry place and not on a concrete floor"? I only ask because my battery has been sitting on the concrete floor in my slightly damp garage for the last 2 months!

Dave

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[VicS]

Vic, you might know the reason for disconnecting the battery negative when working on a car but for the benefit of those who don't it is because with a car when you disconnect the battery you disconnect the negative FIRST. That's because if you accidently 'short' the terminal to the engine, the body or almost anything else while doing so it dosn't matter and once the negative is disconnected then it dosn't matter if you short the positive to the body etc either. If you attempt to disconnect the positve first then touching the body with the spanner while doing so results in a genuine short circuit with posible dire consequences. Once the negative is disconnected everything is safe the positive can be left connected.
None of this applies in a grp or wooden boat of course unless the battery is right next to the engine.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[MainlySteam]

<<<unless the battery is right next to the engine>>>

And can add, then only if the engine is not DC isolated - many pleasure ones now are.

John

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[GARYOFFSHORE]

Your negative system on your boat is also or should be connected to your bonding system which as you know runs to your anode which in turn is very good negative.
If you switch your negative only there are other ways your battery can find it's negative albeit a trickle discharge.
Also remember to fuse the positve cable to protect it, the last thing you want is a unprotected cable travelling through your boat!!!

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[MainlySteam]

<<<Your negative system on your boat is also or should be connected to your bonding system>>>

Why?

I would be interested if you can come up with any good reason. In my experience even big vessels (eg 200 foot motor yachts) frequently now have isolated DC systems.

John

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[VicS]

If there is an open switch in the negative connection to the battery what are the "other ways the battery can find its negative".

You could apply the same illogic to having the switch in the positive or even switches in both. Sounds like the only way of totally isolating the battery is to take it home!

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[Born_Free]

VicS has got it right. However I would always recommend a double pole master switch, which truly isolates the battery, thus avoiding any possibility of a short circuit.

<hr width=100% size=1>I have nothing to declare, except my genius
(With apologies to Oscar Wilde)

 

[andyball]

<A target="_blank" HREF=http://www.ybw.com/cgi-bin/forums/showflat.pl?Cat=&Board=ym&Number=467394&page=&view=&sb=&o=&vc=1>here</A> isthe other thread I mentioned earlier......including this

<font size=1>. If only the negative pole is isolated THE COMPLETE WIRING OF THE WHOLE BOAT IS STILL LIVE and can short out anywhere the water gets to and in the case of a battery that is not in a waterproof box it will run straight back to earth at the negative terminal even if an isolator is fitted...</font size=1>

I disagree,btw.

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[VicS]

Re: Got something right!

thanks for that. Its nice to switch the computer on and find somebody saying I've got something right. Quite a change from realising that what I posted late the previous evening was total 8/3 pi.r^3

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[Benbow]

You are right that this is total nonsense. In fact since electrons actually move from neg to pos this strange image of the positive side of the circuit being somehow full of electricity trying to find its way out, is crazy!

However, while it is true that the battery master switch could be either side, once you have an active system with the boat's ground connected to battery negative it is important to be consistent. For instance fuses are conventionally in the positive side, this is because in boats with seacocks and engines connected to negative there is a good chance of a stray +ve finding its way to ground and causing corrosion if a fuse in the negative side blows. If a fuse in the +ve side blows the protected device won't be +ve with respect to the surrounding (damp) environment.

But certainly the decision to call one side pos and the other neg was completely arbitrary. Somewhere there is a planet where the local beings call the side that emits electrons the positive and the other side negative.



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[GARYOFFSHORE]

The main reason for your negative being run to the bonding system is for safety sake. Only neccesary when your vessel has AC, to protect the DC system from the possibility of chaffing between the two. Giving the DC a ground point.
Most vessels are also negatively earthed to the anode via the engine block anyway!!

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[MainlySteam]

So they say, but the argument is spurious and caters for an extremely unlikely event. When have you last heard of that situation happening?

You will find it is frequently not done on modern properly fitted vessels and those will likely also have have an insulated shaft coupling electrically isolating the engine from the sea, making bonding it pointless, or/and also a DC isolated engine so even if it is bonded, the bonding is isolated from the DC negative.

It has been previously claimed on these forums that class requires it, but that is not so - I am familiar with at least Lloyds, DNV and GL vessels which have isolated DC systems. ABYC (which is only a pleasure boat standard) does require it, and that for the reason you state, but that reason is spurious for properly fitted modern vessels and should never be proposed as being necessary, or even as being desirable.

John

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