Re: Which battery lead to disconnect first

[C2slayman@domain.elided]

John,
       I won't copy the whole text, but ....

> Date: Mon, 13 Jan 2003 20:11:44 +0200
> From: "John Fielding" <johnf@domain.elided>
> Subject: RE: Which battery lead to disconnect first
> 
> Hi to all the Digestables,
> 
> 
> When I was at college, many years ago, we learnt about Kirchoff's law which 
> simply stated
> that the current flowing in a circuit was the same at the positive pole as 
> the negative
> pole, so why disconnecting the negative produces no spark?? , but the 
> positive can,  when
> the same current is flowing in both baffles me.
> 

You are absolutely right.  Same current HAS TO FLOW THRU BOTH terminals.  So 
symmetry is perfect with respect to current flow.  However, the electrical 
circuit of the auto itself is not symmetric.  So "perhaps" this asymmetry is 
important.  But I doubt it.

> I also have a very faded Morris Motors workshop manual from the 1930 which 
> deals with
> disconnecting the battery before working on the electrical system.  In this 
> they state
> "always disconnect the negative lead first".  However, in those days cars 
> where fitted
> with positive ground electrical systems, so the negative battery lead was 
> the live one,
> not the ground lead!
> 
> Methinks along the ways somewhere the changeover to negative earth vehicles 
> got muddled in
> the handbooks!
> 

Bingo!  It would make more sense to me to say disconnect the ground first.  
Or disconnect the non-ground first.  The former has been pointed out by other 
Digesti as having the advantage of not shorting out the wrench if you 
accidentally hit a part of the chassis - a definite plus!  If there is some 
current draw, there is going to be minor sparking from the terminal 
regardless of which terminal you remove first.  So I think it's all bogus and 
not based on any electrical engineering principals other than avoiding the 
"sparking wrench" phenomenon.   Fellow Digesti - please debunk me if there is 
another reason!  But in that case, the rule should read "remove ground 
terminal first."

The only logical explanation I have ever heard is to aviod generating a spark 
in close proximity to a battery that is generating gas (big kaboom!)  So when 
you connect jumper cables, connect the ground cable to the chassis (instead 
of the battery terminal) so that the spark is away from the battery.

> 
> So I am still puzzled, no one can give me a definitive answer, not even the 
> local Bosch
> agents.
> 
> The FIA in its many pages of regulations for the various motor racing 
> formulae deals with
> battery isolators.  They state the battery must have an effective isolator 
> "which can be
> fitted in either battery lead".  Battery isolators are used extensively on 
> commercial
> vehicles, all the catalogs I have so far managed to clap eyes on have the 
> main switch in
> the positive lead!  So what gives???
> 

Should be able to connect to the negative also.  I can't think why not.  The 
battery isolator is a simple mechanical switch isn't it?  It doesn't have a 
circuit with relays in it, does it?

> 
> John
> Durban
> South Africa
> Alfetta 1.8L turbo
> 
Cheers,
Charlie
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