RE: Which battery lead to disconnect first

[Jon Pike <jhpike@domain.elided>]

Hmmmm...

I'll throw in a few thoughts on this one..

First off, I'm embarrassed about the "US Fire Department" sites.. 
sheesh, current is current, either way you make the same spark!

Getting down to which one to interrupt..  one way to look at it is, (on 
a negative ground car) pull the positive, and all the wiring is removed 
from a connection to the battery,  pull the negative and the chassis is 
removed, but the rest of the system is attached.  Still not a huge 
difference, but with later cars with semiconductor electronics 
(computers, gauges and sensors) it could be an issue with static 
electricity.  Still not a great one though, a car's electricial system 
is extremely "noisy" with huge spikes and so forth and designers put 
protection in against that.. And you're far more likely to spark to the 
chassis then the wiring, with any static you might generate.

Now, if some of that protection circutry "ages" (MOV transient absorbers 
do) and dosen't work as well you might worry about it.

If one was totally paranoid on that issue, removing the positive and 
grounding it might be an idea.

Overall,  the main thing is to break the battery circuit, and pulling 
either one does the job about as well at preventing wrench fireworks.

Jon



> 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,
>
> I had a good amount of off-line comments which all seemed to support the "negative lead
> first" syndrome.
>
> I went searching on the Internet, just love this modern technology!  I found two sites
> which happened to be USA Fire Departments and each had a catalogue of how to tackle a
> vehicle after a collision.  Both stated that the negative lead should be disconnected
> first.  One even went so far as to say "disconnect the negative lead first as this will
> not produce a spark if current is flowing, whereas the positive lead can produce a spark
> which can ignite the battery gassing products".
>
> 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.
>
> 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!
>
> I have an aquaintance who runs an auto electrical repair shop and I asked him the same
> question, which lead to disconnect first.  He said it didn't matter as long as the battery
> was effectively isolated from the vehicle wiring, but his preference was the positive (on
> a negative earth vehicle).
>
> 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???
>
>
> John
> Durban
> South Africa
> Alfetta 1.8L turbo
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