RE: Which battery lead to disconnect first

["John Fielding" <johnf@domain.elided>]

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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