Does it matter order of battery disconnection in a car?

[deblen]

Originally Posted by AZ-Oldguy

I had a friend who touched the car frame with the spanner and the spanner to his wedding ring and the wedding ring to the battery post and the wedding ring turned instantly white-hot. Fortunately it was not too tight and he was able to jerk his finger out and only had permanent scarring, superficial.

Avoid wearing jewelry or a watch when working on electrical is good safety practice. / Len

[deblen]

Originally Posted by Srah 1953

I'm studying electrics and battery theory at the moment. The question relates to a battery in a car. I'm sure I read somewhere in my "investigations" that there was an order in which leads from a car battery should be disconnected to avoid, I thought, an electric shock. But when I think about it I can't see how it matters. If I disconnect the positive lead first and say touch it to the car metal, how can there be a shock as it's no longer connected to the battery? It seems to me the same as if I touched the negative terminal with my hand. As long as I don't touch the positive terminal at the same time I won't get a shock. Similarly, if I disconnected the negative terminal first and touched it to metal, there won't be any reaction as it is no longer connected to the battery. In either case, it seems to me, that once one lead (whichever) is disconnected from the battery, there is no longer a circuit, but I feel I'm missing something.
Am I wrong?
Thanks

Most people don't feel electric current(shock,tingle) until voltage gets up around 48V.
However,batteries,even 1.5V ones,can create enough heat, when shorted (+ to -)' to start a fire. Avoid putting batteries in your pocket or in trash bins without taping their contacts. / Len

[Time2Go]

Originally Posted by deblen

Avoid wearing jewelry or a watch when working on electrical is good safety practice. / Len

After hearing a few grisly stories of
“Degloving” ” of fingers caused by rings catching
on stuff , I never wear any Jewelry on a boat.
Watch has a velcro band.
Just sayin
Cheers
Neil

[jacktheflyer]

The other rule of thumb is when using jumper leads to make the last connection to the frame away from the battery and disconnect this one first. This will avoid making a spark next to a battery that is possibly gassing.
Scrubby

[taxwizz]

Originally Posted by Srah 1953

I'm studying electrics and battery theory at the moment. The question relates to a battery in a car. I'm sure I read somewhere in my "investigations" that there was an order in which leads from a car battery should be disconnected to avoid, I thought, an electric shock. But when I think about it I can't see how it matters. If I disconnect the positive lead first and say touch it to the car metal, how can there be a shock as it's no longer connected to the battery? It seems to me the same as if I touched the negative terminal with my hand. As long as I don't touch the positive terminal at the same time I won't get a shock. Similarly, if I disconnected the negative terminal first and touched it to metal, there won't be any reaction as it is no longer connected to the battery. In either case, it seems to me, that once one lead (whichever) is disconnected from the battery, there is no longer a circuit, but I feel I'm missing something.
Am I wrong?
Thanks

If you disconnect the positive terminal first you can potentially create A HUGE spark and invoke the Wrath of God.

[waterman46]

Originally Posted by Kalinowski

Always black first. Black (b) comes before R (r) in the alphabet.

This is useful, but not near as interesting as the sentence we used for remembering the resistor color code.

[Wotname]

OK the OP's question has been answered and without debate - always a good sign .

In the interests of furthering the OP's quest of learning more about electricity, allow me to pose the following apparent contradiction as highlighted by the post below.

Originally Posted by taxwizz

If you disconnect the positive terminal first you can potentially create A HUGE spark and invoke the Wrath of God.

Taawizz is correct, a HUGH spark can be created but consider the following.

The battery voltage is nominally 12V (DC).
Air is considered a good insulator and has a dielectric strength of around 3MV/m (that is 3 million volts per meter).
The means a lead taken from the positive terminal has to be closer than 0.004 mm to the negative terminal (or chassis etc) before a spark will occur.

Now 0.004 mm (or about 0.16 of a thousand of an inch) is pretty small and so the spark will be very hard to see.

However most of us has seen much bigger (longer) sparks emanating from a 12 volt battery so given above facts about the dielectric strength of air, how can this be so.

No prizes for the correct answer but your CF electrical credibility will increase!!!

[GILow]

Oh great. I read this at midnight, now I won’t sleep ‘till I know the answer. I have a feeling it lies with the fact that once the spark has started it creates some kind of tunnel. I want to use the word plasma here, but I might have been watching too much Star Trek.

As a regular welder I see the described phenomenon often.

[GILow]

Yes, I just considered this some more and realised I always have to strike the target to get the weld started.

I’m going the plasma option.

Now pour scorn upon me.

[Wotname]

^^ it's part of the explanation but plasma doesn't fully explain why the length of spark (arc) from 12V can massively exceed 0.15 thousand of an inch.

[model 10]

How about creating a capacitor?

[Wotname]

^^ sorry no capacitance involved but here's a hint - it is something similar.

[model 10]

Potential?

[Wotname]

^^ potential is involved

[model 10]

Well por supuesto it's potential. It's the desire that creates the spark?