On Sat, 07 Jan 2012 01:04:13 +0000, Steve Walker
wrote:

Not always a good idea to have electrical conductors in flammable liquids..

That's when you use intrinsically safe devices and either zener barriers
or galvanic isolators - restricting the voltage (typically 28V dc max)
and current (typically 20-odd milliamps) so that there is not enough
energy supplied or stored to ignite any vapours.

I'd understood the vapour concentration in a typical petrol tank was
too high for explosion risk. Otoh, it's not entirely unknown for one
to go up.
Years ago I saw a pic of static discharges in an Avgas tank - slightly
worrying.