On 06/12/2010 21:45, Terry Casey wrote:
In , lid says...

alexander.keys1 wrote:
On Dec 6, 2:18 pm, wrote:
Bob wrote:
in 1119577 20101205 084818 wrote:
On the question of voltage, a relative of mine was killed by an
electric shock from a 12 volt car battery. He had no underlying
health condition. :-(
I can believe that. The worst shock I had was from 28 volts in an RAF
servicing bay (and I've had quite a few from the mains).
It was raining heavily. His car wouldn't start and he flattened the
battery. He told his wife he was going to put the battery on charge.
He had opened the bonnet and removed the battery clamp. No-one knows
exactly what happened next but it is assumed that he had wet hands and
was electrocuted from the battery terminals.

He could have cut himself, the skin has a high resistance, but if it
is broken and a live conductor at any voltage touches the tissue
beneath, then lethal currents, it only takes a fraction of an amp, can
flow. There have been a number of deaths from this cause.
I got the belt of my life breaking a circuit from my toy train
transformer to an electric motor in my Meccano set.

Wasn't till ten years later I learnt about inductive flyback voltages.
Easy to get 250v+ out of breaking an inductive circuit carrying current
from a 12v source,. That's what a car coil is anyway. starts at 12v,
break it, primary goes to -300 to -400v and secondary goes to 25KV or
more if the spark gap don't short it.

IF the starter was stalled and on and someone disconnected the battery,
then easily a very big current 400+ volt spike for a few milliseconds.

Normally not enough to kill, but with a hearty condition..?

We are told that there was no underlying health condition.

Within reason, the body can withstand some very high voltages - the killer is current.

The EHT on monochrome TVs, for example, was from about 10 to 16kV but from a high
impedance source, so you got a belt and cursed, but that was all there was to it.

The introduction of colour with its power hungry shadowmask tubes was a differnt matter
entirely as not only was the voltage much higher, at 25kV, but it was generated by a
relatively low impedance source. It's surprising how safety conscious one becomes
overnight when you realise you are now dealing with something that is lethal ...

Going back to your Meccano motor, I had a Trix construction set from which I built bells
and buzzers. One project in the manual modified a buzzer by adding two wires connected to
metal 'handles' to turn it into an electric shock machine ...!


I remember there being an electric shock machine outside the lower tram
station on the Great Orme in Llandudno. I think it took pennies and
there was a fixed brass handle, a rotating one, and a meter, and I
suppose the idea was to see how high a voltage one could stand.

Regarding shocks from batteries, I recently heard of some chaps who
claim to have been told on a Balfour Beatty site induction that the use
of battery operated tools is not permitted if there is a drop of rain in
the sky- it sounds a bitt OTT and of course they may have misunderstood...

j