electric shock

On Sat, 19 May 2012 13:29:38 +0200
Martin wrote:

On Sat, 19 May 2012 12:06:22 +0100, Stephen Wolstenholme
wrote:

On Fri, 18 May 2012 16:16:14 +0100, "Brian Gaff"
wrote:

Its of course across the heart you need to watch out for. Some
people like the effect of electric shocks, but obviously there are
limits!

Brian

A good shock is also used to start the heart!

About 50 years ago when I was an apprentice TV engineer I often
"showed off" by producing a blue arc with my finger from the EHT. It
was quite safe because the current was very low. Years later I was
working on Ampex computer tape decks and accidentally touched a motor
servo drive. That was a releatively low voltage but high current - I
almost did a somersault!

LOL I did the same thing.

I also picked up a plug of a Honeywell line printer. A capacitor
discharged through me knocking me off my chair.

I have had a few car ignition spark shocks, and at work, one or two
6,000 volt industrial ignition transformer shocks. The common brand
used half of the sine wave, but the Honeywell transformer used
both parts, or something, and produced 15,000 volts. That hurt.
--
Davey.

Brian Gaff wrote:
They must have some strange ideas of how to power mast head amplifiers over
there then.

What i mean is that you can still get devices that create electric shocks.
tens devices and these are not lethal, some find them very pleasent.
Brian

I have a 200 year old device intended to cure neuralgia by means of
electrodes and a small dynamo.

Bill

Martin wrote:

The printer had been switched most of the day, when somebody asked me
to pass him the plug.

The mains plug? How did that work, then? If it were an SMPS the
storage capacitor would have to discharge "backwards" through a bridge
rectifier.

Interesting!

--
SteveT

On Sat, 19 May 2012 18:48:09 +0100, Steve Thackery
wrote:

Martin wrote:

The printer had been switched most of the day, when somebody asked me
to pass him the plug.

The mains plug? How did that work, then? If it were an SMPS the
storage capacitor would have to discharge "backwards" through a bridge
rectifier.

Interesting!

Quite common, it's the capacitors in the EMC filter that remain
charged up when the power is turned off mid cycle.

--
Graham.
%Profound_observation%

On Sat, 19 May 2012 18:08:07 +0100, Bill Wright
wrote:

Brian Gaff wrote:
They must have some strange ideas of how to power mast head amplifiers over
there then.

What i mean is that you can still get devices that create electric shocks.
tens devices and these are not lethal, some find them very pleasent.
Brian

I have a 200 year old device intended to cure neuralgia by means of
electrodes and a small dynamo.

Bill

My Dad was a physiotherapist, and our front room was his treatment
room in the late 50s - early 60s.

The electroheropy console was bigger than me, and my dad would talk
about galvanism and faradism and also shortwave diathermy (although I
don't think he had a machine for that)

He also had a poweful UVA lamp that I had until comparativly recently.
It eas great for curing Loctite Glass-Bond when there was no sun.



--
Graham.
%Profound_observation%

Graham. wrote:

Quite common, it's the capacitors in the EMC filter that remain
charged up when the power is turned off mid cycle.

Ah! Thank you.

--
SteveT

"Davey" wrote in message
...
On Fri, 18 May 2012 16:06:31 +0100
Bill Wright wrote:

I was working on a 55VAC line-powered TV system the other day and I
could feel the power through dry fingers. I got the impression 55V
could be quite nasty with wet hands and a good earth. It was
interesting to feel this attenuated mains. The individual cycles can
be felt quite clearly. It made me wonder if the feeling of touch has
less persistence than vision. Of course the ears would turn the 50Hz
into a note.

Bill

The 'nasty' value would also depend on the available current flow,
surely? 20 Amps would do serious damage, even 2 will kill you.


Quoting figures like "20A" is pointless - a few tens of mA can kill, but you
have to have enough voltage to develop the current through the resistance.

With dry unbroken skin the resistance can be as high as a few hundred k
Ohms - some people can survive mains voltage for a short while.

With wet skin mains shocks are frequently fatal and with broken skin, the
55V might be enough to do the job.

With 55V, the sensation it causes might be enough to cause an increase in
perspiration - then you have wet skin. That may lead to passing enough
current to damage the epidermus, and then you're down to broken skin!

In message , Steve Thackery
writes
Graham. wrote:

Quite common, it's the capacitors in the EMC filter that remain
charged up when the power is turned off mid cycle.

Ah! Thank you.

There should, of course, be a law which requires that there be a (say)
100k bleed resistors across each EMC capacitor.
--
Ian

On 19/05/2012 23:00, Ian Jackson wrote:

There should, of course, be a law which requires that there be a (say)
100k bleed resistors across each EMC capacitor.

There is, in effect. The two main safety standards, viz. EN 60065 for
AV & similar equipment and EN 60950 for IT equipment both have
requirements in this respect:

- EN 60065:2010 (clause 9.1.6) requires that 2 s after withdrawal of the
mains plug the voltage at its pins shall be less than that defined as
'hazardous live' - essentially 60 V DC;

- EN 60950:2006 (2.1.1.7) expresses it in terms of the discharge time
constant, which has to be 1 s (or 10 s if the mains plug is an
industrial IEC 60309 type);

- in both cases if the capacitance is less than 100 nF you don't have to
worry.

Since compliance with a harmonised standard is the usual way of meeting
the LVD requirements (implemented as the Electrical Equipment Safety
Regulations in the UK), the above is effectively a legal requirement.

--
Andy

Ian Jackson wrote:
In message , Bill Wright
writes
Richard Tobin wrote:
In article ,
Bill Wright wrote:
The individual cycles can be felt quite clearly. It made me wonder
if the feeling of touch has less persistence than vision. Of course
the ears would turn the 50Hz into a note.
Presumably you feel it as 100Hz, since all you can detect is the
magnitude.
-- Richard

I have very bad skin. It only conducts one way.

You ought to see a dermatologist and see if he can rectify the problem.
Oh God here we go....
OK. "I have considerable resistance to the idea."

Bill