🚀 go-pugleaf

On 2008-11-10, Dave Plowman (News) <dave@davenoise.co.uk> wrote:
> In article <slrnghhb1o.bes.catwheezel@ID-107770.user.individual.net>,
>    Whiskers <catwheezel@operamail.com> wrote:
>> Our power sockets are only 3kW each - but it isn't the Watts that really
>> hurt, or even the Volts; it's the Amps you really need to be careful
>> with. 60 Amps at a Japanese 100V would be a lot more dangerous than 30
>> Amps at a European 230V.  (Although either could kill you, so the
>> difference could be academic as far as the one grasping the wires is
>> concerned).
>
> Not quite sure what you mean. The current drawn by a given resistance is
> proportional to the voltage - and touching a 1000 amp supply is no more
> dangerous to a human than touching a 1 amp one.

Well you go right ahead and play with the 1000 Amp wires if you want to,
but wait for me to get out of sight and earshot first - I'm a bit
squeamish.

> And since the body resistance would be a constant 

It isn't.  There's a threshold at around 50V at which the body's internal 
resistance drops considerably; after that, the resistance will depend on 
the path taken through and/or over the body and the amount of tissue 
damage caused - those things being dependent on the energy (ie Amps) 
available rather than on the voltage.  AC and DC current also have 
different effects.

> 230 volts would cause
> more current to flow than 100 - so more likely to kill. 

Think of water pipes.  A narrow pipe can be likened to a low current cable 
(few Amps) and a wide pipe is like a cable carrying a large current (many 
Amps).  Both can be at the same pressure (Voltage) but one will cause a lot 
more damage when it bursts than the other.  Even if the wide pipe is at a 
lower pressure, it can still deliver a lot more water than the narrow 
pipe; likewise, even at a lower voltage a cable carrying a lot of current 
will deliver a lot more energy than one with less capacity but at a higer 
voltage.

> Indeed in the UK
> 110 volts is used on building sites for safety reasons. This is obtained
> usually via an isolating transformer so there is no potential to ground.

I don't know what the regulations are for temporary electrical 
installations on construction sites in the UK, but I suspect they aren't 
much different from those relating to permanent installations.  The use of 
110V equipment may have more to do with avoiding such stringent inspection 
and installation standards as apply to higher voltages - but the safety
measures in use will be related to the current available, not the voltage.  

If there is "no potential to ground" then the system will only be safe 
with "double-insulated" appliances.

To deliver a given amount of power, a low voltage supply has to carry more 
current (more Amps) than a higher voltage supply, so in that respect it 
can be more dangerous.

Once the current is flowing, what matters is the amount of energy (Amps) 
not the 'pressure' (Volts) - and where the energy flows.  A few milliamps 
will stop the heart, if that current flows along a path that disrupts the 
tiny nerve signals involved.

Walking across a synthetic carpet can generate a charge of thousands of 
Volts - enough to create a spark when you get near another person or a
metal fixture or piece of furniture.  But there is very little energy
involved.

See <http://www.pa.msu.edu/sciencet/ask_st/010892.html>.

-- 
-- ^^^^^^^^^^
--  Whiskers 
-- ~~~~~~~~~~