🚀 go-pugleaf

"Whiskers" <catwheezel@operamail.com> wrote in message 
news:slrnghja8j.e5j.catwheezel@ID-107770.user.individual.net...
> 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
> -- ~~~~~~~~~~

I've got to agree with Dave on this one. Much of what you say appears to be 
based on an incorrect understanding of Ohms Law, and how it applies to the 
potential for causing electrocution of the human body. The water and pipes 
analogy is good for some simple college explanations involving DC systems 
with fixed parameters, but its validity for explaining complex dynamic 
systems, is tenuous at best.

A system carrying a lot of amps to the load that's drawing it, is no more or 
less dangerous than one that's only carrying a small current, except in as 
much as there is a greater potential for heat failure at connection points 
within that system. A high voltage system, irrespective of how many amps it 
is capable of carrying above a few milliamps, is far more dangerous to a 
human, than a low voltage system good for a few hundred amps. Given the 
(reasonably) constant resistance of any described path through the human 
body, a voltage of a hundred volts with a current availability of as little 
as 50mA, may be enough to kill under the right (wrong!) circumstances. On 
the other hand, a low voltage welding supply, will not have enough voltage 
behind it to push enough current through that same path to kill you, even 
though that supply is good for 200 amps or more.

As far as your contention that a transformer isolated supply is only safe 
with double insulated equipment, that simply isn't true. The only way that 
you can drive current through the body from such an isolation transformer, 
is to hang yourself across both output terminals. Either terminal to ground 
will present no electric shock hazard at all.

Arfa