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On 2008-11-12, Arfa Daily <arfa.daily@ntlworld.com> wrote:
>
> "Whiskers" <catwheezel@operamail.com> wrote in message 
> news:slrnghjjbg.e5j.catwheezel@ID-107770.user.individual.net...
>> On 2008-11-11, Arfa Daily <arfa.daily@ntlworld.com> wrote:
>>> "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:
>>
>> [...]
>>
>>> 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.
>>
>> So a charge of 10kV from walking across a carpet (DC) , or a "non-lethal"
>> 50kV "Tazer" (AC), are more dangerous than a 1kV power line?  The danger
>> from 'High Voltage' power lines comes from the vast amount of current they
>> can deliver.
>
> You see, there you go getting your apples and oranges mixed up again. A 
> static charge from a carpet is not lethal to you because it is an extremely 
> high resistance source. That is the nature of a static charge. No matter 
> whether it is 1kV, 10kV or 40kV, the source resistance will ensure that only 
> uA will flow for decimals of a uS. Enough to give you a nasty little crack, 
> yes, but not enough to come within several orders of magnitude of being 
> lethal. The same applies to the taser. It is designed to not be able to 
> supply enough current (theoretically) to be lethal. However, its aim is to 
> incapacitate by disrupting nerve activity and causing extreme pain. To do 
> this, the charge is applied to the receiving body in multiple short-duration 
> pulses. In some cases, this has proven to be fatal, I believe, due to the 
> length of time that the pulses are delivered for. On the other hand, a high 
> voltage power line, or even a fairly low voltage household supply, *is* 
> potentially lethal not because it can deliver huge amounts of amps, but 
> because the voltage that is present, is sufficient to drive *enough* (mili) 
> amps from that low resistance source through the conduction path within the 
> body.

Precisely; it isn't the voltage that's lethal, it's the current.  If the 
current isn't available, the voltage is pretty harmless.  High-voltage 
mains power lines carry a /lot/ of current, whereas high-voltage 'Tazers' 
don't.

>>> 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.
>>
>> What voltage does a 'low voltage' welder operate at?  Don't they use
>> capacitors to raise the supply voltage to at least 40kV?  Or can welding
>> really happen at 12V?
>
> Yes, arc welding can occur at 12v and, in fact, at lower voltages than this, 
> although there are many different methods these days of generating and 
> controlling the welding arc, though none that operate by charging caps to 
> the sorts of voltages that you are talking about, as far as I am aware. In 
> general, arc welding is a high current rather than high voltage system. Spot 
> welders sometimes work by charging very large capacitors to low voltages. 
> The reason that the low voltage arc welder is not capable of killing you, 
> any more than a car battery can, is because although both are extremely low 
> internal resistance sources, and hence capable of supplying large amounts of 
> current to a low resistance load, the human body is a much higher resistance 
> load, so the 12v is not capable of pushing enough miliamps through that 
> path, to cause electrocution - or even electric shock.

OK, I'm not familiar with arc welding, and I'm surprised that 12V is 
enough to strike an arc.  But your comment that it is the high current 
that matters for creating a weld, rather supports my contention that 
current is more dangerous than voltage.

Electron beam welding (which you probaby won't find happening in a garden 
shed or on a building site) requires many thousand volts, but only a small 
current (less than 1A).  

The only welding I've ever done myself, involved a coke-burning forge and 
a heavy hammer.

>>> 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
>>
>> Sounds too good to be true.
>
> Well, if you understood the principles of isolation transformers, which as 
> well as being used on building sites for obvious reasons, are also to be 
> found in every professional electronics workshop, to render safe, the need 
> to work on live equipment employing such potentially lethal nasties, as 
> switch mode power supplies, then you would know that it isn't too good to be 
> true - it simply *is* true. Sketch it down on a piece of paper then have a 
> good think about it.
>
> Arfa

The only 'isolation transformers' I've come across, are earthed - and 
either have an output restricted to 50V or less (that threshold at which 
the human body's resistance drops substantially) or provide a good earth 
connection for the appliance.  An isolation transformer /does/ disconnect 
the connection that exists between the power station and the ground and is
carried by the 'live' or 'hot' cable, but that is /not/ the same as the
safety 'earth' connection which is meant to provide a relatively safe 
return path in case exposed metal parts of the appliance accidentally 
become connected to the power supply.  

The 'earth' connection provided by an 'isolation transformer' to the
appliances running off it, may be arranged to effectively create a
'short-circuit' on the output windings of the transformer if any current
flows through it, and that should trigger the safety cut-off (probably a
'residual current device').  Or the 'earth' connection will pass straight 
through to the same 'earth' as the mains supply provides.

A double-insulated appliance has no exposed metal parts that can become
connected to the power supply, and that's why they are permitted to be
used with no 'earth'.

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