🚀 go-pugleaf

"Whiskers" <catwheezel@operamail.com> wrote in message 
news:slrnghlo20.bmt.catwheezel@ID-107770.user.individual.net...
> 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.


Neutral and ground are bonded together at the substation


>
> 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
> -- ~~~~~~~~~~

Hmmmm. Your understanding of the principles of ground isolation by use of a 
transformer seem very flawed I'm afraid, as also is your knowledge of 
electricity distribution schemes, and earth / neutral commonality.

Arfa