is third transformer hole an earth?

I always thought they were 31.07 amp ringmains.

Some peoples knowledge is really rubbish.

There are defined tables and graphs which accurately describe fusing capacity of fuses, it is known that a 5A fuse will carry 10A - but not for very long! If the kettle boils before the fuse wire works up to its melting temperature then the fuse wire cools again before it blows so you get away with it another day, but each time the fuse is used above its rated current it degrades a little and sooner or later will fail within the time it takes the kettle to boil.

Actually a 5A fuse carrying exactly 5A has a finite fife expectancy - which actually isn't all that long, unfortunately I can't find the graphs to look up the exact life expectancy.

[...]

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).

Notice that fuses and cut-outs are rated in Amps, and that "non-lethal" stunners for the cops are rated at tens of thousands of volts. In the right (or wrong!) circumstances, the current that will make a torch bulb glow can be enough to stop the human heart.

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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.

And since the body resistance would be a constant 230 volts would cause more current to flow than 100 - so more likely to kill. 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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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that do?

Internal surge supression/EMC circuitry.

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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.

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.

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.

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 .

--
-- ^^^^^^^^^^
--  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

I know, but the reference was to Japanese RINGS.

It's both. 1 billion amps at 0.5 volts definitely won't do you any harm - mainly because you have too much resistance to conduct 1 billion amps without a much bigger voltage. I'd actually go for the watts being the best measure of danger (but you have to take into account how many watts you will dissipate, not how many are available). I can draw 5kW off a decent car battery into an invertor to power mains appliances, but if I touch the two terminals, I feel nothing, and I certainly don't dissiapte 5kW!

They are mainly to stop a fire from a short. The safety circuit breaker is the earth leakage one.

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that do?

Ah, required to pass the tests to be able to sell it, but of no concern to the end user.

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Err, your car has one totally uninsulated conductor carrying many hundreds of amps when you start it. Perhaps you need to avoid driving...

Can you give a reference for this?

It only takes milliamps to kill you under the right conditions. That's why RCDs are set as they are.

Not many homes have high voltage DC supplies.

Electricity doesn't spill out when a cable breaks. ;-)

All of which is irrelevant when considering potentially lethal current.

They are very different.

You're talking nonsense.

More nonsense.

Only in terms of a fire risk.

And you can't have that current flow without the voltage.

You seem to be technical terms around without understanding their meaning.

Have you actually read it?

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

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.

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?

Sounds too good to be true.

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--  Whiskers 
-- ~~~~~~~~~~

[...]

You can touch the 12V battery terminals reasonably safely, but don't try it with the 240V terminals. That voltage is well in excess of the body's

50V limit for being a non-conductor, and the battery can deliver plenty of Amps to be lethal. It isn't the 240V that kill, it's the 21A.

Turning fresh meat into smoke and ash can dissipate a great many Watts.

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--  Whiskers 
-- ~~~~~~~~~~

Sigh. You don't need 'vast amounts of current to kill'. That's why RCDs are set at 30 mA. Less than that is *generally* safe.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Makes you wonder why they have neutral in households as ground potential= .

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A tazer limits the current to a lot less than your body could conduct at that voltage. A 110 volt power line and a 1000 volt power line both have more currrent than your body will draw, so the limit is your resistance. And 1000 volts will give you more current for the same resistance.

I think it's a pretty low voltage, hence the very thick cables! From memory it's something like 12 volts at 500 amps. And no it's not dangerous to touch, from an electrical point of view - the heating when welding is in progress might be a tad sore though.

Why?

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Do you know what a Jewish dilemma is?
Free ham.

See:

and scroll down to "Lethality of a shock".

From the table there it is apparent that passing a current anything over about 0.1A is quite likely to be fatal, so it makes little difference whether the voltage source is capable of supplying 1A or 1000A.

The lethality of any voltage source that can supply more than about 100mA will depend on there being sufficient voltage to pass lethal current through the body according to Ohms law (V/R=I).

Typical estimates for human body resistance:

Unbroken dry skin = about 100,000 - 500,000 Ohms Unbroken wet skin= About 1000 Ohms Broken skin= As low as 100 Ohms

The higher the voltage, the greater the likelihood that localised burning will break the skin resulting in a rapid drop in resistance with consequent increase in current flow.

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.

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.

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

This is unmitigated nonsense. You clearly have little understanding of how the terms that you are bandying about, relate to one another, and more importantly, to current flow within the human body. I don't mean to be offensive, but there's a good old phrase which goes something like -

"When the hole that you're digging gets too deep to climb out of, stop ... "

'Nuff said, I think.

Arfa