Nigel Goodwin said:
Under very rare conditions perhaps, but with DC it's almost certain to freeze you one the wire and kill you! - I'd sooner take the 99% chance of been thrown free, than the 99% chance of death!.
120VDC is unlikely to kill you 120VAC is pretty likely to kill you, 60VDC probably won't even shock you, 60VAC might even kill you.
The idea the AC makes you let go and DC makes you freeze is a complete myth. My mum's neighbour was nearly electrocuted after using a faulty drill and not being able to let go; it was only her turning off the poer that saved his life. I don't know how many anecdotes of people freezing up when being shocked by AC currrent I've heard but here's one I read recently.
The first occurred when I was eight years old. My brother was trying to learn about electrical circuits by wiring bells and lights in various circuits and then energizing them with 115 V ac. The circuit was on a large round oak table in our dining room and power came through two long wires from the chandelier above the table and ending in non-insulated alligator clips.
The thing I did not notice was that my brother always put on rubber gloves before picking up the alligator clips. After showing me what fun it was, he cautioned me not to touch anything when he left the room. I immediately picked up the clips to try my hand with the lights and bells. The first shock involuntarily caused a spasm in my hands that would not allow me to let go of the electrodes. 115Vac went from one hand through my arm, upper torso, arm, and out the other hand. Unable to let go and shouting at the top of my lungs, I made two quick involuntary circles of the table and was then thrown deep into the fire place where I was pressed between the back wall and grate -- still unable to let go or even move in my tight fire-place-cage. It was probably less than 15 seconds before my brother responded to my yelling and pulled the wires out of the chandelier, but it seemed to me more like a lifetime.
**broken link removed**
I'm sure a multi-KW switchmode power supply, fed from 300 or 4 thousand volts, is going to give similar reliability to a simple transformer!
That's the only valid point you've made so far.
Where's my databook?, I seem to have missed thouse 500KV 1000A MOSFET's
Why would you need transistors with such a high rating?
For a start IGBTs and SCRs are used for high voltage not MOSFETs.
Apart from safety, the main advantage I can see is that it would mean that you wouldn't have to worry about harmonics or power factor correction when designing a switch mode power supply for something like a PC.
and what's the source of the DC going to be?.
Also, I'm only talking about low voltage supplies, medium voltage will still be AC. Low voltage DC can be derived using a phase shifting transformers and rectifiers, no capacitors or regulators are required. The three phase supply is converted to a 24 phase supply which gives vertually no ripple on the output and the input harmonic currents are very low.
Pommie said:
While we're on the subject, which is better, earthing the neutral or not? (don't know what the US call it - the one that isn't hot)
Bonding the neutral to earth is normally done for the following reasons:
- If a an earth fault occurs on any live conductor, the other lives will float at 400V.
- If you're powering a device that uses an autotransformer to get a high voltage (e.g a neon sign transformer which runs at 20kV) and the live output developed an earth fault the neutral will float at 20kV which would damage the insulation in the distribution system and cause a fire.
- There's no earth path for high voltages induced from lightning strikes.
The only advantage of not earth bonding the neutral is that you can't recieve a shock by only touching one live conductor.
You might think that earth bonding the neutral increased the risk of shock but it doesn't if an RCD is used and all live connections are suitably insulated from the user.
Non-earth-bonded supplies are normally used in places like hospitals where the tripping of an RCD could be life threatening as it might cut power to a life support system. In such case a single phase isolation transformer is used and a device to warn the staff of an earth fault is installed.
In some cases isolation transformers are used to power tools on a building site but they're generally only small and not used for powering more than a couple of tools from. Normally 110V AC is used on construction sites and the centre tap of the transformer is earth-bonded giving 55-0-55V.