Could be and that explains the higher leakage current associated with the higher frequency.

 

Thanks for this information! Always nice to learn something new!
Although nowadays this interconnection is made with thyristor frequency converters...no moving parts. This is done at the Itaipu hydro-electric station on the border of Paraguay (50Hz) and Brazil (60 Hz). 18 turbines of 700 MW each! Impressive!

 

Sound advice.

I was actually curious because I can see the advantages of both systems.

Mike.

 

I mostly agree with Nigel. Because AC alternates through zero, any loose connection will be much safer when it carries AC rather than DC. Once DC starts to arc, it just gets worse and will burn your house down quickly!

As far as "feasible distribution" is concerned, it is cheaper to distribute DC over long distances (only 2 conductors) than AC (3 conductors). DC is transmitted via underwater cable between Britain and France, also between Mozambique and South Africa and probably in some other places too. (Positive and Negative with physical conductors and a common Ground at midpoint) - (using the soil with grounding/earthing mats at both ends of the transmission system).

 

I fully agree with your statement as I know from experience. A collegue of mine was outside installing a car radio standing in a puddle of water. When he turned on the drill he immediately froze to the position he had. He was thrown free when I pulled the plug off the socket. Although the time of contact was very short he needed intense medical care to recover from the shock.

 

Don't you guys know how to spell in English???
A mussel is a hard black shell fish.
A muscle moves your arms, legs and other parts.

__________________
Uncle $crooge

 

Maybe a return to the old "knob and tube" wiring method?

__________________
Inside every little problem, is a big problem trying to get out.

 

... ruins my English knowledge too.

 

The IEEE has a good article about Rotary Frequency Changers. These were used in the early days of electricity generation when mains (utility) frequencies were not standardised at 50 or 60 Hz.



Low frequencies were favoured for motors/traction (16⅔Hz is still used on some European railways, 25Hz is still used in some places in the US*). Higher frequencies were used for lighting (133⅓Hz was popular). It was the introduction of the induction motor that led to the final choice of 50 or 60 Hz which was also a compromise between the above 2 conflicting requirements.
The cinema industry had determined that 48Hz was the minimum frequency to avoid perception of flicker (thats why 16fps was popular for silent films: 16x3=48. A higher speed was needed to record sound on film and 24fps was chosen: 24x2=48).
London was supposed to have had 10 frequencies and over 20 different voltages in the 1910s. Coventry used 87Hz then 50Hz single phase followed by 2 phase before they settled on 3 phase.
It's not very clear why some places chose 50 or 60 Hz (Both are used in Japan: 60Hz in the west and 50Hz in the east). Plenty of rumours and urban myths abound.

The USA standardised on 60Hz and rotary frequency converters were used to convert and synchronise between frequencies. The original design for Boulder dam did include some turbines for California which untill 1948 (approx) used 50Hz.


DC transmission can now be economic where there is a long distance and/or the connected networks are not synchronised. For a given breakdown voltage, √2 more DC power can be sent compared to AC. DC transmission is used is between UK and France and will be used between UK and the Netherlands.



*New York Times "4,400 Manhattan buildings use D.C. power"

 

Has anyone tried connecting their nutts to a high voltage/power source?

 

I suspect you could find some fools on youtube that have tried it

__________________
Measurement changes behavior

 

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.

http://www.smpstech.com/mtblog/power_supply_safety.html

That's the only valid point you've made so far.

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.

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.

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.

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I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

Certainly not, I personally know of many instances were it has applied. Basing your premise on one single incident hardly makes it a myth.

As for what voltages kill people?, I would refer you to the original arguments of Edison and Westinghouse - but certainly you have to work pretty hard to get killed by 240V AC.

Oh, a quick tip for anyone who doesn't know?, never touch any exposed metalwork with the palm side of your hand - stroke it with the back of your fingers - if it's live it will feel rough.

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It is a myth.

Both DC and AC can cause your mussels to lock up and make you stick to the power supply and both AC and DC can throw you free, as your mussels contract. The myth that AC throws you free is based around the theory that the zero corssing period gives you time to break free but they only lasts for a few miliseconds and it takes much longer for your mussels to relax. There might be a grain of truth in this as the DC 'can't let go current' is slightly lower than AC but that's offset by the fact that the peak AC voltage is 1.414 times higher than DC and that ventricular fibrillation can occur at one fith of the current than a DC supply. This is why 25VAC 50Hz is widely regarded to be as dangerous as 50VDC.

Try it yourself, connect an isolation transformer with a 55V secondary to a variac, hold either side of the secondary in each hand and get a friend to crank up the votage until you get shocked. Now connect a bridge rectifier and capacitor to the output and repeat the experiment. You'll notice that on DC you probably won't feel a shock until about 50V and on AC you'll get shocked at a surprisingly low voltage. Just make sure you trust the person at the variac with your life.

I've had a worse shock from a 30VAC transformer than I've ever had from 48V vehichle supplies at work which can often reach 56V.

I wouldn't rely on this, the skin on the back of the hand is normally dry and might have a resistance of >1Mhm: and the skin on the palm is invariably moist and will have a much lower resistance. It's quite possible that you wouldn't even be shocked if you touched something with the back of your hand, but will be electrocuted if you grasped it in the plam of your hand.

Better still, wear rubber gloves but don't rely on them for total protection and you'll never be shocked in the first place. If you want to check if something's live, then connect your multimeter between it and a known good earth.

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I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.