[prateeksikka]

HI ALL!
YOU MUST BE KNOWING THAT ELECTRIC POWER IS TRANSMITTED AT HIGH VOLTAGE AND CURRENT IS KEPT LOW.REASON IN SUPPORT OF THIS IS THAT HIGH CURRENT GIVES MORE I*I*R LOSS IN POWER.I WISH TO KNOW THAT WHY WONT HIGH
V*V/R LOSS BE SIGNIFICANT HERE WHICH IS ALSO A FORMULA FOR POWER LOSS AND REMEMBER V IS IN kVs(KILO VOLTS)? ???
PRATEEK

 

[Alun]

Because V2/R losses are unaffected by the cable resistance, but more by the insulation between the cables, and since the distance between each neighbouring conductor is so large the insulation loss is not a big factor.

ps prateeksikka,
Please don't use BLOCK CAPITALS it makes your posts harder to read and it looks like YOU'RE SHOUTING. ;D

 

[prateeksikka]

hi alun!
i apologize for using block letters. believe me i am not shouting  and i am as soft as a cotton ;D.
can u tell me when a person gets killed by a shock, is it the high voltage which kills the person or high current.A book in physics by resnick and halliday which i studied in school shows a line man repairing a high voltage wire without switching off the power sitting in a helicopter?
hows that?

 

[Alun]

prateeksikka,

You need a high voltage to penetrate the skin, but you need current to kill injure or shock.

For example a static shock from a car on a hot day migh be >25kV but the current is far to low to kill but you'll feel a short sharp shock.

If you're connected to 240VAC the voltage might be lower but the current will be a lot higher - high enough to kill.

Frequency is also important, with DC your mussels will lock up and you'll stick to the power, with 50Hz AC the mussel spasms will shake you and might push you away. In either case the electric current will upset your caridac rythm and possible stop your heart and then you're dead for sure. Higher frequencyies above 10kHz or so are less of a risk because they're above the frequency response of your nerves. The skin effect also diverts the currents away from your internal organs but they still cause heating of your body tissue which can cause severe burns and possibly cook you alive.

The man could repair the power line because he's isn't earthed and he'll be also wearing a special conductive suit that diverts the current around his body this is called the faraday cage effect.

 

[prateeksikka]

hi alun!
yes in continuation with that you might have noticee it with a 6V battery ,even if you short it with a wire u see a slight spark or somewhat heating effect and tingling sensation if you touch it to your sensative part like the tongue, in other words it is insufficient to kill you.while the current may be as high as 6A(if the resistance is 1ohm).
Agreed!!!!!!!~!!!!!!!
But the car example you gave,do we really have 25kV inside the car?
why would it not kill a person?dont you think that the person who touches 25kV will
draw a current depending upon his body resistance and will be torn to ashes?
25kV/1M ohm=25mA sufficient to kill!!!no? :
prateek

 

[Alun]

This resistance of the human body is just 3k ohms but the skin has a much higher resistance so you're correct that holding a pair of multimeter probes you'll measure about 1M. But this is at lo voltages, the resistance of the skin goes down with voltage so at 25kV the overall resistance might be <25K which would draw 1A and kill you for sure. Having said that often high voltage sources have a very high internal impedance and this normally limits the current to a non-lethal value.

Pulse lengh and duty cycle are also a factor, if the spark in a car ignition only lasts for 2ms and the current is limited to 30mA by the impedance of the ignition then you'll recieve a nasty shock though it won't be fatal.

 

[EdwardM]

Hi Folks

I just checked the Manchester University, Dept. of Chemistry site and they say:

Voltages in excess of 50 V can be lethal

Which confirms what I was taught many years ago when at Radio college.

Ed

 

[Alun]

I've heard that before, and I agree I wouldn't play with voltages >50V I would always turn the power off first, unless the power supply is high impedance of course.

For AC I'd 35V RMS as the peak voltage is much higher.

How as with lots of things there's not threshold, you can't say with 49.9V you'll be ok and 50V you're dead, it depends on the moisture of your skin and it you have any cuts also your general health. I would imagine 50V was chosen because there's a big safety margin it's actually uncommon for mains shocks to cause death I've had a few myself.

On a hot and humid summer's day I've been playing with voltages as low as 28V and I've been able to feel a tingeing sensation on my fingers. This worst shock I've ever had is from a microwave transformer and fortunately I wasn't earthed  because if had been earthed well enough I would've been surverly injured or even killed.

 

[EdwardM]

Hi Aun

For AC I'd 35V RMS as the peak voltage is much higher

The site mentions 40 something volts ac, being a uni site means there's probably some integral calculus going on ;D

Just to add to what you've said, I've never had a bad shock (well 1, more later) and that's because I had a good teacher - he advised that I keep one hand in my pocket when doing anything where there was a possibility of high voltage being present.

The only painful one was a bit odd: I'd spent some time trying to solve a power supply problem in a satellite receiver out in my garage workshop but left it as I had to move on to other things.  Anyway, about a month later I was at a loose end and decided to have another look at said receiver.  You can probably guess what happened next...

I picked up the circuit board with both hands (it wasn't connected to anything) and jumped a foot or two and probably said something mild like oops.  The reservoir capacitor had obviously retained a *lot* of its charge from the previous power-up.  Ever since I've been even more careful, even when something hasn't been plugged-in for months!

Ed

 

[audioguru2]

There are also limits for PA levels being distributed in open ceiling spaces without conduit. In Canada in schools it is 25VRMS, offices 70.7V but ordinary telephones use 90V for ringing. In Japan the PA voltage limit is 100V.
I found an intermittert PA problem one time by turning off the lights and seeing the spark. The very poor joint in the cable was done by a licenced electrician! ;D

 

[EdwardM]

Hi audioguru

here in the UK we've recently had a law passed that only licensed electricians can work on many types of home electrics.  In all the houses I've had over the years, the only wiring problems I've found are those caused by licensed electricians!

AAAAArgh  :

Ed

 

[Alun]

I don't care, it's not going to stop me doing the odd wireing job, after all there's no proof whether's it's been done 10 years ago or last week, so you'll be very unlucky if you're caught.

 

[EdwardM]

Hi Alun

couldn't agree more, the loonies are taking over the asylum

Ed

 

[audioguru2]

couldn't agree more, the loonies are taking over the asylum

In all the houses I've had over the years, the only wiring problems I've found are those caused by licensed electricians!

Years ago, I had a house with aluminum wiring that was approved by the loonies in my country's government. Big thick wires that didn't fit the screws on receptacles, a maze of loose wires joined by the biggest twist-on wire connector that I've ever seen and burnt-off insulation at every joint because aluminum "moves" very slowly.
Of course, the house was wired by licensed electricians. ;D

 

[prateeksikka]

hi alun!
you are again and again talking about the source impedance in supplying lethal current.do u think a 5V battery open cicuit without internal impedance would be
sufficient to kill a person? ;D
how much min voltage source can kill a person if no resistance is b/w the body and source?
:-\

 

[audioguru2]

Every voltage source has a source impedance. Even a brand new little 9V alkaline battery might burn you to death if you short it and place it near a vital organ in your pocket. The short won't get hot but the battery certainly will.
The opposite will occur with a powerful car battery. A short will certainly get very hot (don't try it, the battery might explode).
The important source impedance causes a voltage-divider with the load. It also limits the current. ;D

 

[Alun]

hi alun!
you are again and again talking about the source impedance in supplying lethal current.do u think a 5V battery open cicuit without internal impedance would be
sufficient to kill a person? ;D
how much min voltage source can kill a person if no resistance is b/w the body and source?
:-\

Apart form short circuits and heating as audioguru sais, low voltages present little or no risk because the voltage is too low to break down the skin's insulation and cause a harmful level of current to flow.

 

[prateeksikka]

hi there!
basically we conclude it is neither voltage nor current but the power which kills the person
;D

 

[Alun]

No prateeksikka you need both current and voltage.

You need a high enough voltage souce to break down the insulation of the skin and it's impedance needs to be low enough to allow suffiecnt current to flow to kill someone.

The current needs to be DC or low frequency AC to have the worst effect, higher frequencies >10kHz burn but don't shock as they're above the frequency response of your nerves.