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Author Topic: Safety of 120V vs 240V  (Read 6379 times)

Jonathan Johnson

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Re: Safety of 120V vs 240V
« Reply #10 on: June 15, 2016, 01:24:43 pm »

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Mike Sokol

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Re: Safety of 120V vs 240V
« Reply #11 on: June 16, 2016, 07:37:08 am »

Of course, if you are working with a 277/480 system, then you are looking at 277 volts to ground- but that would be uncommon for most people.
Steve is 100% correct. It's almost impossible for a consumer to get a line-to-line shock in the USA, so most shocks are limted to 120 volts. However, in industrial situations with 208/120-volt high-leg delta or 480/277 -volt Wye distribution, it's pretty easy to get between 208 or 277 volts to ground. And JR is correct that 2x the voltage equals 2x the current. And at these relatively low voltages, current is what causes heart fibrillation problems.

As far as the physiological problems from shock, at somewhere around 20 mA of current, the muscles in your hand that make a fist are stronger than the ones that open up your fingers. That 20 mA is enough to overwhelm your nervous system, making ALL of your muscles contract. So there's a tug of war happening with your muscles, and the ones that make a fist win. So when you grab onto something like an aluminum ladder that's electrified, you can't let go. Secondly, it appears that 60 Hz is just the right frequency to interrupt your hearth rhythm somewhere around 30 mA across your chest cavity. Of course, your mileage will vary depending on age, heart condition, etc... But if you figure that the human body (meat puppet) has a hand-to-hand resistance somewhere around 1,000 to 1,500 ohms, at 120 volts you can easily get 100 mA fault current though your chest cavity (and your heart) with a hand-to-hand or hand-to-foot shock. The last part of the equation is that your skin acts like a non-linear resistor. Once you get somewhere around 60 volts or so, the voltage will punch through the surface of your skin and more current will flow. So 120-volts produces way more than 10 times the current of a 12-volt shock. That is, at low voltages dry skin is relatively high in resistance so you don't feel a shock when touching a 9-volt battery. But touch it to your wet tongue and you'll get a real wake up shock.
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frank kayser

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Re: Safety of 120V vs 240V
« Reply #12 on: June 16, 2016, 12:19:35 pm »

<snip>
 Secondly, it appears that 60 Hz is just the right frequency to interrupt your hearth rhythm somewhere around 30 mA across your chest cavity.
<snip>
For our friends elsewhere in the world where 50 Hz is the norm, is that 30 mA threshold any different, or is it comme ci, comme ca?
frank
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Mike Sokol

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Re: Safety of 120V vs 240V
« Reply #13 on: June 16, 2016, 01:19:07 pm »

For our friends elsewhere in the world where 50 Hz is the norm, is that 30 mA threshold any different, or is it comme ci, comme ca?
frank
I've often wondered that, but there's not enough data out there for a comparison. And the 30 mA threshold is time dependent, so a few cycles of 60 Hz at 30 mA may not cause your heart to go into fibrillation. But a few minutes of it would probably be life threatening.

What bothers me the most is that in many cases of home or recreational electrocution, someone witnessed the shock, but yet the victims still died. Just calling 911 and alerting the emergency responders, then starting compression only CPR immediately would would probably save some lives.
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John Roberts {JR}

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Re: Safety of 120V vs 240V
« Reply #14 on: June 16, 2016, 01:57:34 pm »

I've often wondered that, but there's not enough data out there for a comparison. And the 30 mA threshold is time dependent, so a few cycles of 60 Hz at 30 mA may not cause your heart to go into fibrillation. But a few minutes of it would probably be life threatening.

What bothers me the most is that in many cases of home or recreational electrocution, someone witnessed the shock, but yet the victims still died. Just calling 911 and alerting the emergency responders, then starting compression only CPR immediately would would probably save some lives.
Thresholds can vary widely from person to person (and situation, like how sweaty etc). I am not aware of any significant difference between 50Hz and 60Hz AC, but AC and DC are likely different.

 The scary thing about electrical shock events, is you can get stuck to the same electrical fault, trying to save a friend. Think a few seconds before just grabbing somebody in obvious distress.

JR
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Mike Sokol

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Re: Safety of 120V vs 240V
« Reply #15 on: June 16, 2016, 02:38:08 pm »

The scary thing about electrical shock events, is you can get stuck to the same electrical fault, trying to save a friend. Think a few seconds before just grabbing somebody in obvious distress.
Best practice is to turn off the circuit breaker first, THEN get the victim clear of the fault. However, if that's not possible, then use a piece of dry wood if it's available to knock the person clear of the electrical fault.

Possibly the worst case of multiple electrocutions was at a ditch with an irrigation pump that wasn't properly grounded. A woman went into the ditch to rescue a dog that had been shocked, she was electrocuted. A few hours after she was reported missing a man found her body and went into the ditch to rescue her, and HE was electrocuted. Then a second man tried to rescue the first two victims, and HE was electrocuted. So, three people were killed after the first one tried to rescue the dog without turning off the power first. https://www.youtube.com/watch?v=RDs8a_ym3Zc
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David Buckley

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Re: Safety of 120V vs 240V
« Reply #16 on: June 16, 2016, 04:53:30 pm »

For our friends elsewhere in the world where 50 Hz is the norm, is that 30 mA threshold any different, or is it comme ci, comme ca?

No idea, but there used to be a published statement in the UK that no-one had ever died of electric shock from a source being protected by a 30mA RCD.  Japan would be the place to ask, as they have 50Hz on one side, and 60Hz on the other.

Here in New Zealand, we have the 30mA RCDs, except in patient treatment areas, 10mA RCDs are required.  I think 30mA has been settled on internationally in 220/230/240-ville.  And for some years now, its been mandatory to have home socket outlets RCD protected.  There's loads of old houses, of course, so its far from universal, but if more sockets are added in a house, then RCD protection has to be fitted to the feeds to the new stuff, which generally protects (some of the) old.
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Re: Safety of 120V vs 240V
« Reply #16 on: June 16, 2016, 04:53:30 pm »


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