Electrical shock?? 8

[MichaelkSA]

Good day people.

My first post here.

I was looking for a general electrics thread but couldnt find anything, so I hope you guys dont mind helping.

What I would like to know is the following:

If I accidentally touch a live wire and am not insulated from ground, what would the current flowing through me be, and would it make a difference if there is current flowing through the wire at the time (eg if the motor is off at the time or is drawing say 10A)? Would that affect the severity of the shock?

And secondly, why is it that 3-phase power is more dangerous than single?

3-phase line voltage is much higher yes, so I understand that if I touch two 3-phase live wires I will get a much worse shock, but even with 3-phase the live-ground voltage is exactly the same as single phase (220V in my country). Would I get a worse shock? And if yes, why?

Thanks for any replies.

 

[MikeHalloran]

>>>If I accidentally touch a live wire and am not insulated from ground, what would the current flowing through me be, ...<<<
It's impossible to predict, because of the number and nature of impedances in the circuit you form. Usually the soles of your shoes protect you, but not always.

>>>... and would it make a difference if there is current flowing through the wire at the time ...<<<
No.



Mike Halloran
Pembroke Pines, FL, USA

 

[DRWeig]

>>>And secondly, why is it that 3-phase power is more dangerous than single?

3-phase line voltage is much higher yes, so I understand that if I touch two 3-phase live wires I will get a much worse shock, but even with 3-phase the live-ground voltage is exactly the same as single phase (220V in my country). Would I get a worse shock? And if yes, why?<<<

Line-ground 220V is Line-ground 220V. Shock is no worse. Three phase same as single phase.

Do not try to measure this.




Best to you,

Goober Dave

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[itsmoked]

Voltage is voltage. Number of phases doesn't matter. Motor running or not, doesn't matter. Existing current in the circuit doesn't matter as it takes so little to kill someone, as to be insignificant.

As for severity it's all about the insulation involved and the specific path through the body that the current travels through. Dry human skin has a resistance of about 10,000ohms. So at a minimum you're probably talking about 20,000ohms. With 120Vac as the source the peak current could be as little as Vpeak = √2 x 120 or 170V/20,000 = 0.0085Amps

Since 0.005A across the heart is the generally accepted level to cause death you can see where you are. It's tenuous. Add shoes and you're probably way below 5mA. Run the current between hands and you're back to seriously dangerous. Add water.. Add sweat. Raise the voltage..

A favorite peeve of mine is hearing people say, "Every time I touch the 'fill in device' I get a shock". These people are fools. Why? because all they need to do is change up one aspect like wet carpet, or wet shoes, or no shoes, a wet hand, or more pressure somewhere, and they easily pile on several more milliamps and a shock transitions to an electrocution.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikekilroy]

as well stated so far. If you want to know more, MEASURE YOURSELF. Use an ohmeter and measure your resistance to see how you stack up to the norm. Ohms law is in effect here: 220vac means 325v peak as Itsmoked showed, so 325v/YOUR_OHMS= current flow....

 

[jraef]

I have been shocked by 480V; in through my thumb on a screwdriver, out my back into an expanded metal grate I was laying on, so right across my heart (as the crow flies anyway). I survived, but only by a miracle. I do not recommend it... (And for the purists, yes it WAS 480V not 277V because it was an ungrounded delta system).

To elaborate a little on the "if there is current flowing through the wire at the time (eg if the motor is off at the time or is drawing say 10A)?"

As was said if there is current flowing, the wire is live so it would not make a difference what else was connected and you can be killed. If a motor is "off" and you grab the motor lead, DOWN STREAM of whatever is rendering it "off", then there would likely be no voltage, so no shock. But that is a horrible idea to test unless you can ENSURE that whatever is keeping that circuit open will REMAIN open. Here in North America, we call that "Lock-Out/Tag-Out" because you open a disconnect switch, test that it is really open, then place YOUR own lock on the handle so that the disconnect cannot be closed while you are working on the circuit, and a tag on the lock with the date and time telling anyone that you are working there (in case they feel inclined to cut your lock off).

One related aspect of 3 phase that MIGHT make it seem more dangerous is that if you have a 3 phase motor running, and you open only 2 of the 3 phases, the motor will stop and someone may BELIEVE that the circuit is safe. But that 3rd phase will still be live and lethal.

"Will work for (the memory of) salami"

 

[MichaelkSA]

Thank you all. All your replies were very helpfull.

I think the reason why there is a stigma around the danger of 3-phase is simply because most accidents and deaths happen in industrial applications and 3-phase is the norm in industrial applications.

Also in factories there are much more wires lying around wanting to be touched by an uninformed person, where in say, housing, where single phase is the norm, lots more effort is done to take all wires out of site.

 

[sibeen]

I'd better throw in a supplementary question, as I must admit I'd never thought about it before.

The 5 mA 'rule of thumb'. Is it peak, or is it RMS? The average heart rate is about 80 beats per minute. Would that 1.414 really make a difference?

I suspect that getting approval for a double blind medical experiment may be a tad problematic.

As an aside, my worst ever encounter was placing my forearms across 415V P-P busbars many moons ago. Completely my fault. I was hungover; I'd "isolated" the machine, and forgot it was dual fed. Threw me back and only a nice sturdy concrete wall stopped me going back even further. I was lucky, I was about 30 and as fit as a mallee bull at the time. That was well over twenty years ago now, and I suspect the outcome may be slightly different if I tried it again.

 

[electricpete]

When people give ac voltages they typically mean rms.

I know this doesn't apply to anyone in the thread, but I have to mention anyway - It gets me worried when we talk about sharpening pencils to factor of 2 or doing measurement of our body resistance as if to say we may reach a conclusion about what is dangerous through these calculations. Follow standard safety procedures, not homemade calculations.


=====================================
(2B)+(2B)' ?

 

[waross]

An interesting note on body resistance once the skin is broken the body resistance drops dramatically. I don't remember the figure, but I came across it a few decades ago when designing some some wiring for an operating theatre. This was for a tiny hospital in the jungles of the Moskito Coast. (Named after the Moskito Indians, not the insect.)
If a shock is the result of an incident where the skin is punctured, BEWARE.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

To follow up on Bill's comment, time is a factor as well. Current passing through a body will ionize stuff in its path, thus lowering the resistance, thus increasing current, and so on. The longer you hang on to that wire, the more damage is done but it's exponential not linear.

Best to you,

Goober Dave

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[cranky108]

I think 480 is one of the worst voltages to get into because it is at the peak of the voltages that won't allow you to let go (Go adead and argue that 415, or 600 is worst, it makes little difference if you are dead). At higher voltages there is more of a factor that you will be throwen, and therefor out of the danger zone. Also at 480 an arc is less likely to self disconnect than that of higher or lower voltages, which is why it is more dangrous. So drop a wrench into a 480 bus and it will not only vaperise, whith an gas expansion factor simular to TNT, but that the arc-plasma won't disipate fast enough to allow the arc to go out.

jraef, how did you get phase to phase voltage on a delta system if you were the only fault?

Also on higher voltages you don't need to be connected physically to ground to become electricuted. The capactance of your body to ground will conduct enough current to electricute a person (notice birds are much smaller than us, before anyone asks).

 

[electricpete]

In addition to hazard from direct contact, there is hazard from arc flash. This tends to be higher at higher voltage levels and higher source strengths.

So for example while inadvertantly shorting across 120volt might typically cause some sparks (*), shorting across 480volt might cause a flash or blast.

(*I'm not saying it's safe to short across 120volt either).


=====================================
(2B)+(2B)' ?

 

[cranky108]

Isen't that how those carbon arc lights work? Just a big arc between carbon rods. Makes a bunch of light.

 

[ScottyUK]

Don't see many arc lights in the multi-megawatt range though...

 

[cranky108]

They were replaced with radar (or microwave) after WWII. The last carbon-arc lamp I saw was being used to sell cars at some end-of-year sale.

In fact I haven't seen very many large loads, just gobs of smaller load located in one place. Server farms, lights, pumps, and a few chillers. No large compressors, pumps, etc.

 

[waross]

On a large (lots of energized branch circuits in grounded raceways) delta system the system capacitance charging current will generally peg the voltage to ground at 277 Volts. However there is no guarantee. The voltage may be anywhere up to 480 Volts.
I once got 600 Volts phase to phase between my thumb and finger. (LOTO error due to a faulty meter.) It burned a small hole in my thumb and a matching hole in my finger. My choice of words WAS NOT politically correct.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

Agreed that delta systems are difficult to work on. And for longer delta circuts it is possible to ignite dried vegestation from the capactance/arcing to ground, and still not be able to detect you have a fault.
Although I am taking about a 7 kV delta system, it can still apply to 480 V, but to a lesser extent.

Politically correct? I would have said something R rated or something about our creater, but nothing political.

 

[jraef]

I guess now, after all these years, I have to admit to not really thinking through the voltage potential I receive through my Engineer's eyes. I was an Electrician at the time, so I was taught that in a floating ground delta system, the phase to ground was potentially* the line voltage, so I always assumed it actually was. After finishing school, I guess I never revisited that story line to think about adjusting it...

*Side note: Would that be the potential potential? Then if so, wouldn't it be the potential²? Then if so, would that make it 230.4kV? Inquiring minds want to know!



"Will work for (the memory of) salami"

 

[cranky108]

The good thing about a delta system is the first fault should not cause much damage, unless you start the forrest on fire. The bad thing about a delta system is that you don't know where the fault is.

In other countries they use delta secondaries, but they don't look for ground faults. So if you have a ground in a light for example, you may only get a shock in the shower if that light is on. Or it could be a ground in the house next door. They clame the delta system is safer, but with the reduced insulation they use it may not be so.

jraef, don't worry I can't read the voltage by toughing the wire either.