Question about dangerous voltages? - Page 1
Forum section: electronics / beginners

author: H2Woah
author_rank: Newbie
timestamp: December 22, 2013, 08:10:44 pm
content: After following along with this online textbook (http://www.allaboutcircuits.com/vol_1/chpt_3/4.html) I was reading up on how to calculate the maximum safe voltage to be shocked with. I don't think I'm understanding it correctly.Using Ohms law, the page basically says that V = (8mA)(1Mohm) , so the voltage required for AC pain is 8000 volts. (Assuming your hands are dry and such)Does this mean that if you were to touch the positive and negative terminals of a (lets say 110v) 60hz wall outlet with each hand that you would only fell pain at the very worst? This doesnt seem right to me...I'm sure I have a logic error somewhere in my understanding haha. If anyone could help me out, that would be great. Thanks!

author: c4757p
author_rank: Super Contributor
timestamp: December 22, 2013, 08:24:21 pm
content: 1) The resistance of your skin is highly variable. 1Meg is a really high estimate.2) High voltage can break down the little bit of insulation provided by the outer layers of skin, suddenly dropping the resistance significantly.Your resistance can be anywhere from half a kilohm to megohms. Basically, about half the whole range of resistors that you can buy.My rule of thumb is this: If it's over 30V, don't touch it on purpose. If it's over 45-50V, take care to completely avoid touching it.

author: Unknown
author_rank: Unknown
timestamp: December 22, 2013, 08:34:38 pm
content: The 1 Mohm figure might be accurate for dry skin at the ~ 1 V a DMM uses to measure resistance, but will be much lower for higher voltages that break down the skin. I find this a stupid way to illustrate the application of Ohm's law. The calculations don't provide just rough estimates, they are off by orders of magnitude. If you want experimental evidence, I'm sure you can find some videos of people climbing on high voltage lines.This Wikipedia pageprovide some data of how the resistance of the human body varies with voltage.

author: johnh
author_rank: Regular Contributor
timestamp: December 22, 2013, 09:14:49 pm
content: Yep. I have had a tingle from 50v Dc.

author: Bored@Work
author_rank: Super Contributor
timestamp: December 22, 2013, 09:49:21 pm
content: Quote from: H2Woah on December 22, 2013, 08:10:44 pmI would drop that online book ASAP! Not only because of the silliness you are dealing with in that particular chapter. But also because the original author felt the need to vent his spleen and used electron flow instead of conventional current direction. Just because he didn't get conventional current direction and thought it is shit.

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2013, 10:10:03 pm
content: It depends on the current. Low frequency AC gives the most dangerous shock, followed by DC. High frequency AC (above 20kHz) doesn't shock because it's above the frequency response of the nervous system but it causes nasty, deep RF burns, similar to being microwaved.

author: hikariuk
author_rank: Unknown
timestamp: December 22, 2013, 10:51:17 pm
content: Quote from: Hero999 on December 22, 2013, 10:10:03 pmWhich is something people who sit on Tesla coils should really think about, I suspect.

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2013, 11:25:11 pm
content: Sitting on the coil with a metal cage or woven metal clothing is fine, it's only when the energy goes through your body and is concentrated to a tiny point (entry/exit) that you get burns.With the metal around your body the spark jump to/from that, since the metal is the path of least resistance compared to your body.

author: andtfoot
author_rank: Unknown
timestamp: December 23, 2013, 12:19:46 am
content: Quote from: Psi on December 22, 2013, 11:25:11 pmI have a question about that actually.I hear of electricity taking the path of least resistance a lot, but basic circuit theory suggests to me that the electricity would take all paths at equal voltage and that the current through each path would be determined by its resistance. Am I missing something really obvious?

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2013, 12:24:42 am
content: Electricity does flow through all paths but most of it flows in the lowest resistance nodes of the circuit: connect a 1R resistor in parallel with a 99R resistor and 99% of the current flows through the 1R resistor. The same is true for a human with a resistance of 100k, wrapped in a metal cage with a resistance of under 1R.

author: andtfoot
author_rank: Unknown
timestamp: December 23, 2013, 12:33:31 am
content: Quote from: Hero999 on December 23, 2013, 12:24:42 amSo that is reliant on there being an overall current limit for the source?If not, then theoretically the 99R resistor that is in parallel with the 1R resistor would get the same amount of current as the 99R by itself. There would just be just be more overall current drawn from the source with the parallel resistor.

author: Psi
author_rank: Super Contributor
timestamp: December 23, 2013, 12:50:16 am
content: yep, in order to have a large current flowing through the 99R you have to have a massive amount of current flowing through the 1R.In the case of a 1R metal cage vs a 100K person the cage would probably turn to molten metal from the heat before the person would get noticeable current through them.

author: pickle9000
author_rank: Super Contributor
timestamp: December 23, 2013, 01:39:31 am
content: Being punctured with a live wire is of course possible as well. That would normally happen if a muscle contracted in an undesirable way while a person touches a live circuit.

author: Maxlor
author_rank: Frequent Contributor
timestamp: December 23, 2013, 02:02:24 am
content: Quote from: pickle9000 on December 23, 2013, 01:39:31 amAnd if you're unlucky enough to get up to the 600V range, the sheer voltage will puncture your skin for you (source). One more reason for gloves.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 23, 2013, 09:43:22 am
content: Quote from: Bored@Work on December 22, 2013, 09:49:21 pmConventional current flow isn't shit--it's just pretend,like the Easter Bunny & Santa!

author: Tepe
author_rank: Frequent Contributor
timestamp: December 23, 2013, 09:59:28 am
content: Quote from: vk6zgo on December 23, 2013, 09:43:22 amNope, it's just a sign convention. Changing it now would just lead to a world ofpainconfusion.

author: Rerouter
author_rank: Super Contributor
timestamp: December 23, 2013, 10:14:08 am
content: equally and this is something youll pick up more on later, even with a current limited high voltage supply, any capacitance, even pico-farads can give a large enough pulse of current as it discharges through you to fry yourself,a real world example of this is a CRT monitor, the high voltage fly back is generally current limited between 5-50mA, so approaching a garenteed kill, but possible to walk away from if you stick to perfect beginners theory,however the moment you contact that voltage it ionizes a spark right though your skin dropping your resistance to a very low value, discharging possibly 10's of amps through you for a brief second until the capacitance is used up and you fall back to the current limited source, which not only kills you but nicely barbeques the current path.

author: Psi
author_rank: Super Contributor
timestamp: December 23, 2013, 12:17:33 pm
content: Death from electrocution is a bit more random than that.You get people dying from 48V and people surviving a direct hit by lightning.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 23, 2013, 01:05:35 pm
content: Quote from: Tepe on December 23, 2013, 09:59:28 amNobody is seriously talking about changing it,I'm just "rattling his cage"!Back in the late '50s it took about 5 minutes to explain to 1st year students doing Elementary Electrical Theory,that all the Right Hand Rules & stuff were devised using Conventional Current Flow,so you needed to use it in those cases.It is,on the other hand,very hard to explain the operation of Thermionic devices using Conventional Current Flow.Some organisations tried to promote "LH Rules" to work with Electron Flow,but it never took off,as Techs happily used Electron Flow & Conventional Current Flow interchangeably over many years.Confusion? Not a bit!-----It seems it's only Engineers & the AAC bloke that have a "bit of a thing" about this.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 23, 2013, 01:24:03 pm
content: Quote from: H2Woah on December 22, 2013, 08:10:44 pmYou only read the first part,if you had continued,you'd have found that the author pointed out thatthe 1.0 MOhm figure was not a realistic one for normal working conditions,where the skin resistance can fall to a very much lower figure.The voltage necessary for a dangerous current was calculated for a number of scenarios.It always pays to read the whole thing!!!

author: Shock
author_rank: Super Contributor
timestamp: December 23, 2013, 01:48:44 pm
content: Quote from: Bored@Work on December 22, 2013, 09:49:21 pmCould you point to the most comprehensive source that covers electrical safety or whats incorrect in that chapter?

author: vk6zgo
author_rank: Super Contributor
timestamp: December 26, 2013, 12:58:43 pm
content: Quote from: Rerouter on December 23, 2013, 10:14:08 amStatistically,your contention just doesn't hold up!Thousands of people have been zapped by CRT EHT supplies & are still here to tell the tale.The regulation of the supply isterrible,so the "you" load causes an immediate drop in the output voltage.At the same time,the very small capacitance of the CRT,which doesn't really store a lot of energy,discharges,partially through you,& partially through the beam current of the tube .Does it hurt?-------Hell yes!,but it is unlikely to kill you.Very early colour TV tube EHT suppliesdidhave better regulation than the later solid state supplies,but even with these,there wasn't an epidemic of electrocuted Techs.