Milliamps and death - Page 1
Forum section: electronics / beginners

author: windmill john
author_rank: Contributor
timestamp: January 14, 2023, 02:11:46 pm
content: Catchy title huh!Okay, I’m old enough not to be embarrassed by this question. Let’s not get off topic and talk about AC and its issues, I’m sticking with DC for this question.I need some clarity… as a beginner… If 100 milliamps can kill, how come in some videos you can see the host twisting wires together in a live circuit at 100 milliamps with no issue?Now the voltage was 12 volts. I just need clarity. The circuit was complete, no faults. At what point does the danger become an issue? Does it depend on voltage? I’m aware that’s the force etc.Can this be answered simply without anecdotal stories? Thanks.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 14, 2023, 02:16:14 pm
content: With 12V, 100mA was not flowing. If somebody claimed it, they measured wrong. I = U/R. Human body has some certain R.

author: Ian.M
author_rank: Super Contributor
timestamp: January 14, 2023, 02:39:34 pm
content: ... or more correctly, has some variable R, depending on contact area, skin thickness, condition and dampness, or a much lower R if a conductor penetrates through the skin or the skin is significantly damaged. Your personal R isn'treliablyhigh enough to protect against shock, so don't count on it to save you when working on circuits carrying more than a few tens of volts.

author: bostonman
author_rank: Super Contributor
timestamp: January 14, 2023, 02:47:28 pm
content: Low DC voltage can't break skin resistance. Although, it's probably safe to practice never touching live wires.DC is dangerous because, unlike AC, it never crosses zero. Every 8.3ms AC crosses zero, so you theoretically have a moment to pull away. DC remains constant (i.e. direct current), so you can become fused to it.Correct me if I'm wrong, but wet hands touching the positive of a car battery terminal (while some other part of the body is touching battery ground through the metal of the car - or directly to the negative battery terminal) can be deadly because the liquid would reduce the resistance, correct? For this reason, it's why you can lay your finger across a 9-volt battery without getting a shock, but touch the terminals to your tongue and you'll get a jolt.

author: TimFox
author_rank: Super Contributor
timestamp: January 14, 2023, 03:25:29 pm
content: As a generalization, a circuit through the human body will see relatively high resistance through the skin (which depends on how wet it is) and relatively low resistance through the interior (wet tissues with lots of salt).

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 14, 2023, 03:33:54 pm
content: Quote from: windmill john on January 14, 2023, 02:11:46 pmThere is no 100mA going thru the host body. In fact he wouldn't be able to hold it if 5mA is going thru his body. So when you watch the video what make you think there is a 100mA or more going thru the host body? I have work on some static eliminator and other high voltage system (20,000V and more) but with the current limit to 5mA. I do not know how much current would go thru a person body when he/she touches it but it would be less than 5mA. It doesn't actually kill anyone with the shock but the reaction of the person getting the shock have been known to cause serious accident.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 14, 2023, 03:36:23 pm
content: The typical resistivity of dry skin is about 25,000Ω cm, too high to allow much current to flow. Dry skin has modest insulating properties but this will break down easily when enough voltage is applied. 30V is now considered the upper bound for safe working. (eg a truck battery). Put your fingers across the phone line for a big 48V suprise. Dont.Even microamps can kill under adverse conditions. A current entering via a skin lesion or puncture wound can affect the nervous system. Put your fingers across a 9V battery. Nothing. Then put it on your tongue. Crikey! Wanna try that with 24V or maybe 48V Now imagine what a few volts in your precious wet innards. Nerves get twitchy at about 60mV.Even a very small DC current will cause electrolytic burns and necrosis.A small, usually pulsed DC current sometimes used to move charged drug molecules under the skin. (Iontophoresis)

author: Siwastaja
author_rank: Super Contributor
timestamp: January 14, 2023, 03:45:39 pm
content: Quote from: Ian.M on January 14, 2023, 02:39:34 pmAnd many, many other things, as equivalent circuit not being anywhere near a resistance, but we need to start from somewhere and get to the simplest possible model that explains away all the "it's amps that kill and volts that jolt" type absolute bullshit.And constant resistance model is pretty good first order approximation for that. There is voltage, and current relative to it results. Period.

author: IanB
author_rank: Super Contributor
timestamp: January 14, 2023, 03:49:48 pm
content: Quote from: windmill john on January 14, 2023, 02:11:46 pmYes, it can be answered very simply. The measure that counts is the number of milliamps going through your heart, and also for how long. A hundred milliamps flowing in a wire won't mean a thing. A hundred milliamps flowing through your fingers might hurt, but it won't kill you.What causes current to flow though your chest? A combination of high enough voltage and low enough skin resistance, with a circuit that goes from hand to hand, or hand to feet. Skin resistance matters, so that wet skin is much worse than dry skin. That's why electricity and bathrooms don't mix.What is a high enough voltage? Roughly speaking, more than 50 V, but there is no specific value. On the other hand, 12 V is not enough in normal circumstances.

author: iMo
author_rank: Super Contributor
timestamp: January 14, 2023, 04:17:01 pm
content: The damage is a combination of the current and voltage and the resistivity of your body parts exposed and the path through your body.A couple of uAmps through your heart may stop it beating (ie. the pace maker regulates the heart rhythm where the currents are almost nil).The power_loss=voltage*current creates a heat, which may evaporate your fingers or your leg in a fraction of a second. Also the current can make chemical changes in the body liquids/electrolytes (DC especially).So - it depends. 100mAps through your body would require the voltage and the resistivity such the 100mA could flow. The U*I creates heat which could smoke your tissues. Your nerves are sensitive to the currents as well (your nerves conduct electrical impulses from/to your brain), the path of the current colliding with the important nerves could immobilize you, or stop your breathing, heart, etc.Thus you cannot say "this current or voltage will do this or that" to your organism, without knowing above details.

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2023, 04:49:39 pm
content: Quote from: windmill john on January 14, 2023, 02:11:46 pmThe simple answer is that lower voltages can't cause enough current to flowthrough your bodyto cause harm. The current in the conductors is irrelevant.The complex answer is that there is quite a bit of variance in results depending on the situation and thus there is not a well defined voltage cut-off line below which you are safe. The human body isn't really a resistor--maybe it is more like an electrolytic capacitor. There are any number of standards related to the acceptable voltages in directexternalcontact with people. Internal contact is a different matter and swallowing even a 3V coin cell can cause serious medical issues.https://en.wikipedia.org/wiki/Extra-low_voltage

author: windmill john
author_rank: Contributor
timestamp: January 14, 2023, 05:17:54 pm
content: Thanks all, much appreciated.It all looked safe, but that was visual, I just needed the technical/theoretical answer.To answer one point above, the circuit was tested and showed 100 milliamps. The poster then undid the connection by hand, hence my question.If you are happy to accept silly questions (not silly to me ) it’s appreciated. I’m covering all safety aspects before I delve deeper into the subject.

author: nightfire
author_rank: Frequent Contributor
timestamp: January 14, 2023, 05:40:42 pm
content: In appliance testing for work safety, the "Berührungsstrom (german) " (touch current) is measured via a 2k Resistor against earth to simulate the human body.In reality, depending on some conditions like hand to hand or feet, or adult or child, it obviously varies a bit, but for own rough "over the thumb" calculations something like 1k Ohm is a pragmatical value to use.And with skin, the voltage needed to push the first current through is a bit higher than what is needed to sustain the current flow.Doing some math: 12V=1000 Ohms * x Amps=>0,012 AWith 2000 Ohms obviously half, so 6mA.And not to forget: If someone touches a live circuit, we talk about a voltage divider the human body presents, so not all current will flow through the human body, becaus according to Kirchhoffs Law it will divide proportionally to the conductivity of the current paths.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 14, 2023, 05:43:19 pm
content: Quote from: windmill john on January 14, 2023, 05:17:54 pmYou wrote "twisting wires together", so it seems 100mA was flowing through the wires which were twisted together, never through the human being.

author: jonpaul
author_rank: Super Contributor
timestamp: January 14, 2023, 05:53:20 pm
content: 5 ma is sufficient to cause ear disruption.Resistance of body depends on probe area, skin penetration, humidity, etc.Pins inseteed just below the skin surface can get substantial current flow with just 10..30VSELV = Safety Extra Low Voltage is defined as 42 V DC or peak.Huge amount of shock/hear/medical /safety info/regs on this topic.100 Ma is lethals 100%Jon

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 14, 2023, 05:53:24 pm
content: Quote from: windmill john on January 14, 2023, 05:17:54 pmThe current if flowing thru the circuit whatever it is but not the host body. I have a heater that draw 500A at 5V and people touching the bare cable without getting shock. Because the 500A is going thru the heater and not the body. There is some current going from the 5V bare wire thru the human body and to the ground but it's very very small and he doesn't get shock.

author: bostonman
author_rank: Super Contributor
timestamp: January 14, 2023, 05:59:56 pm
content: Also, a good practice is to keep your left hand in your pocket when dealing with high voltage.Easier said than done, but, if you accidentally touch a life wire, it will hopefully go down the right side of your body thus bypassing your heart.

author: IanB
author_rank: Super Contributor
timestamp: January 14, 2023, 06:11:52 pm
content: Quote from: BeBuLamar on January 14, 2023, 05:53:24 pmWhy do you say that? Most low voltage supplies are isolated, so there is no path to ground for any current to flow.

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2023, 06:43:54 pm
content: Quote from: bostonman on January 14, 2023, 02:47:28 pmYou're wrong on both counts.DC is actually less dangerous than mains frequency AC, for the same current. AC doesn't pass through zero for anywhere near long enough to mitigate any shock. It has a higher peak voltage, compared to DC. 240V is 325V peak, whist 240VDC, never gets above 240V. 50 or 60Hz also disrupts the rhythm of the heart, better than steady DC.Even if the skin is very wet, it's very difficult to difficult to get a lethal shock from 12VDC. It might be possible if a large area of skin is broken, but still highly unlikely.

author: Traceless
author_rank: Frequent Contributor
timestamp: January 14, 2023, 06:53:26 pm
content: There is an"urban legend"that someone was killed by their handheld DMM which was powered by a 9V battery (I'll leave it to the professionals here to confirm or debunk that). According tothis sourceeven a current as small as 0.01mA can cause ventricular fibrillation and thus potentially kill a human under certain circumstances. On the other end of the spectrum most of us have already received static shocks for instance after walking on a carpet and touching some metal object afterwards. Apparently the voltage built up prior to those shocks can beeasily several kV.

author: TimFox
author_rank: Super Contributor
timestamp: January 14, 2023, 07:03:15 pm
content: Electroplating commonly uses direct current, and there is a safety hazard due to liquids, DC, and other chemical problems (especially with cyanide solutions).

author: jpanhalt
author_rank: Super Contributor
timestamp: January 14, 2023, 07:09:54 pm
content: Quote from: bostonman on January 14, 2023, 05:59:56 pmThat "one-hand" theory is often cited and unfortunately is interpreted wrong. I researched its history once. The real reason for recommending "one hand" is to lessen the chance of touching a live wire (e.g., two hands have twice the probability).Of course, one is often wearing shoes with low conductivity. The presumed path is probably wrong. Remember, the heart is not directly between the two arms, it is located quite low in the chest in what is called the mediastinum. If the geometric path makes a difference, arm to contralateral/opposite leg should be most dangerous. There is some evidence for that in humans, but the strongest evidence is from early experiments in dogs. See cited references here:https://www.researchgate.net/publication/242705238_A_review_of_hazards_associated_with_exposure_to_low_voltagesQuoteIt is often stated that DC is more dangerous than AC, based on the theory that one can release from AC easier at the zero crossing point, but the evidence from actual electrocution is very weak. Here is what the previously cited review states (page3):QuoteThis review gives more insight into the reason AC is more dangerous:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/Quote

author: ejeffrey
author_rank: Super Contributor
timestamp: January 14, 2023, 07:13:36 pm
content: Quote from: jonpaul on January 14, 2023, 05:53:20 pmActually not really. There is an urban legend about a sailor being killed by a Simpson DMM puncturing the skin with the probes but this is easily refuted.Get a small jar of water and prepare an isotonic (0.9% by weight) saline solution. Warm the water in a microwave to about 38C and insert your multimeter probes a few cm apart. You will get a surprisingly large resistance because the spreading resistance in the neighborhood of the probes is still quite large.Now get some copper braid and clip that to the probe ends and you will see much lower resistance due to the dramatically greater surface area. .Make sure to thoroughly rinse your probes immediately afterwards to avoid corrosion.

author: Gyro
author_rank: Super Contributor
timestamp: January 14, 2023, 07:18:11 pm
content: Quote from: windmill john on January 14, 2023, 02:11:46 pmUnfortunately you cannot separate the two.As Zero999 and jpanhalt say, it's AC that causes the heart to fibrilate and results in death at relatively low currents, certainly sub 100mA, depending on conditions - hand to hand vs same hand etc. At the same time, there are occasional stories on kids, and occasionally adults, straying onto 3rd rail railway lines and coming into contact with 600 -750V DC traction current on the conductor rail. These inevitably lead to severe burns. A lot of victims die from such injuries, particularly if they fall across the rail, but there are survivors, maimed and scarred but alive. From the severity of these burns, and calculating the likely current, far more than 100mA has passed.At some point, you have to clarify what you mean by "If 100 milliamps can kill...". AC currents levels causing the heart to fibrilate, leading to rapid death are rather well documented. When it comes to DC current, injury due to severe burns, is highly variable both in survival chances and time to death. The survivability of burns [Edit: of any cause], fluid loss, infection etc. is far more variable, anecdotal stories become inevitable.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 14, 2023, 07:20:34 pm
content: Quote from: ejeffrey on January 14, 2023, 07:13:36 pmExactly this. Sharp tips get better contact to the tissue by bypassing the skin, but at the same time, exactly due to the sharpness, you sacrifice all the surface area.Safety extra low voltage limits are well thought out and any skin damage won't make them dangerous.

author: Rick Law
author_rank: Super Contributor
timestamp: January 14, 2023, 08:34:31 pm
content: Perhaps the most important electrical signal of a human's life is the heart beat waves typically shown in the EKG. The EKG is a complex non-sinusoidal wave but a regularly repeating pulse sequences. The strongest pulse is probably the "QRS Complex" and the "R Wave". The tail of the "R Wave" is the start of "S Wave" which is the negative deflection that follows the "R Wave".The maximum delta typically would be between the R Wave's peak and S Wave's bottom, it is less than 100mV. Typically 50-70 mV. If an external current passing your body creates a signal of near that magnitude, it will interfere with your heart pulse.So, when "applied" properly, your body can reach room temperature with just a small current. I don't know exact how much it will take, but I sure am not in a hurry to reach room temperature.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 14, 2023, 09:09:04 pm
content: Post #21, perhaps the main reason people don't discuss DC shocks is because DC at high voltages is rather rarer than AC at high voltages. Afterall the whole point of AC is the ease with which you can step AC voltages up and down with transformers. There are some HVDC power lines thesedays in a very few places, and maybe some high voltage DC in some parts of some modern electric cars (400V ish), but otherwise it virtually doesn't exist, and until recently existed even less.

author: TimFox
author_rank: Super Contributor
timestamp: January 14, 2023, 09:23:23 pm
content: Quote from: Infraviolet on January 14, 2023, 09:09:04 pmA normal third-rail supply to subway trains and similar applications is 600 V DC, which can be quite lethal.This has been true since the 19th century, and continues to the present day.Overhead wire electric trains usually run on even higher voltages, and traditionally that was DC or low-frequency AC.Overhead wires are now commonly at 50 or 60 Hz.A summary (from Wikipedia) on electrification for ground-level (third-rail) and overhead wire in use now in UK:3 Existing systems – overhead line (OHL)3.1 National Rail: 25 kV, 50 Hz AC overhead3.1.1 Existing3.1.2 2010s Network Rail electrification programme3.2 Other systems3.2.1 1,500 V DC, overhead3.2.2 750 V DC, overhead3.2.3 Other overhead systems4 Existing systems - third and fourth rails4.1 National Rail: 650 V - 750 V DC, third rail (top contact)4.2 630 V DC, fourth rail (top contact)4.3 750 V DC, third rail (bottom contact)4.4 750 V DC, fourth rail (top contact)4.5 600 V DC, third rail (top contact)4.6 250 V DC, third rail (top contact)4.7 110 V DC, third rail (top contact)4.8 100 V DC, four rail

author: nightfire
author_rank: Frequent Contributor
timestamp: January 14, 2023, 09:32:13 pm
content: The fact that DC (and frequencies significantly apart from 50 Hz) is less dangerous to humans is also reflected in appliance testing. Here the so called "50Hz filter" is employed in device/appliance testers, that essentially filters low and high frequencies in an attempt to reflect the sensitivity of a human body.This is/was in the EN61010 Picture A1, if I remember correctly.

author: geggi1
author_rank: Frequent Contributor
timestamp: January 14, 2023, 09:35:01 pm
content: Gottfried Biegelmeier the father of RCD

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 15, 2023, 12:38:14 am
content: Quote from: IanB on January 14, 2023, 06:11:52 pmAlthough the 5V is derived from a 100:1 ratio transformer but I measure I do have 5VAC to ground.

author: IanB
author_rank: Super Contributor
timestamp: January 15, 2023, 12:43:54 am
content: Quote from: BeBuLamar on January 15, 2023, 12:38:14 amWhat do you mean by "derived from"? What what do you mean by "ground"?There are too many things that are unknown in your statement.I can assure you, however, that if I take a properly constructed step down transformer with 120 V AC on the primary, and 5 V AC on the secondary, there is no voltage between the secondary side and the earth beneath our feet. That is the whole point of isolation in transformers.

author: james_s
author_rank: Super Contributor
timestamp: January 15, 2023, 12:54:03 am
content: 100mA flowing through a circuit doesn't make it dangerous, the current has to flow through your body to hurt you. If you insert yourself into a low voltage circuit you are not going to have anywhere near 100mA, you need a much higher voltage than 12V to push 100mA through the resistance of your body.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 15, 2023, 01:03:47 am
content: Quote from: windmill john on January 14, 2023, 02:11:46 pmOhms Law! I=V/R, where I is current in Amps, V is voltage (obviously enough, in Volts), & R is resistance in Ohms.For a current of 100mA (0.1A), R would need to be 120 ohms.The device, which is being connected does offer that resistance, but a human appears as a very much higher resistance, so the 12 V cannot cause a 100mA flow through the human.For a Mains supply of 230v, or even 120v, a current of 100mA could flow through a person, as 2300ohms or perhaps 1200 ohms are feasible resistances for skin contact resistances.There is a lot of literature on this stuff, but suffice to say, you can pretty much fart around with 12v & under till you are blue in the face, & your chance of injury is up there with being hit by a meteorite.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 15, 2023, 01:15:13 am
content: Quote from: Rick Law on January 14, 2023, 08:34:31 pmYou are already "above room temperature", unless you have just come in from a blizzard.

author: IanB
author_rank: Super Contributor
timestamp: January 15, 2023, 01:52:30 am
content: Quote from: vk6zgo on January 15, 2023, 01:15:13 amI think you missed the point. If you are "at room temperature" you are dead. The point being, it doesn't necessarily take a large current through your body for you to end up cooling down to room temperature and being taken to the mortuary

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 15, 2023, 03:23:25 am
content: Quote from: IanB on January 15, 2023, 12:43:54 amIt's a transformer stepping down from 480VAC.The voltage we have is actually slightly over 490VAC and the output is quite close to 5V. One end of the output is connected to the ground wire which is connected to the ground wire all the way to the panel and also all the metal frame of the machine. Measuring from one of the wire that supplying 5VAC and the ground wire show about 5VAC.One end show 0V.

author: jonpaul
author_rank: Super Contributor
timestamp: January 15, 2023, 03:26:46 am
content: AC frequency....mains vs RF vs HF all different issues.Tesla experienced RF burnsJon

author: james_s
author_rank: Super Contributor
timestamp: January 15, 2023, 05:34:14 am
content: RF burns suck. I got one once while playing with a Tesla coil. I tried to turn on the light and when my finger touched the grounded screw holding the cover plate on the light switch I got a burn.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2023, 09:19:05 am
content: Quote from: BeBuLamar on January 15, 2023, 03:23:25 amThere will be a tiny interwinding capacitance between the primary and secondary. 5VAC is probably even too good to be true. If the meter has an input impedance of 10M, that would be just 500nA of leakage. The transformer probably has a screen, which is coupling the secondary to earth.

author: 8753ES
author_rank: Newbie
timestamp: January 15, 2023, 06:44:16 pm
content: Typically, a person can sense currents of 1mA or more, with currents over 40mA being possibly lethal as they pass through the heart. Therefore, it is how much current flows through the body and where it flows that is of concern. When a person becomes part of a circuit, there are four parameters that determine the current:1) source voltage2) source resistance3) contact resistance, and4) internal body resistance for the current path.The second two parameters are a function of body physiology. The contact resistance is mainly caused by the dead skin layer where contact is made. Contact resistance is typically on the order of 100 ohms for sweaty skin to 100 kohms for very dry skin. Beware that if the skin is cut, the contact resistance becomes negligible. The internal body resistance is fairly low due to the fact that nerves and blood vessels make good conductors. Any limb-to-limb internal resistance can be approximated as about 500ohms.The effects of electricity are felt differently, depending on frequency. The most dangerous frequency range is from about 5Hz to about 500Hz and peaks in danger right at about 50/60Hz, the frequency of power lines. The frequency sensitivity has to do with the physiology of the human nervous system, which typically communicates via pulse trains in this range of frequencies. A 60Hz current has approximately two to three times the danger as the same current at DC. As opposed to DC signals, these AC signals can cause muscles to lock up, leaving a person unable to let go of the voltage source.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2023, 07:17:56 pm
content: A common myth is the skin effect protects against electric shock, or harm from AC. The skin effect depth even at 2.45 GHz is around 10cm or so in human flesh. If this wasn't the case, you wouldn't be able to cook meat in the middle in a microwave oven. It's probably more true of the higher microwave bands above 100GHz which behave more like infrared, causing surface, rather than deep burns, but will still damage the eyes and testicles