Can a switching power supply kill you?

Question

I'm curious about one thing. Can a switching power supply with 24V - 4.5A output can kill you if you touch its outputs directly?

As I know, just a 100mA may kill you too, so is it safe to use switching power supply without using any safety equipment or what should I care about?

A bullet can kill you. But if I am holding it in my hand and just pushing it into your skin there is not enough energy to get it inside you, even though it would do a lot of damage if it did get inside you. 100mA with 24V behind it is the same thing. The 100mA would be dangerous if it got inside you, but with something as weak as 24V trying to push it in, it's not getting past your skin. — DKNguyen, Jan 30 '20 at 22:46
As I know, no.. But I don't know much about electricity and I read some Quora answers and they say even 12V can kill you too. So before working with power supplies and on electronics, I wanted to be sure or take necessary measures. For example, what if a stepper motor draws 1A from 24V supply and you touch the cables with bare hand? Does it change anything or your skin will create resistance and avoid shock. — she hates me, Jan 30 '20 at 22:48
Just don't connect the power supply to some needles than then stab it under your skin with them, and certainly not on either side your body so the current passes through your heart. The resistance of your insides is much lower than that of your skin and 24V will be able to push more current if it gets past your skin. You're pretty safe up until at least 50V-60V. — DKNguyen, Jan 30 '20 at 22:49
If this is true... https://darwinawards.com/darwin/darwin1999-50.html — Sredni Vashtar, Jan 30 '20 at 22:50
Check this https://en.wikipedia.org/wiki/Electrical_injury#Voltage-current_characteristic_of_skin — jDAQ, Jan 30 '20 at 22:53
@shehatesme also, there is no way the motor would be pulling 1A @ 24V and that would suddenly "jump through you". There would have to be some sort of voltage spike or other fault, but the current of the motor itself would no go through you, also, resistance is a conductivity property you won't "create resistance and avoid shock". — jDAQ, Jan 30 '20 at 22:57
@jDAQ For example, I'm playing with several step motors now and their datasheet says they are 0.4A/12V, I thought some powerful motors can be 1A too. Of course it won't jump through me but what if I touch their naked connection cables with both hands? Not because it's easy to happen, just to learn... — she hates me, Jan 30 '20 at 23:03
@shehatesme You've already asked this 3 times and you got the same answer each time. — DKNguyen, Jan 30 '20 at 23:05
@shehatesme please, don't use EESE as you electric safety training, you will probably get hurt. Also, if your power supply is outputting **24V and no more AC voltages** then when you touch the wires you are like a "resistor" and Ohm's law can be used to calculate the current going through you. — jDAQ, Jan 30 '20 at 23:07
_"so is it safe to use switching power supply without using any safety equipment or what should I care about?"_ - I knew some one who was killed by a 24V power supply, but that was because they contacted the 240V mains side (bare feet and an ungrounded metal bench didn't help). If the power supply is properly insulated with no access to internal parts you are as safe as any user - but if it's an 'open frame' unit be extra careful. Always consider the possibility that mains-powered equipment could be 'live' on _any_ metal part. — Bruce Abbott, Jan 31 '20 at 00:17
Does this answer your question? [How much voltage/current is "dangerous"?](https://electronics.stackexchange.com/questions/19103/how-much-voltage-current-is-dangerous) — NStorm, Jan 31 '20 at 07:33

score 5 · Answer 1 · edited Jan 31 '20 at 00:04

5

The thing that is dangerous for the human body is current. While its effect depend on path (though heart or not), duration of the exposition and frequency (in your case DC) in general currents above 1 A are considered deadly.

AC-1 zone: Imperceptible

AC-2 zone: Perceptible

AC-3 zone: Reversible effects: muscular contraction

AC-4 zone: Possibility of irreversible effects

AC-4-1 zone: Up to 5% probability of heart fibrillation

AC-4-2 zone: Up to 50% probability of heart fibrillation

AC-4-3 zone: More than 50% probability of heart fibrillation

Source: https://www.electrical-installation.org/enwiki/Protection_against_electric_shock

On the other hand you have to consider that the human body-resistance is voltage dependent (higher voltages = lower body resistance).Below 25 V it is around 60 kOhm. In general, DC voltages below 120 V are not considered deadly, that’s why you can hold a battery with your hands and no current will flow through you, but if you short circuit a battery you can create high currents indeed.

Long story short: the 24 V output of your power supply won’t kill anybody.

edited Jan 31 '20 at 00:04

Bruce Abbott

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answered Jan 30 '20 at 22:55

Ken Grimes

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If you touch +V with one hand and the -V with other hand, does that change anything? What needs to be done to flow current through the heart? For example when a motor draws 1A from 24V, if you touch it's cables (lets say they are not insulated), will it shock? – she hates me Jan 30 '20 at 22:59
@shehatesme No, because 24V isn't enough to get past your skin. – DKNguyen Jan 30 '20 at 23:01
The *motor* draws the current, not you. – Eugene Sh. Jan 30 '20 at 23:02
@Ken currents as low as 40 mA can kill you. – jDAQ Jan 30 '20 at 23:02
@EugeneSh. but isn't is flowing over me when I touch the all cables? I guess my logic is wrong about this subject – she hates me Jan 30 '20 at 23:05
@shehatesme No, because it is too difficult for it to flow over or through your skin. – DKNguyen Jan 30 '20 at 23:06
2

Ohms law lets you calculate the current a particular voltage will push through a particular resistance. 24 volts divided by 60k ohms (from comment above) = 0.0004 A or 0.4 mA. That puts the current in the very green "imperceptible" zone of the chart in the answer above. If you wet your skin you might get it into the perceptible range. – George White Jan 30 '20 at 23:38
All of this is based on the assumption that the skin is integer. Resistance of the body under the skin can be as low as a hundred ohms. So, if the OP does not pushes the terminal inside his/her fingers to the point of breaking their skin, they should be fine. – Sredni Vashtar Jan 30 '20 at 23:41
@shehatesme This answer is helpful BUT LETHALLY DANGEROUSLY WRONG. Despite the graph and its source, low voltage DC FAR below that shown MAY kill you. A 24 V bench supply is almost certain to do you know harm at all. But in worst case conditions 24 VDC MIGHT kill you. VERY unusual. Not unknown alas. Documented cases can be found. Helicopters have 36 VDC batteries. Fatalities have occurred. Wet, salty, cuts, blood, ... make it more likely. | I've spent 50+ years working with DC at 50V or less and never had a substantial shock from it. At 50 VDC on a humid day on a telecom wiring frame I ... – Russell McMahon Jan 31 '20 at 00:37
@shehatesme ... could feel nuisance shocks on the back of my hands. I'd not 'grab' 50V hand to hand - but would expect contacts across the back of one hand to annoy at best. usually. || 24 VDC - treat with care. Expect it never to hurt you. Know that worst case it could. – Russell McMahon Jan 31 '20 at 00:39

score 3 · Answer 2 · answered Jan 31 '20 at 01:26

THIS MACHINE (ONLY VERY VERY OCCASIONALLY) KILLS.
BE SENSIBLE - DON'T LET IT

Under MOST conditions a 24 VDC supply cannot cause the lethal conditions shown.
Under exceptional circumstances, it can.
DO NOT be scared of such supplies.
I have used similar for 50+ years without problem - as have many other people.
But be aware that a combination of high humidity, damp hands, maybe cuts or blood on hands MAY cause problems. It's very unusual. But, always treat anything "energetic" with respect.

In day to day life you can be killed by an angle-grinder, skillsaw, power-drill (it's hard), stationary bus (it happens), not looking both ways when crossing the road (4 times in China), bicycle (on foot or riding), skateboard (same), and much more.
All these are hazards of modern society. Add the power supply to the list. it's well down in danger from angle grinders and bicycles - but don't leave it off.

From this page that Ken Grimes cited.

When a current exceeding 30 mA passes near the heart of a human body, the person concerned is in serious danger if the current is not interrupted in a very short time.

The protection of persons against electric shock in LV installations must be provided in conformity with appropriate national standards, statutory regulations, codes of practice, official guides and circulars etc.

Relevant IEC standards include: IEC 61140, 60364, IEC 60479, IEC 61008, IEC 61009 and IEC 60947 series.

The graph that Ken published from that page needs careful inspection.
Under MOST conditions a 24 VDC supply cannot cause the lethal conditions shown.
Under exceptional circumstances, it can.
DO NOT be scared of such supplies. I have used similar for 50+ years without problem - as have many other people.
But be aware that a combination of high humidity, damp hands, maybe cuts or blood on hands MAY cause problems. It's very unusual. But, always treat anything "energetic" with respect.

Here is the same chart with some annotations:

LETHALITY AND MUSCLE CONTRACTION 'KEY' BELOW.
This is for hand to foot shocks.
Hand to hand is easier on a bench with grounded surfaces.

Note the added red and blue lines and green circles. Ventricular fibrillation will usually kill you if IMMEDIATE medical attention is not available ("crash box").
At 50 mA you hit the "may die" (5% VF chance) at 1 second.
You'll USUALLY survive this.
You are well into the muscle contraction effects area but can probably pull free. DC is harder to self release from than AC.

At 100 mA you hit "may die" at half a second, 50/50 chance of death at still under a second, and "probably die" at under 2 seconds. Muscle contraction is substantial and well into the maybe-cannot-let-go zone by now. Believe me :-) :-(.

At 200 mA maybe / 50-50 and probably arrive at about 1/4 second, 1/2 second and 1.5 seconds.
You don't want to try that.
And it hurts! (as a bonus)

At 500 mA - forget it (50 / 200 / 400 ms).

Notes:

A 24 V bench supply is almost certain to do you know harm at all.
But in worst case conditions 24 VDC MIGHT kill you.
VERY unusual. Not unknown alas. Documented cases can be found.
Helicopters have 36 VDC batteries. Fatalities have occurred.

Wet, salty, cuts, blood, ... make it more likely.

I've spent 50+ years working with DC at 50V or less and never had a substantial shock from it. At 50 VDC on a humid day on a telecom wiring frame I could feel nuisance shocks on the back of my hands. I'd not 'grab' 50V hand to hand - but would expect contacts across the back of one hand to annoy at best. usually.

So - 24 VDC - treat with care. Expect it never to hurt you. Know that worst case it could.

I had a friend who took a 12V LED lamp flounder fishing. Standing in salt water with the lamp on a aluminum pole he managed "muscle lockup" when something in the lamp shorted.
12 Volts!!! But, standing in salt water. No harm done. But a lesson.

Had a friend (townie) who peed on an electric cattle fence - shocking but funny for us :) no harm though... — Solar Mike, Jan 31 '20 at 05:41

Can a switching power supply kill you?

2 Answers2