How to calculate the lethal amount of current for given amount of output voltage?

Question

I have some electrical parts that I was planning to reuse, things like laptop chargers and some parts from a printer. I searched google for what's the lethal amount of current and they mention 10ma is lethal. But they didn't mention for what voltage it is lethal. My phone charger outputs 2Amp at 5 volts and I know it's not harmful. So I have these parts that output 24V 0.6Amps and I wanna know if there's a way I can know this will be harmful or not?

In general is there a formula I can use to calculate a threshold amp value for given DC/AC voltage?

Answer 1

The formula is very basic Ohm's law:

$$I=\dfrac{V}{R}$$ or $$V=I\cdot R$$

Anyway, the 10mA isn't lethal, a current more than 30mA through the heart may cause permanent damage to heart or infarct.

There is no specific lethal voltage and current, only the recommendation of the max. current that doesn't cause permanent damage. It depends on size of the body, hydration, ..etc.. on what is the body impedance. It also matters how the body is connected to the electrical circuit: hands-foot, head-foot, ... it also differs if you hold the wire with left hand or with right hand.

Generally any DC voltage below 48V is to be considered safe voltage, or AC voltage below 34V.

Answer 2

Per legend, someone was once electrocuted by a 48V telephone ringer voltage. However, that is extremely rare. Typically, anything less than 50 V is unlikely to cause electrocution, although it may cause painful shocks.

Although 10 mA may kill you, if you are lucky, it won't. That is, 10 mA probably won't kill you unless a significant portion of that passes through your heart. 100 mA is much more likely to kill you, especially if it is continuous.

Others have mentioned Ohm's law, V = I * R. A value sometimes given for the resistance of a human body from say, hand to foot, or hand to hand, is 1000 Ohm. 50 volts through 1000 ohms gives 50 mA. and 100 volts through 1000 ohms is 100 mA. But a human can have lower resistance, for example if wet or sweaty. So, even at 50 volts, someone unlucky could have 100 mA.

If you are not experienced working with higher voltages, I would recommend that you keep your circuits below 50 volts.

Other recommendations include using fuses appropriate for your wires and equipment so you don't start fires, and use ground fault circuit interrupters, or GFCIs (that is what they are called in US, they are called something different in Europe), so that if you should accidentally make a mistake in your experimenting, you will have a better chance of surviving a shock from the mains supply.

Answer 3

A power supply's current rating is telling you how much current it can supply, not how much current it does supply. It's actual output current is determined by whatever you connect to it, called the load. That current rating is only a maximum, so if you have a supply rated at "12V, 2A", and you connect a human body across it with a resistance of 100kΩ, the current that will pass through the body can be calculated by Ohm's law:

$$ I = \frac{V}{R} = \frac{12V}{100k\Omega} = 120\mu A $$

That's way less than the 2A that the supply is capable of. If you wanted the entire 2A to flow, you would need a load of:

$$ R = \frac{V}{I} = \frac{12V}{2A} = 6\Omega $$

So, if the current required to kill you is 10mA, you would need your body to have a resistance of:

$$ R = \frac{V}{I} = \frac{12V}{10mA} = 1.2k\Omega $$

In other words, if you place your left hand on one terminal of a 12V battery, and the right hand on the other, and the total electrical resistance between the two teminals (consisting of terminal-to-skin resistance, and the resistance from left hand to right via your hands, arms and torso) is 1200Ω, the resulting current through your arms and torso will be 10mA, and you will die, possibly. This is a very simplistic model, though.

It's very rare (though not impossible) to find such a low body resistance. It would normally be in the tens or hundreds of kilohms, meaning that 12V batteries are not considered to be dangerous. Even 48V would probably not be enough to do any harm, since the determining factors are both voltage and body resistance, not voltage alone, and body resistance is generally too high to draw any harmful amount of current from the supply.

This doesn't mean you won't feel anything. The various nerves in your body (particularly near the skin) are sensitive to potential (as opposed to flowing current), and any voltage difference will stimulate them, resulting in pain and muscular spasm. So even if your body resistance is high enough to avoid lethal currents, you can still get a "shock" and the associated spasms.

The heart's rhythm can easily be upset by potential differences messing with the potentials that are supposed to be there. This can occur even if the duration of voltage applied to the body is very short, too short to cause any physical damage. Voltage is a killer in that respect.

Physical damage (burning) occurs through heating, which is related to power being dissipated in the body. Since the current is the same along its entire path through the body, heating due to \$P = I^2 \times R\$ power dissipation is greatest where resistance is greatest, which is usually at the skin. Heating will continue for as long as current flows, so where a short high-voltage shock can stop the heart, burn severity will depend on the duration of the shock, and the current flowing. In this sense, it's current that kills.

As you can see, any idea that it's voltage that kills, or current that kills, is over-simplistic. I suppose you could generalise that the culprit can be either; voltage upsets the nervous system, and current burns, both of which can be fatal. However, current depends on body resistance and voltage, so you could argue that it's neither current nor voltage, it's actually hydration and electrolytes that kill.

I feel comfortable with voltages up to 60V or so. I will happily touch AC or DC voltage sources up to that point, though I would never knowingly touch any voltage source over 30V with wet hands, since clearly that will increase current flow. I will have no problem touching a 10A 24V supply, because that will not cause 10A to flow through me.

Answer 4

Depends on the current resistance and voltage which amounts into power (W)vs(R) So assume a 19v DC charger with 19a of current can hurt bad and possibly kill you under the condition of 300W~ being passed through the resistance of your skin dry or wet. Wet, possibly can kill you. Dry skin, will hurt a lot but may not kill you, unless most of the current went into a vital organ which usually are nerves, vanes or heart, in that case there will be permanent damage or can be fatal.