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I am building a prototype rig. It includes: 1) A "naked" 24/48 V motion control evaluation board. 2) A I=3.6 A stepper motor. 3) A power supply.

I wired up the motor to the board with four wires (A-, A+, B-, B+). I wired the board to the power supply: source (24 V) and ground wires.

The power supply allows to set both voltage and current. I am planning to set the power supply voltage to 24 V, not sure about current. By default it's at I=0.1 A, but I guess I might need at least 1 A to activate the stepper.

Now, a pessimistic assessment of the skin resistance is 500 Ohms (Wikipedia article on electric shocks). At 24 V, this would give a through current of 48 mA, causing heart fibrillation, breathing difficulties and inability to let go of the wire -- not cool.

How do I play it safe?

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  • \$\begingroup\$ Anecdotal observations from my past experiences: 1) I measured my dry skin resistance to be 600kΩ; 2) I have received -180VDC across my fingers via stupidly tiny BNC connector without problem. On 2) I was very embarrassed but glad to have that event go unobserved, as I would have failed that practical had the examiner noticed... \$\endgroup\$
    – Chris K
    Jan 2 '15 at 11:05
  • \$\begingroup\$ I agree with the comments below, but I'll add that being organized, avoiding conductive surfaces, fastening down the boards in some manner, controlling wires and turning the power off positively before fooling with it will also help avoid a much more likely outcome- zapped parts and air filled with the acrid odor of electrical disappointment. \$\endgroup\$ Oct 22 '16 at 9:06
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There is a whole body of knowledge about how to keep from getting zapped, and I'll give you a few highlights.

1) At 24 volts, if your skin is dry you won't feel more than a tingle. Don't play with electricity in the rain and don't spit on your hands before you start trouble-shooting. Go back to the Wiki article and you'll see that the 500 ohms figure was for wet or broken skin, so don't let live wires stick into any open wounds you might have.

2) If you're seriously paranoid about getting shocked, put the circuit in a box with a switch attached to the lid, so if the lid is opened the power is turned off.

3) Poking around a live circuit is normally necessary as part of troubleshooting. First, make sure you're sitting or standing surface is dry and not conducting. Second, keep one hand in your pocket at all times. (I'm not kidding about this.) That way, you cannot make a circuit from one hand to the other. At 24 volts, this is generally unnecessary, but it's a good practice at higher voltages.

4) Never build a circuit which is directly connected to the AC power lines. Always use an isolated power supply.

5) Note that there is such thing as a "let-go threshold". When the power is on, you can poke and prod with a probe to your heart's content - just don't grab anything. Again, at 24 volts, this isn't a problem (unless you're violating rule 1 and playing in the rain), but it's still a real good policy. If you see a loose connection, or a screw that needs tightening, stop and turn off power, then fix the problem.

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500 ohms is into the body, one way. Not only does it have to come back out, but it also has to go across the heart. The total resistance it has to cross is closer to 10kohm, which at 24V gives 2.4mA, a barely perceptible current. This is why even 24V is considered low voltage by building codes.

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  • \$\begingroup\$ In Australia, even 50V is considered low voltage by building codes. \$\endgroup\$ Jan 1 '15 at 23:58

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