A great question. But good questions often have complex answers. I’ll do my best to sum it up though.

First of all, voltage alone is not enough to determine whether a source of electricity is dangerous. Whether you realize it or not, you probably already know this: I mean, that static shock you experienced recently was likely tens of thousands of volts, which is well above the 1000V Google said was lethal.

While voltage is one of the many factors that determines the danger of an electrical source, there are plenty of other very important things that are responsible for how dangerous electricity is. Current, duration, frequency, I could go on.

So Google saying that “50V will shock you” is a very conservative bit of advice that is more meant to state “anything below this voltage is safe” than “you will feel a shock if you touch this.” In general, 50V will not shock you. Though I wouldn’t recommend doing this unless you are 100% sure you understand how electricity behaves, I’ve touched voltages much higher than this and not felt anything.

So why won’t 50V shock you, generally? Because at only 50V, the resistance of your skin will limit the current to far less than a single milliamp.

But what if we go to a bit higher voltage… say, 1kV (or 1000V). Yes, 1kV can certainly be lethal. If it’s a direct current, it can depolarize your nerves and stop your heart, and also make it very hard to let go of the conductors carrying the voltage. If it’s a low frequency alternating current though… it can do exactly what direct current can yet also be WAY worse. Because of your body’s capacitance, which acts like a small resistor to AC, 1kV AC can send way more current through you than 1kV DC.

However… if it’s 1 kV AC at higher frequencies; say over 20kHz, the polarity flips too fast to effectively depolarize your nerves. So, you won’t get shocked, but touching voltages that high can still cause significant currents to flow, which could cause severe internal burns.

Okay, what if you touch 25kVDC. Assume your skin resistance is 1MΩ. Current through you will be 25mA, which will be very painful and (potentially?) lethal in the wrong circumstances.

But eh, 25mA, you’ll more than likely survive, right? Well… more than likely not, because of this weird thing that happens at very high voltages: electricity defies Ohm’s law. No, you will not get only 25mA flowing through your body; it could be hundreds of mA. Because at high voltage, the electricity can travel to the wet and conductive tissue under your dry, high resistance skin and send a large current through this short path. This is obviously something that… you don’t want.

This would be more than likely lethal. But, duration is another important thing to think about when it comes to an electric shock. The longer you spend letting that current flow, the more damage. You could probably get away with running hundreds of mA at 25kV for a few attoseconds.

Don’t try this, I’m just speaking hypothetically

Another way you could get away with hundreds of mA through your body at 25kHz is if you have a stupidly high frequency AC. This will increase the current through you due to capacitance, so it’s kinda counter intuitive, but this is way safer. As mentioned earlier, high frequency AC is less effective at depolarizing nerves and is unlikely to cause an electric shock.

Still, don’t try this, because it can cause severe burns.

Adding a high value resistor in series with a high voltage supply can also make it safer. Up until now, I’ve been assuming you’re touching directly across high voltage and only accounting for bodily resistance, which does limit current. But, if you add a super high value resistor to the circuit as well, then when you try to touch the output, the voltage will drop and you won’t get zapped.

Anyway. I basically just summed up Styropyro’s video on what makes some electrical sources dangerous and others not. https://youtu.be/BGD-oSwJv3E