Are anti-static wrist bands really safe?

Question

Is grounding oneself using anti-static wrist band (connected to ground wire & having a protective resistor) still dangerous in case a very high current flows through the ground wire (due to some short-circuit/ faulty lines)?

Will the power rating of the resistor (generally 1 Mega ohm 1/2 Watt resistor used) be sufficient in case a very large current surges through the ground wire & a person is grounded with it (via anti-static wrist band)? Or do we need to use a higher Watt resistor (power rating > 1/2 Watt) to take care of this particular situation?

Answer 1

Think this out a little bit. The wrist strap is there to tap off static you've built up through your clothing, shoe interaction with carpet, etc.

In a scenario where you grab hold of a 220V cable and are safely grounded through nothing but that wrist strap, if we were to assume that your body magically had zero resistance, then 0.00022 Amp would flow through that 1M Ohm resistor to ground (i=E/r or 220V/1,000,000 Ohms) saving you from heart fibrillation. It is of high resistance because that prevents you from seeing a high current flow at lines voltage but presents a ready path to ground for static.

That 1M Ohm resistor would be dissipating 0.0484 Watt if it were to bridge a 220 mains by itself (P=E*i or 0.00022A*220V), so the day one burns out is the day that Ohm's law has collapsed into the dust or that you were really unlucky and met up with something in the kV range.

Moral of the story, Wrist straps are there to prevent you from zapping silicon. System grounds are there to protect you from having hot chassis conditions that cause you to become the ground circuit. Shields, covers and interlocks are there to prevent you from sticking your hands into mains voltages which could zap you and kill you if your wrist strap was some piece of cheap junk that didn't have a proper high resistance, low wattage resistor in it.

One of our avionics shop routines is to check all new wrist straps for proper resistance (quality check that they're made properly) and all used ones to make sure the conductor path didn't break.

Answer 2

An anti-static wristband is meant to protect electronic components from being damaged by static electricity from your body. Ever shuffle your feet across a carpet and then "zap" the next person you touch? That amount of current is harmless to a human, but potentially damaging to electronic parts, so the wristband prevents that.

It's not meant to protect you from being shocked by your electronics. If you're concerned about faulty wiring causing "very large current surges", you need to consult a qualified electrician. Don't rely on an anti-static wristband to protect you from harm.

Answer 3

A "ground" is, generally speaking, where you want the electricity to go. Otherwise (and I'm grossly oversimplifying) it basically goes "everywhere".

If you wear a grounding strap, virtually all of the electricity goes into YOU. Which, when you're working with the extremely low amounts of electricity generated by static whilst you're working on a computer, is less harmful to you than it is to the components you're messing with (you literally won't even feel it most of the time, it can fry unprotected electronics).

However, in a situation where you're working with more power (something plugged into a wall, or even the unshielded capacitors inside a computer's power supply - which is why you're NEVER supposed to open them up), all that electricity going into YOU is a major problem - one that usually leads to a charbroiled technician.

So in a nutshell: DO use grounding straps in situations where the voltages/amps are very low and dangers of shock are more concern to the machine than you. Know that you WILL take all the shocks.

DON'T use a grounding strap in situations where your body can't take the potential shock.

When in doubt, don't use a strap. Accidentally frying a component inside the computer isn't nearly as bad (or as expensive) as accidentally frying the technician.