Quote from: davidlawson on Today at 04:46:07 PMHi there!
I have a question regarding static discharge. My current understanding is that voltages are all relative – if you 'zap' someone when you touch them, it's because they had a different static charge to you.
This requires that both you and the other person are conductive. If you charge up an insulator, by rubbing a rubber balloon against a cat, for example, and then touch the balloon--no current will flow and no shock, because the charge in the rubber stays where it is. When you and the other person with a different charge come close together, your conductivity allows those charges to move. The electric field between the different charges causes them to concentrate in the part of your body that's nearest the person, like your finger. This concentration of charge gets greater the nearer you get to them, as the electric field gets stronger still. In effect you have formed a capacitor between you and them. But before you actually touch them the field overcomes the breakdown voltage of air, and the spark travels across the gap.
Voltages seem like they're relative, because the definition of a voltage is the integral of the electric field between two points. Charge isn't relative: an object is electrically either positive, negative, or neutral, without relation to any other object. Static charges are produced by triboelectricity, the greater affinity of some materials for electrons against other materials.
How does that work with circuits then? What if a PCB has a higher static charge than you, and you touch it while wearing a grounded wrist strap? If voltages are all relative, why do we assume it's safe to handle electronics when we just have no potential difference between ourselves and 'ground'?
This is an example of an unsafe ESD environment, as the short path to ground can cause high currents to flow in a device when static charges are present. To prevent ESD events the PCB must be kept in a static-safe (equipotential) environment, and many more precautions are needed besides a wrist strap. The use of a correct wrist strap with a 1 megaohm resistor would limit the discharge current in this scenario, however.