EMC Question of the Week: September 2, 2024
On a dry day, a person can experience an electrostatic discharge when reaching out to grasp a metal doorknob in a wooden door because the doorknob has a
- resistance to the earth
- capacitance to the earth
- inductance to the earth
- safety ground connection
Answer
The best answer is “b.” The doorknob accepts charge quickly because of its self-capacitance (also called its capacitance to the earth). Charge flows from the person to the doorknob until they both reach the same electrical potential.
Yes, the doorknob has a resistance and inductance to the earth, because even a wooden door has some amount of conductivity. Over time (perhaps milliseconds), the charge on the doorknob will dissipate. But the relatively high resistance of the door prevents the charge from moving quickly enough to allow an electrostatic discharge. This is the reason you don't experience a discharge when you reach out and touch the wooden surfaces of the door.
Finally, it would be extremely unlikely for a doorknob in a wooden door to have a safety ground connection. And even if, for some reason, the doorknob did have a safety ground connected to the earth, that connection would have inductance. The initial flow of charge (and the reason for the ESD event) would still be the doorknob's capacitance.
Note: The term "capacitance to earth" can be a little misleading, because the doorknob would accept the same amount of charge if it was on the moon or in the middle of an empty universe. For this reason, it is often more instructive to use the term "absolute capacitance" or, in a system of conductors, "self-capacitance".
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