QT designed instrumentation
A (wrist)groundstrap can be used to prevent damaging sensitive circuits
by the person handling them. However it's purpose is to prevent ElectroStatic
Discharge (ESD). By mechanical friction people can be charged up to 25kV.
If they touch another object the effect of charge displacement (or discharge)
is called ESD. For safety reasons a 1Mohm series resistance is placed in a
groundstrap's grounding lead. It is therefore still possible for a person with
a groundstrap to be on a potential of several volts by capacitive coupling to the power-lines.
Below are scope pictures of the induced voltage when touching a 10Mohm resistor without/with
wearing a groundstrap.
induced voltage (by capacitive coupling, no ESD) on a 10Mohm sample when touching it, no groundstrap
induced voltage (by capacitive coupling, no ESD) on a 10Mohm sample when touching it, with grounstrap