Siemens Simatic M7-400 Reference Manual page 600

Guidelines for Handling Electrostatic Sensitive Devices (ESD)
D.1 What is ESD?
Definition
All electronic modules are equipped with large-scale integrated ICs or components.
Due to their design, these electronic elements are very sensitive to overvoltages
and thus to any electrostatic discharge.
These Electrostatic Sensitive Devices are commonly referred to by the
abbreviation ESD.
Electrostatic sensitive devices are labelled with the following symbol:
Caution
!
Electrostatic sensitive devices are subject to voltages that are far below the
voltage values that can still be perceived by human beings. These voltages are
present if you touch a component or the electrical connections of a module without
previously being electrostatically discharged. In most cases, the damage caused
by an overvoltage is not immediately noticeable and results in total damage only
after a prolonged period of operation.
D-2
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07