Siemens Simatic M7-400 Reference Manual page 92

Power Supply Modules
Cover
The battery compartment, battery selector switch, voltage selector switch and
power connection are housed under one cover. The cover must remain closed
during operation in order to protect these operator controls and to prevent static
electricity from affecting the battery connections.
If you have to carry out measurements on a module, you must discharge your body
before you start the measurement by touching grounded metallic parts. Use
grounded measuring devices only.
3-12
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07