Mitsubishi Electric Q26UD(E)HCPU User Manual page 668

(4) Isolation transformer
An isolation transformer has an effect on reducing conducted noise (especially, lightning surge).
Lightning surge may cause a malfunction of the programmable controller.
As a measure against lightning surge, connect an isolation transformer as shown below.
The use of an isolation transformer reduces an impact of lightning.
666
Programmable
Main
controller power
power supply
supply
100VAC
200VAC
Relay
terminal block
I/O power supply
Motor power supply
Inside a control panel
Isolation
transformer
Programmable
controller
T1
I/O equipment
Motor equipment