I think your question is probably more like how YOU can protect the PLC from outside inteference?
The PLC is tested to withstand the following environmental stress:
a. Storage temperature: -20 to +85 degree C
b. Running temperature: 0 to 70 degree C.
c. Noise resistance: IEC801-4 (Fast transient)
2KV to power supply, 50 microsecond pulse width, 1 minute
1KV to I/O by capacitive coupling, 50 microsecond pulse width,
1 minute.
These specs are usually good enough for most industrial applications without additional shielding. However, like all electronics devices if your environment contains high voltage spike, lightning strike, high energy electromagnetic pulses, high current noise generator such as VFD then you may have to take additional steps to shield them from such interference. This includes filtered power supply, grounded metal shield, separation of noise generator from control devices into different control panel - the usuall industrial practices.