Whether to install SPD or not is generally determined by two aspects. First, it’s the probability of equipment being invaded by surge, such as local thunderstorm days, system environment, equipment location, etc. Second, it’s the importance of the equipment, such as the value of the equipment itself, the safety requirements of the system, the importance of the equipment in the system control, etc. 

Standards refered:

1、General standards

IEC 62305-4 2010, Protection against lightning - Part 4: Electrical and electronic systems within structures

2、SPD selection guide

IEC 61643-12, Low-voltage surge protective devices - Part 12: Surge protective devicesconnected to low-voltage power distribution systems – Selection and application principles.

IEC 61643-22,Low-voltage surge protective devices - Part 12: Surge protective devices

connected to telecommunications and signalling networks - Selection and application principles.