What minimum (Imax) rating is required for SPDs at the service entrance?

The minimum (Imax) rating required for Surge Protective Devices (SPDs) at the service entrance is based on the expected maximum fault current and the protection level necessary for safety and equipment reliability. A rating of 40 kA/20 kA reflects a robust standard in the context of lightning protection and surge suppression.

This rating signifies that an SPD can handle a surge current of up to 40 kA without failure at the main service entrance, which is a critical point in a power distribution system where the electricity enters a building. In the event of a lightning strike or other high-energy surge event, having a SPD with this rating ensures that it can absorb significant surges and protect downstream equipment from damage.

The selected rating represents an appropriate balance of protection and survivability against the most common surge events encountered in typical electrical installations. It assumes a baseline level of risk while integrating the necessary components to mitigate potential harm to the electrical system.

Understanding these ratings helps in selecting the right SPDs for specific installations, which is essential for maintaining the integrity and longevity of electrical equipment. The other options provide various ratings; however, they do not match the current standards required for effective surge protection at the service entrance.