Protecting load devices from the effects of low-side surges

Wherever lightning and poor utility power system grounds exist, distribution secondary systems are subjected to high-voltage surges due to lightning current seeking ground through low-voltage circuits. Utilities are becoming aware of this low-side surge phenomena and are taking measures to protect their distribution transformers´ secondary windings. These measures can increase the voltage stress at the customer service entrance. If any ground paths exist on the customer side of the service entrance, surges can penetrate further into the customer´s system and damage loads. Damage caused by low-side surges can be avoided if properly coordinated arresters are installed at the transformer secondary, at the service entrance, and at load devices. This work describes the secondary surge phenomena and the importance of protecting the service entrance and critical load devices effectively, especially when the transformer secondary is protected. A properly coordinated and effective protection scheme is described and recommended