Surge Protective Device (SPD): Also known as a surge protector, over-voltage protector, or commonly referred to as a lightning arrester or surge arrester. It is primarily used to protect electrical devices from damage caused by lightning-induced electromagnetic pulses or operational over-voltages. It limits transient over-voltages that surge into power lines or signal transmission lines to a voltage level that the equipment or system can handle, or diverts powerful lightning currents to the ground, thus protecting the equipment or system from damage due to such surges.