Need for Multiple Layers of Protection.
A cascaded installation with Surge Protective Devices (SPDs) at the Service Entrance and downstream at the protected equipment Distribution and Branch Circuit panelboards is required for effective surge protection. This coordination using multiple SPD units, provides a protective barrier at the Service Entrance to protect from externally generated transients, i.e. lightning, utility grid switching, high winds causing power line arcing, and electrical accidents. These sources represent a low percentage (less than 20%) of all transient activity, but can be of typically higher magnitude causing immediate hardware damage to electrical systems and destroying sensitive electronic equipment.