The transient overvoltages or surge events that are described and discussed in this guide are those that originate outside of a building or facility and impinge on a power distribution system (PDS) through the service entrance conductors. Transient overvoltages or surge events that originate from equipment within a specific facility are not within the scope of this document. This guide applies to surge-protective devices (SPDs) that are manufactured for connections to 50 Hz or 60 Hz ac power circuits that are rated between 100 V rms and 1000 V rms. This guide applies to SPDs that are specifically identified, labeled, or listed for connections on the load side of the service entrance main overcurrent protective device. This guide does not cover those SPDs identified, labeled, or tested as a secondary surge arrester intended for connections on the line side of the service entrance main overcurrent protective device. The SPDs covered in this guide are those manufactured for use in an association with electrical power distribution equipment such as load centers, motor control centers, panelboards, switchboards, switchgear, and end-use equipment installed in commercial and industrial facilities. This guide excludes SPDs associated with retail and consumer appliances and components for residential use. This guide does not specify or set limits on insulation levels of any components associated with power distribution systems or end-use equipment. In addition, it is not the intent of this guide to address individual SPD component specifications associated with any specific manufacturer of surge protection products.The SPDs discussed in this guide contain at least one nonlinear component for either diverting surge currents and/or dissipating surge energy. Examples of such nonlinear components are metal-oxide varistors (MOVs), silicon avalanche diodes (SADs), spark gap tubes, or thyristors. Ferroresonators, motor-generators, uninterruptible power supplies, and filters containing only inductive or capacitive components are not considered SPDs in the guide.