This standard applies to two or three terminal, four or five layer, thyristor surge protection devices (SPDs) for application on systems with voltages equal to or less than 1000 V rms or 1200 V dc. These protective devices are designed to limit voltage surges on communication circuits and on power circuits operating from direct current (dc) to 420 Hz. The thyristor SPD can be manufactured with unidirectional or bidirectional, symmetrical, or asymmetrical V-I characteristics. This standard contains definitions, service conditions, and a series of test criteria for determining the characteristics of a thyristor SPD. If the characteristics differ with the direction of conduction, each polarity shall be separately specified. New IEEE Standard - Inactive-Reserved. This standard applies to two or three terminal, four or five layer, thyristor surge protection devices (SPDs) for application on systems with voltages equal to or less than 1000 V rms or 1200 V dc.