Analysis of coordination between primary and secondary protectors

This paper provides circuit models for analyzing the coordination between primary and secondary surge protectors. Although the analysis is performed in the context of lightning surges on telecommunication circuits, the models developed are generic in nature and can be used to analyze the interaction of cascaded surge protectors under voltages produced either by lightning or by power line disturbances. Equivalent circuits are developed for calculation of voltages and currents at primary and secondary protectors. Formulas are provided and procedures are described for achieving the best coordination between primary and secondary protectors. Two different configurations are analyzed for protection coordination. In the first configuration, the primary and secondary protectors connect between each conductor of the cable pair to ground. In the second configuration, a secondary protector is located between two conductors of a cable pair while the primary protector is located from each conductor to ground.