Power Over Ethernet Interoperability Guide

Secondary Discharge

This is a common occurrence in troubleshooting field returns. For example, PSEs with their 100 V ceramic port capacitors literally charred and cracked have shown up. How could that happen? To crack open a 100-V ceramic-port cap would perhaps require 2 or 3 times its rated voltage delivered instantantaeously on it. Surges could account for that, but the unit had correctly rated Y-caps and so on, so the differential voltage across the port could really never be that high. This could very well be a failure from secondary discharge. This is best explained by Teseq (formerly Schaffner Test Systems) on Page 9 of the Application Note at http://www.teseq.com/com/en/service_support/technical_information/01_Transient_immunity_testing_e.pdf ...