Power Over Ethernet Interoperability Guide

Brief Overview of Overloads and Shorts as Per AF Standard

In Fig. 6.1 we show what happens as per the AF standard, when the port current I_PORT, ratchets upward. I_CONT is the maximum continuous current rating and equals 15.4 W/V_PORT (default Class 0/3 assumed). In the figure we also see I_CUT, the overload current, and T_CUT, the overload timer (which later became T_OVLD in the AT standard). In addition, the AF standard has a T_LIM timer of 50 to 75 ms too, which starts when the current limit is encountered. As indicated, in the figure, we have sketched the actual I_CUT and I_LIM of a given chip, as well as its actual T_CUT. As the port encounters these levels, timers are actuated. If for example, the overload persists long enough that T_CUT timer times ...