The Ethernet physical layer involves the Ethernet phy (or controller), a transformer and a RJ-45 connector as can be seen in Figure 2-1. Additionally there are shunt capacitors placed on the phy side while there is an RC filter placed on the center tap of the transformer towards the connector side. When placing an surge protection diode in the schematic, there is the option to place it before or after the transformer. On the connector side of the transformer, the common mode voltage is uncontrolled. In the typical application the voltages will be low but in power over Ethernet (PoE) or a high voltage line coupling onto wires extending from the connector can mean that the line is subject to very high common mode voltages. Therefore the selection of surge protection diodes is extremely limited to ones that have a high enough breakdown voltage to not turn on during the high common mode voltages but absorb the transients. Additionally, having surge protection devices mean that they have to take the entirety of the surge pulse. This is extremely problematic as surge events can go up to 6 kV with currents reaching 100 A. Extremely few surge diodes are capable of handling that much energy and would have so much capacitance that it would be impossible to have a high speed signal sent through.