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ESD Electrostatic discharge (ESD) occurs as a result of the buildup of charge from the contact and separation of two insulating materials discharging when brought into close proximity to another object of lower potential. A person walking across a carpet can generate an ESD of some 15kV or more. If a PoE installation is subject to ESD the voltage is usually dissipated to earth. ESDs are characterized by very rapid rise times (typically less than 1ns) and a total duration of around 150ns. A related source of electrical transient is a cable discharge event (CDE). Such events occur when a passive Ethernet cable is charged by dragging it across a carpet or from contact with a human who has previously become charged. Connecting a charged cable to a PoE system discharges it into the network. CDEs expose the PoE installation to much higher levels of energy than ESD events because charged cables have a much higher (distributed) capacitance than that typical of an electrostatically charged human. (The actual capacitance of the cable depends on its length and height above ground.) Fast voltage/current transients Fast voltage/current transients occur as a result of arcing across the switches and relays which is common in the commercial and industrial environments where PoE systems are typically installed. The energy from the arcing event is transferred into the Ethernet cable forming the PoE system by capacitive coupling. A typical event reaches peak voltage in around 5ns with the voltage declining to 10 percent of its peak value in 100ns. (Figure 1) Fast voltage/current transients are characterized by a series of high-voltage spikes in the kHz frequency range. The short- circuit current value is calculated by estimating and dividing the open-circuit voltage by a 50Ω source impedance. (The relatively high frequency of the repeating voltage spikes can add to the challenge for the design engineer because they can also compromise the integrity of the PoE communication system.) Voltage/current surges Voltage/current surges present the biggest threat to the sensitive electronic components of a PoE implementation because they expose the system to the highest currents for the longest duration. (However, the rate of current/voltage rise is less steep than other electrical transients such as ESD.) Sources of these voltage/current surges include induced voltages from nearby lightning strikes or load switching of high current power systems. The magnitude of the transient depends on whether the cable is situated indoors or outside, and the problem can be exacerbated by the long cable runs typical of modern installations which act like antenna coupling energy from close proximity electrical transients into the network. (Figure 2) Digi ® XBee ™ Cellular LTE Cat 1 MOV SERIES LEARN MORE 4 • Offer a choice of varistor voltages from 18V to 1820V and Vrms voltages from 11V to 1100V • Feature high current handling, high energy absorption capability, and fast response times • Protect against overvoltage transients such as lightning, power contact, and power induction | 5 | Figure 1: Characteristic of typical fast voltage transient. (Source: Mouser) Figure 2: Characteristic of typical surge current transient. (Source: Mouser)

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