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Guard PoE systems from lightning surges, electrical hazards

Posted: 10 May 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Power over Ethernet? electrostatic discharge? thyristor?

Power over Ethernet (PoE) is becoming popular at a rapid pace. It eliminates the need for separate power supplies for connected equipment, the need to locate equipment near an AC outlet, and the need for separate power cables. Recent increases, in the amount of power that can be delivered by the PoE power supply equipment (PSE), from 15.4W (PoE) to 30W (PoE+), have only increased the number of potential applications. Ethernet now delivers sufficient power for VoIP telephony, to power extended-range wireless access points, and to operate surveillance cameras, where it is used to power "pan and tilt" functions.

Increased use of PoE broadens the range of locations in which Ethernet may be used. It ranges from strictly indoors to campus layouts (building-to-building) or even first-mile/ last mile telephony applications. These outside environments greatly increase exposure to lightning-induced surges and electrostatic discharge (ESD), not to mention accidental power faults due to inadvertent shorts to the AC power line. This article provides guidance on evidence-based design methods that protect PoE equipment from these electrical hazards.

PoE basics
Power is fed into the cable by power sourcing equipment (PSE), either through a switch (also called an endspan) or a midspan (if located anywhere other than at the end of the cable). Devices on the cable that consume power are powered devices, or PDs. These may also have auxiliary ports for backup in case PoE power fails.

PoE power levels. The IEEE 802.af PoE standard limits PD power consumption to 12.95W (360mA), which corresponds to a PSE output limit of 15.4W per port (400mA) after accounting for cable losses. This standard takes into account line losses for maximum loop lengths of up to 100m, thereby allowing up to 57VDC from the PSE. The nominal level is 48VDC.

The PoE+ standard, IEEE 802.at, allows the PSE to deliver up to 30W and the PD to accept up to 25.5W for Type 2 equipment (PoE+ Type 1 is equivalent to PoE); with the PSE supplying up to a maximum of 600mA. PoE+ also requires the use of low impedance wiring (12.5 ohms per loop pair compared to 20 ohms per loop pair for PoE), such as CAT5e or CAT6.

Companies are working to increase this even more; there are PSEs available that promise 60W per port, and one vendor sells midspan PSEs claimed to provide 95W per port using a proprietary discovery process. This may be approaching the physical limits on CAT5 cable, however, which means that for higher power (there has been talk of 200W PoE), some way around this will have to be found. One simple approach is to increase separation between bundled cables, to allow for improved heat dissipation. Beyond that, cable with heavier gauge conductors would be required. These higher voltages are neither IEEE 802.3af nor IEEE 802.3at compliant.

Table 1: PoE PD power classifications.

Table 2: PD power classifications and their signatures for PoE+ (IEEE 802.3at) * The PoE+ Type 2 returns a Class 4 classification signature.

Determining power requirements. It is important for the PSE to deliver the amount of power required for the PD (or PDs) it feeds without causing damage. To determine the power level required, the PSE and PD engage in a back-and-forth signaling handshake routine at turn-on that involves voltage pulses from the PSE that determine the impedance signature of the connected PDs; this discovery process sets the system to one of five classes (table 1). Table 2 below shows the PoE PD classifications for PoE+.

Figure 1: PoE Mode B power is applied over the "spare" data pairs found in 10BaseT or 100BaseTX systems or over pairs 4-5 and 7-8 of a 1000BaseT system. PoE uses the phantom powering technique so that a single pair carries a 0V potential difference between its leads; power supply voltage is derived as the difference between two different wire pairs.

PoE modes (phantom power technique)
Power can be provided in one of two ways over the Ethernet cable. In Mode B power is applied over the "spare" data pair (pair 4-5 & 7-8) found in 10BaseT or 100BaseTX systems since only two pair are used for data delivery (RJ-45 pins: 1-2 and 3-6 are used for data). This leaves RJ-45 pins, 4-5 and 7-8, available for power delivery (figure 1). Notice that PoE uses the phantom powering technique so that a single pair carries a zero DC volt potential difference between its leads. The power supply voltage is derived as the difference between the two center tap connections of the different wire pairs.

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