Issue link: https://www.e-digitaleditions.com/i/1347119
56 I ICT TODAY connection while transmitting PoE. These factors are especially of concern in a project where a wide variety of PoE-enabled devices require different wattage levels, including the highest levels of PoE. Power Needs The first generation of PoE, IEEE 802.3af Type 1, delivers 15 W of power, which can support lower-power devices like IP clocks, VoIP phones, and simple security cameras. The second generation of PoE, IEEE 802.3at Type 2, delivers up to 30 W of power, which is ideal for video IP phones, wireless access points (WAPs), RFID readers, and pan-tilt-zoom cameras. Both Type 1 and Type 2 PoE deliver power over two pairs of a balanced twisted-pair cable. Devices such as PoE lights and high-end video displays require higher wattage that typically ranges anywhere from about 40 W to 70 W depending on the device. These devices require higher level IEEE 802.3bt Type 3 that delivers 60 W of power and Type 4 that delivers 90 W of power, which both deliver power over all four pairs of a twisted-pair cable simultaneously with data. Maintaining the goal of having one type of cabling for an entire building and all devices can be achieved by designing and deploying the system to support the highest level of PoE. Heat Rise With higher levels of PoE comes the potential for heat buildup within cable bundles caused by the power dissipating as heat. When this occurs, the temperature within the cable can increase and lead to higher insertion loss that can degrade signal transmission, as well as cause long-term damage to the cable. The amount of heat rise can also depend on the size of the cable bundle—cables located in the middle of a large cable bundle have a more difficult time dissipating heat and can reach even higher internal temperatures. Higher ambient temperatures that can occur in the ceiling space can further increase these temperatures. Because insertion loss is directly related to the length of the cable, industry standards address these concerns by specifying insertion loss derating at elevated ambient temperatures above 20°C (68°F). This means that cable Maintaining the goal of having one type of cabling for an entire building and all devices can be achieved by designing and deploying the system to support the highest level of PoE. runs for PoE deployments may need to be reduced in length depending on the operating temperature and construction of the cable. For example, cables that are only rated to 60°C (140°F) will require greater derating than a cable that is rated to 75°C (167°F) or higher. In addition, shielded cable requires less length derating than unshielded because the metal of the shield features a higher conductivity that helps to dissipate heat out of the cable. The 2017 National Electric Code (NEC) also addresses heat rise within cable bundles carrying Type 3 or higher PoE by specifying the number of cables allowed in a bundle based on conductor size and temperature rating for a given ampacity per conductor. It can be difficult to determine derating requirements in large PoE deployments with varying wattage requirements, requiring the need to determine bundle sizes, various lengths and ampacities. In a large PoE-of-Everything deployment, this challenge can be overcome by selecting cable with a higher operating temperature and shielded construction. Contact Erosion It is not just the heat within the cable that can be of concern. Contacts in network connecting hardware, such as RJ-45 jacks and plugs, are engineered and plated (typically with gold or palladium) to ensure a reliable, low resistance mating surface. Unmating a jack-plug connection while transmitting PoE power, referred to as unmating under load, produces an arc within the spark gap as the current transitions from flowing through conductive metal to air before becoming an open circuit.