can often be avoided through monitoring. Operational technology (OT) plays an important monitoring role in this context. OT comprises hardware and software that is used to control and monitor devices and systems all over the managed building, including their processes. Unlike IT-based monitoring models, OT uses proprietary technologies, interfaces and protocols. The operating technology virtually controls itself in a closed system according to its own protocol. If, on the other hand, equipment or machine control is carried out with the help of IT, error messages are sent to a central location. These are then rectified directly via control mechanisms or forwarded to another instance to solve the problem. In the past, OT systems were isolated from the IT system usedin companies. However, digitalisation, industrial networking and the Internet of Things are increasingly causing a convergence of classic IT and OT. IT/OT convergence offers many advantages but is also associated with challenges and risks. Convergence is the basis for Industry 4.0 and enables solutions such as predictive maintenance and predictive monitoring. OT in critical infrastructures requires special protection. Typical requirements for OT are high reliability, security and availability. In addition, real-time capable processes must be possible. In order to integrate all systems, even those without a direct or difficult power connection, into the monitoring, PoE lends itself as a network infrastructure. This is also where a lot of potential for PoE lies. DC grids as illustrated in Figure 3 are central DC network infrastructures in industrial or production facilities. Here, all components are supplied within a self-contained, self-sustaining system via a DC voltage source, and not via the common AC grid as was previously the case. There are various reasons for adopting DC grids, first and foremost energy efficiency, cost reduction and energy flexibility in production. 5. PoE COMPONENTS AND STANDARDS The backbone of PoE-based smart solutions is the cabling infrastructure with fail-safe, high-performance copper cables and connectors. This networks sensor devices and lighting elements with the PoE switches and gateways as well as LED drivers. In contrast to the classic installation with different types of cable, with PoE only one type of cable is sufficient, which is compatible with all PoE-based devices. Most of the electronics are in the central rack. Only small parts of the LED driver are installed in the ceiling, which, by the way, is where about 70 percent of PoE installations are located. (Figure 4) With PoE topology, users have the choice between midspan and end-span networks. The dominant end-span architecture uses a network switch that supports PoE. Mid-span, on the other hand, uses an external PoE injector installed between a non-PoE switch and a powered device (PD). Both types of architecture make the transition from the old network environment to a PoE network much easier. PoE is defined in several standards which allow for different levels of power depending on the standard. Table 1 provides an overview. After the IEEE 802.3af standard in 2003 and IEEE 802.3at in 2009, the IEEE adopted the IEEE 802.3bt standard in the third quarter of 2018. By implementing to a DC grid, energy generated on site, both from renewable sources and recuperative processes, can be stored more efficiently. In times of massive energy policy rethinking and technological progress in energy storage and power electronics development, sticking to conventional AC/DC power supply is no longer the preferred choice. This is because, in addition to a significant reduction in costs and a saving in energy demand, a power supply via DC Grid is more robust with regard to fluctuating grid quality. It is also able to react flexibly to changing energy supplies, especially when buffering peak loads and bridging grid failures. PoE proves to be a suitable technology for DC grids to supply power to all participating powered devices. The maximum power that can be transmitted via PoE integrates well for the operation of DC grid infrastructures. Thus, the increase and growing acceptance of DC Grids is an important driver for the installation of PoE infrastructures. 4. PoE AS SUBSTITUTE FOR OPERATIONAL TECHNOLOGY (OT) In building management, devices and machines are typically subject to consistent monitoring. On the one hand, to detect faults at an early stage and to keep the extent of the costs for the necessary repairs as low as possible. On the other hand, downtimes in the operating process Table 1: PoE standards and the different levels of power they allow for Standard Type Class VPSE (min) supply voltage Max. current per pair PPSE max. power of the supplier PPD max. power to the unit Pairs 802.3af Type 1 Class 1 44V 350mA 4W 3.84W 2 Class 2 44V 350mA 7W 6.5W 2 802.3at Type 2 Class 3 50V 600mA 15.4W 12.95W 2 Class 4 50V 600mA 30W 25.5W 2 802.3bt Type 3 Class 5 50V 500mA 45W 40W 4 Class 6 50V 500mA 60W 51W 4 Type 4 Class 7 52V 720mA 75W 62W 4 Class 8 52V 860mA 90W 72W 4 Figure 4 Classical Dali/KNX Typical commercial installation PoE base centralised implementation of LED 6 7
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