Pairing Data And Power

Electrical contractors that are actively involved in the energy services market know the importance of staying abreast of technological advancements and product developments. Staying on top of industry trends enables ECs to provide customers with state-of-the-art energy services, which afford a sustainable competitive advantage. One trend worth watching is power over Ethernet (PoE), which is becoming increasingly popular and making inroads into more building systems.

What is PoE?

In PoE, devices connected to a building’s structured cabling system can be powered directly from the Ethernet switch while maintaining a full data link between the switch and device. The connected device’s power, communications and control are all supplied from the Ethernet switch over a standard unshielded twisted-pair (UTP) cable, which provides a number of advantages for the customer.

First, PoE eliminates the need to power the connected device with the building’s power distribution system. PoE usually eliminates the need for a 120-volt (V), single-phase outlet and a power inverter. In addition to saving money here, PoE cabling, just like standard Ethernet cabling, is a Class 2 power-limited circuit per Article 725 of the National Electrical Code (NEC) and normally is not required to be installed in raceway. As a result, the initial installation and future device moves, adds and changes will be much easier, faster and cheaper.

Second, since the device receives its power directly from the Ethernet switch, if one is powered, so is the other. This is important in cases such as voice over Internet protocol (VoIP) telephony where a loss of the building branch-circuit supplying the telephone unit will result in a total loss of communication. To avoid this problem, the branch circuit serving the device would need to be a circuit originating from an uninterruptible power supply (UPS), further complicating both the original installation and future moves. With PoE, if a UPS powers the Ethernet switch, all the connected devices also will have uninterruptible power.

PoE standards

There are two industry standards for PoE. The Institute of Electrical and Electronics Engineers (IEEE) published the first PoE standard in 2003 as IEEE Standard 802.3af. IEEE Standard 802.3af used two pairs of the four-pair Cat 5 cable to transmit power. The standard power rating at the source was 15.4 watts (W). A number of device-makers adopted this standard and saw the need for higher power to support additional device functions and more power-intensive devices. As a result, in 2009, IEEE Standard 802.3at, also known as POE+, raised the power rating at the source to 30.0W. Both standards were incorporated into the IEEE’s general Ethernet standard 802.3 when it was updated in 2012.

PoE’s popularity continues to grow as more manufacturers explore its potential. Again, there is a demand for increasing the amount of power that PoE can deliver. To meet this demand, in 2011, Cisco Systems Inc. introduced its Universal PoE (UPoE), which increased the power available at the source to 60W. The UPoE increase in power output opened the possibility of using PoE to power a whole new group of devices that are either Ethernet-connected or controlled and those that need more than the previous maximum of 30W. UPoE uses all four conductor pairs in a Cat 5 or 6 cable to transmit power to reduce conductor heating and signal losses. Data transfer for communications and control uses the same conductor pairs with the power. This April, the IEEE also established a study group to consider developing a standard for using all four pairs of conductors in a UTP cable for PoE to increase power transfer like in Cisco’s UPoE system.

PoE and building systems

As PoE’s power-transfer capability increases and approaches the limit for Class 2 power-limited circuits, so will its applications in commercial, institutional and industrial facilities. The ability to provide power, communications and control through a single UTP cable (without needing raceway) to a building load is very attractive in itself. However, add monitoring and control of that building load individually using software, and this technology’s value increases by improving building system integration and efficiency. For example, consider the light-emitting diode (LED), which is increasing in popularity as well. LEDs require direct current voltage and have very low power requirements, and they need to be controlled. These characteristics make LED lighting an ideal load for PoE. Currently, a number of lighting manufacturing firms are developing LED lighting equipment and systems for PoE applications, and some of those products are commercially available. PoE coupled with LED lighting and intelligent control systems could completely revolutionize how we illuminate buildings in the coming decade.

About the Author

Thomas E. Glavinich

Freelance Writer
Thomas E. Glavinich was an associate professor in the Department of Civil, Environmental and Architectural Engineering at the University of Kansas. His tenure as one of Electrical Contractor's most trusted and reliable source of industry research end...

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