Safety and Performance Concerns

Heat is the Main Problem

Organizations involved in wiring standards, electrical codes, and safety issues have expressed concern about PoE or PoH installations in which many twisted pair cables carrying power are bundled into the same tray or plenum space. The bundling and packing of cables in such spaces may reduce the potential for heat dissipation into the surrounding air and therefore increase the risk of cable degradation or fire hazards.

UL Offers a Further Specification

In an October 2017 IWCS presentation on PoH cable testing, UL’s Anthony Tassone said that temperatures greater than 100o C have been observed. The UL recently has developed a new specification, UL4299, which describes procedures for measuring and qualifying cables for PoH. Also in 2017, the HDBaseT Alliance and UL announced a certification program for PoH cables that will test cables against the UL4299 specification. The program is for cable manufacturers, and it will evaluate their cables’ ability to 100 W at certain current levels.

Safety Organizations Are Involved

The UL’s specification defines a “-LP” power level that can be safely used in bundles without risk of overheating. The UL’s research on this subject also is the basis of a new table in the 2017 publication of the US National Electrical Code (NEC). The table shows the relationship between cable conductor ampacity, ambient temperature, and cable temperature rating for wire sized between 22 and 26 AWG and for different bundle sizes (number of four-pair cables in the bundle).

Vendors Also Studying the Heat

In a November 2017 webinar organized by the IWCS, Thibaut Lanoe, senior product development engineer for Superior Essex, described his company’s work to analyze twisted-pair performance under the pending four-pair PoE (4PPoE) standard – also known as IEEE 802.3bt. Mr. Lanoe said main concerns of the standard will include temperature and safety of installation. Other concerns will be power efficiency, operating cost, lifetime of cable components – lifetime when exposed to higher temperatures.

Bundles at Higher Power Levels

Concerning safety, the main issue is injecting power up to 100 W. The issue is that dc resistance generates heat (the Joule effect). This results in lower voltage and power loss, and the components are exposed to higher temperatures. To study this, Superior Essex ran tests with 100 cables in a bundle. Mr. Lanoe said, “we think this is a typical number,” but he noted that UL runs tests with larger bundles.

Optimizing AWG and Materials

Superior Essex ran the tests with different category cable types, different conductor sizes (AWG), and different cable materials. Among the conclusions, power efficiency is affected more by gauge than temperature; and aluminium foil shielding can help with heat dissipation. The most effective combination appeared to be Cat-5e cable with 22 AWG and plenum materials. However, this is not the least expensive selection of AWG and cable materials.

Beyond 100 W? Too Soon To Say

Mr. Lanoe noted that the IEEE standard work was aiming to stay under 100 W, although he was aware of some studies to test the performance up to 200 W. He commented that he was not aware of any concrete plans for 200 W equipment. In general, his presentation indicated that work on applications using the IEEE 802.3bt standard or higher powers will require cable manufacturers to undertake studies to balance the AWG, materials costs, and other factors.

PoE has Distance Limitations

All of the standards specify 100-meter spans, although there are possible ways to extend this with mid-span power sources and repeaters. In 2014, TE Connectivity, now part of CommScope, developed a hybrid fiber-plus-power cable to substitute for PoE installations over spans up to 3 km. When TE officials presented the concept during sessions of the IWCS 2014 annual conference, they described it as a “PoE extension cord.” CommScope now refers to this system as its “Powered Fiber Cable System.” The design can support either single-mode or multimode fibres in a central tube. For the power, TE has versions with 12 or 16 AWG conductors to provide -48V dc power over different distance ranges. The result is a compact flat cable that incorporates fibre and conductors to deliver PoE functions over longer distance.

PoE and PoE+ Can Be Extended

CommScope offers this system with a mix of optical power (distance) and conductor sizes (power levels). The system is supplied with the cable, power supply, connection accessories, and a unit containing the fiber-to-TWP data interface, the DC power management, and electrical protection circuits. TE supplies the system with options for conductor size and DC-to-DC converters to cover a range of power requirements, or distances from 0.4 to 3.0 km.

Certifying the Equipment Too

In May of 2017, the Ethernet Alliance launched a PoE Certification Program. The goal is to assure that power devices and power sources from different equipment manufacturers are interoperable. The program also will allow users to rapidly distinguish between proprietary and standards-based equipment. The products are tested against the Ethernet Alliance PoE certification specification, which is based on the current IEEE PoE standards. Tests may be performed at the University of New Hampshire InterOperability Laboratory, or vendors can conduct their own tests using approved equipment. After successful tests, equipment vendors can use the Ethernet Alliance’s certification logo and can be listed to a registry of PoE products.

In December of 2017, the Ethernet Alliance said it would include pre-standard testing of products against the draft of the new IEEE 802.3bt standard. This will further help the expansion of commercial markets and stimulate demand for Cat-5e, 6, and 6A.