Power over Ethernet (PoE) – Finding the Right Fit

PoE is not a new technology; but its adoption is expanding quickly, as increased energy efficiency has become a common consideration when developing any new electrical product. Power over Ethernet (PoE) allows devices to be powered efficiently over 4 pair CAT cables. These cables are installed to provide power and control to dozens of devices in technologies such as lighting systems, access control, security systems with cameras, access points for wireless networks etc. Though the use of the PoE technology is seen as a cost effective solution, there are number of factors that must be considered to ensure the chosen CAT is the correct fit.

Type 1 PoE
The type 1 PoE is commonly known as 2-pair PoE, it utilizes two pairs to connect many types of low-powered devices to the network. Based on the initial IEEE 802.3af-2003 standard, it provides up to 15.4W of DC power to each PoE port (up to 12.95W of power for each device). PoE Type 1 can support devices such as VoIP phones, sensors/meters, wireless access points and static surveillance cameras.

Type 2 PoE
Type 2 PoE is commonly known as PoE+/PoE Plus. Higher-powered devices are connected to the network using type 2 PoE, based on the IEEE 802.3at-2009 standard. It is backward compatible (meaning that it can also support the types of devices typically supported by PoE Type 1), and provides 30W of DC power to each PoE port (up to 25.5W of power for each device). PoE Type 2 can support devices such as more complex surveillance cameras that pan, tilt or zoom, as well as wireless access points, LCD displays, biometric sensors and tablets.

Type 3 PoE
The third type of PoE is well known for 4-pair PoE, 4P PoE, PoE++, UPoE. PoE Type 3 uses all four pairs in a copper cable. It provides 60W of DC power to each PoE port (up to 51W of power for each device). PoE Type 3 can support devices such as videoconferencing system components and building management devices.

Type 4 PoE
PoE type 4 is commonly known as higher-power PoE. The growing power requirements of network devices are pushing the need for higher power delivered through network cabling – which is where PoE Type 4 comes into play. It provides up to 100W of DC power to each PoE port (up to 71W of power for each device). PoE Type 4 can support devices such as laptops and TVs.

PoE Cable Safety Standards
The table illustrated below resides in the section 725 and 800 of the NEC (National Electrical Code), which shows that not every cable is supportive of the high power PoE types.

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The above table clearly illustrates the characteristics of each cable, including conductor gauge and temperature rating, so that a safe power rating can be established for the cable. The goal of the above table is to ensure that the cables chosen for PoE can be based on how much power they can safely handle.

Key Parameters when selecting cables for PoE
There are four main key parameters for each cable type, which determine the cables capability to support PoE applications. Each one of these parameters is vital to ensure reliable PoE operations.
-Temperature rating
-Cable construction
-Conductor Size
-Installation type

  •  Temperature rating
    The rating temperature is often confused with ambient temperature (cable immediate surrounding temperature), especially in the user base. The reason for this is a past oversight of all standard organisations. The currents were so low, that in fact the temperature difference, rating temperature minus ambient temperature, (Temperature rise) was so small that it fell into the temperature range of measurement accuracy but, most of the deployed cables are rated to 60°, 75° and 90°C conductor surface temperature. As the temperature of a cable rises, the electrical performance degrades, as excessive temperature rise in a cable can be detrimental to the cable’s physical and longevity. Shielded constructions have been observed to be significantly less affected by temperature induced electrical degradation than unshielded constructions.
  • Cable construction
    The rise in temperature of a cable in PoE application depends on the overall construction of the cable. As mentioned previously, metallic shielded cables have shown to dissipate heat better than unshielded twisted pair cables. Therefore it has been established that the presence of metallic shield or foil help dissipate heat more efficiently.
  • Conductor Size
    In PoE applications the conductor resistance drives the amount of heat generated in the cable. It has been observed that the larger conductor size in CAT 6, 6A and 7A reduce the DC resistance, resulting in power lost within the cable itself. The overall reduction in conductor resistance is directly proportional to the ratio of reduction of dissipated power within the cable. CAT 6 cables have been observed to have about 80 percent of the direct conductor resistance compared to CAT 5e.
  • Installation Type
    The installation also plays a vital role as it affects the degree of thermal resistance. Large cable bundles, bundles in tight or close places that trap heat within the cable have the tendency to have higher thermal resistance and higher conductor temperatures. The impact of bundle size in cable installation is very significant, as incremental temperature increase due to an increase in bundle size regardless of cable category and construction.
    In conclusion, more and more devices are now offering PoE as a possible data and power source, which means that the infrastructure of buildings are evolving rapidly. Keeping the future of PoE in mind, shielded CATEGORY cables such as CAT6 and CAT6A offer some of the best heat dissipation performance available and also a wide range of application support.

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