Por que toda a agitação sobre a certificação de potência limitada?
You maybe have heard the recent buzz about LP-certified cable for power over Ethernet (PoE) applications and be wondering if you need this cable for your PoE installations. Let’s take a closer look at this new UL certification, its relationship to the 2017 National Electric Code, why it was introduced and why you still need to test.
An Optional Alternative
The increasing number of PoE-enabled devices and increasing power levels associated with the upcoming IEEE 802.3bt PoE standard for 60W (Type 3) and 100W (Type 4) has given rise to concern about heat rise within cable bundles. When multiple cables are formed into a bundle, the cables in the middle of the bundle can’t dissipate the heat caused by PoE, which can cause the cables to exceed the specified operating temperature and result in poor network performance since heat is directly related to insertion loss. This is even more of a concern in warmer installation environments.
Following a fact finding study to investigate the effects of higher levels of PoE applied over cables in a bundle, UL introduced a Limited Power (LP) Certification to help simplify cable choice for PoE applications. The LP certification indicates that a cable has been tested to carry PoE under worst-case installation scenarios without exceeding the temperature rating of the cable. The certification accounts for large bundle sizes, high ambient temperatures and other environmental effects such as enclosed spaces or conduits.
It’s important to understand that LP is a certification, not a listing or a rating. So unlike other UL listings or plenum or riser ratings that are required by the NFPA 70 National Electric Code®, LP-certified cable is an option—not a requirement. And speaking of the NEC®, the 2017 edition contains new requirements that also address the heat rise issue—but only when power is greater than 60W (Type 3). For these cases, NEC includes ampacity tables that specify the maximum ampacity allowed for a certain cable bundle size, conductor gauge and cable temperature rating installed in an ambient temperature of 30°C (86°F). Because the NEC® is law, complying with these ampacity tables is required. However, the NEC does allow the use of an LP-certified cable as an alternative to following the ampacity table.
The good news is that you only need to worry about this issue if you’re planning to run PoE above 60W, and most PoE-enabled devices, including LED lights, require less than that. The bad news is that you never really know how much power might eventually be delivered over the cable so following ampacity tables or using LP-certified cable is a good method for future proofing. Other options include using cables with larger diameter conductors, higher temperature ratings or shielded construction, as well as simply choosing to not use cable bundles.
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Resistance Unbalance Still Matters
Regardless of how you choose to deal with potential heat rise in cables carrying higher levels of PoE, one cannot ignore the need to test for DC resistance unbalance which can distort the waveform of Ethernet data signals, causing bit errors, retransmits and even non-functioning data links. When DC power is delivered over a pair, it should be done using common-mode voltage so the current is evenly split and the resistance between the two conductors remains as close to zero as possible. That’s why IEEE Std 802.3-2012 specifies a maximum DC resistance unbalance of 3% between conductors.
While DC resistance unbalance is not typically an issue in higher quality cables that would have a higher temperature rating or LP certification, poor workmanship can still cause too much resistance unbalance. Thankfully, DC resistance unbalance testing with the DSX Series CableAnalyzers verifies that conductors have equal resistance to enable the common-mode current needed to effectively support PoE and avoid distortion of the data signals transmitting on the same pair. This becomes even more important as power increases and as the number of PoE devices running gigabit Ethernet continues to rise.