Successfully deploying PoE and POH
A BICSI presentation at the recent Integrate expo and conference in Sydney provided some much-needed guidance on cabling for PoE and POH applications.
Power over HDBASE-T (POH) shares similar characteristics to Power over Ethernet (PoE), so the same cabling principles apply.
Put simply, POH/PoE delivers up to 100 W of power from a digital power source (eg, Ethernet switch) to a powered device (eg, camera, digital display) over all four pairs of the data cable, simultaneously with data transmission.
Power is applied as ‘common-mode’ voltage, whereas data is transmitted using ‘differential-mode’ signalling, so data isn’t affected by power and vice versa. A centre tap on the Ethernet pulse transformer extracts the common-mode voltage for power at the receiver.
Done right, POH/PoE is a brilliant means of using existing cabling to power equipment without needing AC mains cabling.
PoE/POH can introduce problems if the cabling can’t handle the power. That’s because data cabling was originally designed to just carry burst power, not continuous currents and voltages.
Delivering 100 W can mean 0.5 A per conductor at 50 V, and that creates heat. Large cable bundles carrying that much voltage and current yield significant temperature rise and eventually fail to transmit data and/or power. It’s like drinking hot coffee through a paper straw — it’s not fit for its intended purpose and eventually falls apart.
The same is true of the thin gold-plated contacts in RJ45 connectors — they were never designed to carry high currents, which deteriorate when arcing occurs when disconnected under load.
Solutions for existing cabling
This is perhaps the riskiest deployment of POH/PoE, because you can’t just assume existing cabling will support POH/PoE. Thankfully, you can quickly get a reasonable idea by inspecting the cabling.
Start by reading what’s printed every metre along the cable — conductor diameter (often listed as AWG or American wire gauge) and temperature rating (listed as XXoC).
Conductors of 24 AWG or less have a decent chance of supporting higher currents without generating too much heat. Data cables are commonly 60°C rated. Since temperature is directly related to insertion loss, it’s important not to exceed the cable’s maximum temperature, so some 60°C-rated cables in high-ambient temperature environments may not support POH/PoE. In such cases, 75°C-rated cables would be far better.
Resistance unbalance is a major issue for POH/PoE, so it’s recommended to test the cabling for that with a suitable network analyser/cable tester. These devices should have the right metrics in them to immediately determine ability to support POH/PoE or not.
A quality installation also contributes to supporting POH/PoE, so inspect the cabling. Look for cable-bundle sizes — the bigger the bundles, the less likelihood to carry POH/PoE. Heat rise in cable bundles is the biggest concern for POH/PoE, so bundles should be limited to no more than 24 cables. Also look for adequate spacing for heat dissipation between bundles.
Solutions using new cabling
Installing cabling affords you the opportunity to factor POH/PoE, and this all involves making informed choices:
- Choose cable expressly designed for PoE/POH with minimum conductor diameter and higher operating temperature ratings. Most ‘PoE-rated’ cables are 23 AWG (0.26 mm2) and 75°C rated.
- Choose connectors expressly designed for PoE/POH, with pin-contact profiles to minimise arcing on disconnect.
- Choose appropriate cabling support that allows adequate spacing for airflow and heat dissipation. Explore the options afforded by catenaries, mesh trays, open/closed trays, open/closed ducts and sealed/unsealed conduits.
- Limit cable bundles to no more than 24 cables. It may even need to be less, so ‘do the math’ to work out the maximum number of cables in a bundle to limit heat rise. For large cable runs, it’s best to install multiple bundles, reasonably separated to allow airflow and heat dissipation.
- The longer the cable, the greater the potential heat rise. So it may be necessary to reduce long cable lengths to minimise the effect of resistance on heat rise over distance.
- Consider using a higher category cable — eg, Cat 6A instead of Cat 6 — as the conductors are often bigger and the materials are less susceptible to heat and have a larger cable cross-section, all of which provides better heat dissipation.
We referred several standards to the AV technicians for more guidance on cabling for POH/PoE, and we recommend them to you too:
- IEEE 802.3bt for PoE;
- IEEE 1911.2 for POH;
- AS/NZS 11801.1:2019 for cabling design;
- AS/CA S009:2019 for cabling installation (Wiring Rules); and
- AS/NZS 62368.1:2018 for AV/ICT equipment safety requirements.
You may not need to buy and read all standards thoroughly, but at least be familiar with them and learn more about what’s said about cabling for POH/PoE.
The BICSI presentation reinforced the importance of checking and choosing to AV technicians — following that advice will go a long way to helping successfully deploy POH/PoE and mitigate the associated risks. Anyone wishing to deploy POH/PoE should follow that same advice.
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