The Hazards of Passive PoE

By Ethernet Alliance


There is nothing more frustrating for the consumer or end-user than when they open up a product thinking it worked with a standard only to find that there are interoperability issues. This happens with lots of products, but it is particularly pervasive in Power over Ethernet (PoE) products. This is because the term PoE has evolved to be synonymous with IEEE 802.3 compliant Power over Ethernet standards (i.e. 802.3af or 802.3at).

The problem arises because products are free to state they support PoE simply because they merge data and power on the same Category cable. This doesn’t necessarily mean they are IEEE 802.3 compliant PoE, or even interoperable. One example of this is “Passive PoE”. IEEE standard compliant PoE is required to detect the desire for power before actually applying power, and avoids powering devices not designed to the standard. In a “Passive PoE” system, power is always present on the powering port. Another difference is IEEE 802.3 complaint PoE has specific allowable voltage and current ranges, along with which wire pairs power is supplied on. In “Passive PoE” voltage, current and powered pairs range across the spectrum, confusing users and even risking damaged devices.

What’s the problem with always having voltage present on the powering port? There are lots of Ethernet connected products that aren’t designed to support power at their Ethernet interface. What happens when you connect a “Passive PoE” port to these products? Usually, you let the ‘magic smoke’ out of some termination components and your product will no longer comply to emission limits required by many government entities – and you are sad about damaging an expensive piece of gear.

What if I use something with the wrong voltage range or current limits? Most likely, your equipment will not power because the voltage or current will be lower than required for the application. But the voltage could be higher, and could cause permanent damage (along with some more of that sadness mentioned before). If the “Passive PoE” device is a “PD” and the Power Source is IEEE 802.3 PoE, you simply won’t get power; but if the power source is another flavor of “Passive PoE”, who knows what could occur.

How do we solve this? One way is to make IEEE 802.3 PoE devices more easily identifiable through a certification program and a logo that demonstrates products are designed to a released standard; one that requires demonstrated compliance to a test plan. Luckily, the Ethernet Alliance saw the need for such a program and implemented one, see https://ethernetalliance.org/blog/2017/06/13/power-to-the-people-delivering-on-the-promise-of-poe/ for more information about the Ethernet Alliance PoE certification program.

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