At SmartHomePerfected our information is accurate and unbiased. If you buy through links on our site, we may earn a small affiliate commission at no extra cost to you. Learn more

PoE (Power over Ethernet) is a superior alternative to Coaxial wires, in that it can transfer both power and data over a single ethernet cable with significantly reduced degradation. PoE has made installing internet-enabled devices much simpler with fewer maintenance costs. But most end users are unaware of the risks associated with PoE cables, in particular passive PoE.

What is Passive PoE?

A crucial aspect of PoE is how power is delivered via Ethernet cables. Passive PoE describes ethernet power supplies (like PoE switches or PoE injectors) that send raw unnegotiated energy down ethernet cables to connected devices. The device connected to that cable will receive the electricity, whether it is able to handle it or not. This is why Passive PoE is dubbed as “Always ON.”

PoE Switch
A typical PoE Switch

Active PoE describes ethernet power supplies that negotiate how much power should be sent to the connected devices. This handshake process ensures the device does not receive energy it is unable to handle, and eliminates the risk of burning out the device. There are three IEEE 802.3 standards, for Active PoE.

  • PoE (802.3af) – Provides 15.4 W of power
  • PoE+(802.3at) – Provides 30 W of power
  • PoE++(802.3bt) – Provides 60 W or 90 W of power

Differences between Active PoE vs Passive PoE

Passive PoE

If you have an electronic gadget that uses Active PoE, it will not power up until it completes a handshake between the PoE power supply and the device. This ensures the device gets the right amount of power inflow (voltage).  After you connect a device to an ethernet cable powered by an Active PoE supply, it first determines the characteristics of the incoming current flow.

After that, it matches it with the requirements of the gadget itself. If it senses any discrepancy in current flow, it simply won’t flow the current to the gadget. This is a great safety measure as it can prevent fire and other hazards.

How power is delivered over Ethernet

PoE (Power over Ethernet) cables serve two purposes. First, they transfer power over an Ethernet cable. And second, they use the same Ethernet cable to also ensure data transfer. To serve both these purposes, the wire pairs inside a PoE cable are divided into distinct pairs.

Almost all PoE cables have eight Ethernet wires inside them. Instead of being organized in a parallel fashion, these eight wires are further divided into four twisted pairs. Among these four pairs, two are used for data transfer and the other two are spare.

Twisted Pairs in an Ethernet Cable
Twisted Pairs in an Ethernet Cable

The two pairs that transfer data are referred to as the Data Pairs. As the name suggests, these two are responsible for transferring data over the cable. The remaining two pairs are referred to as spare pairs. Now you must be wondering which pair is responsible for flowing current through them? Well, a PoE wire can use any of the four pairs to transmit electricity flow.

Electrical current flows in a loop. That’s why you need two conductors to establish current flow over a cable. A PoE wire can use any of the wire pairs as a single conductor. This means PoE wires can use either the Data pair or the Spare pair to flow current through them.

Risks associated with Passive PoE

PoE wires have grown in popularity mainly because they reduce the need for electrical wirework in the home. With every new technological innovation, something new is introduced. And when we are talking about the power supply of electrical gadgets, shifting from hard-wiring to ethernet powered or wireless is the latest trend.

PoE cables have many advantages. Users don’t need to worry about having a power socket close to their devices anymore as this is taken care of over one single wire. However, there is a flip side to it too. If you are using a Passive PoE, you need to make sure you properly understand the voltage requirement for your gadget before you plug it in.

A wireless access point powered by a PoE Injector
A wireless access point powered by a PoE Switch

There are many risks associated with plugging in gadgets to power outlets that exceed the required voltage threshold. For instance, if you plug in an appliance that requires 120V power to a 240V outlet, you are exposed to severe accidents and the device burning out. Your appliance might get damaged beyond repair, or on the other extreme, it can catch fire.

The exact same goes for plugging in an electronic device to an ethernet cable powered by a Passive PoE power supply.

Ways to mitigate the risk of Passive PoE

To mitigate the risk of Passive PoEs, always make sure you are connecting your device to an ethernet power supply that provides the exact voltage the device requires. If in any doubt, consult a professional electrician before installation. Also, always read the instructions written on the product packaging as well as in the user manual.

With ethernet cables powered by an Active PoE power supply, there is a handshake process in place that helps gauge the voltage of the required current inflow. If there is a mismatch or an anomaly is detected, it will simply stop the current flow from reaching your device. However, that is not the case with Passive PoEs as there is no such safety mechanism.

Our Recommendation

PoE ethernet cables are widely used in smart IoT devices. You will find them in IP phones, smart lighting systems, wireless LAN, surveillance cameras, and other household gadgets. 

When using PoE to power your devices, we recommend you use an Active PoE power supply to deliver power. Check the voltage requirement for your appliance, and then match it with what your PoE power supply can provide. For instance, a PoE that supports IEEE 802.3af can provide an electricity supply of 15.4W. Any appliance that requires more than that should not be connected to this PoE supply so instead, you need to look at PoE+ or PoE ++ which have higher wattages.

You can connect multiple devices using a PoE switch. However, while doing so make sure the cumulative power consumption does not exceed the total power limit of the PoE switch. For instance, if the maximum power limit of your PoE is 90W, even if you add 10+ devices to it, make sure the combined power consumption stays below 90W. While connecting an ethernet cable to a Passive PoE power source, always ensure that the power supply has galvanic isolation and short protection. 

Conclusion

We hope you learned something new from this article. In a nutshell, you should now understand what PoE is, how it works, what the difference is between Active vs Passive PoE. And lastly, we looked at some recommendations for using PoE with different Electrical devices. We hope you found this article helpful. For any queries, leave a comment below!

Sources

Netgear

Show CommentsClose Comments

Leave a comment