Power over Ethernet (PoE) has been around for several years. Type 1 PoE (IEEE 802.3af), which was first introduced in 2003, can provide up to 15.4 W of power, of which 13 W can be used for equipment. Then there is Type 2 PoE, sometimes called PoE Plus, which can provide up to 30 W of power, which can provide 25.5 W of power to the device.

Under the above-mentioned DC power level, no one has really questioned the safety of PoE. However, as soon as the four-wire pair Type 3 and Type 4 PoE were launched in 2018, Type 3 and Type 4 PoE provided 60 W and 90 W power respectively, and many people began to doubt whether PoE was safe. We think it is necessary to explore this issue carefully.

Alternating Current (AC) vs. Direct Current (DC)

Although it is absolutely unsafe for your ears to play the AC/DC masterpiece “Back in Black” at a volume of 70 decibels, people have long believed that DC power is safe while AC Power supply is not the case. The fact behind their belief is that alternating current is considered to be about 3 to 5 times more dangerous than direct current. Therefore, at the same voltage, a much larger direct current than alternating current is required to cause a fatal threat to people. In order to better understand the reasons, we need to understand the difference between the two.

Refer to the figure above, the current direction of the AC power supply changes periodically, while the current direction of the DC power supply is always the same. This is why AC power has a frequency and DC power has no frequency. A simple way to visualize this difference is to graph it-alternating current produces a wavy (sinusoidal) pattern, while direct current is just a straight line.

When you are accidentally shocked by an alternating current that reaches a dangerous current level, this periodic change in the direction of the alternating current can cause atrial fibrillation in the heart. The continuous flow of DC power is not that dangerous to the heart, but it can cause spasmodic contractions of the heart, and even if the voltage is high enough, it can cause death. In addition to the current level, the resistance of the human body is also an important factor, which is affected by humidity, skin thickness, weight, age and even gender. Dry skin has a greater resistance than wet skin. When the resistance drops, the current flowing through the human body will rise. Because women have lower overall resistance, they are more susceptible to electrical damage than men.

In addition, different paths will also cause different results. Only when the human body has a complete path and has two contact points for current to enter and flow out, an electric shock will occur, and the current will always take the easiest path to ground. The path that the current travels is closely related to the degree of danger of an electric shock-the current flowing from one hand to the other will flow through the heart, so it is much more dangerous than the current flowing from the finger to the elbow.

What does this mean for PoE?

According to the IEEE standard, the voltage when PoE is injected into the cable is 44 to 57V DC (typically 48V DC). Under normal circumstances, any voltage lower than 35V AC or 60V DC is regarded as a safety extra low voltage (SELV), so by definition, the port that supports PoE is SELV. This is not to say that 48V DC will not cause you to be shocked. If you touched a 9-volt battery with your tongue when you were a child, you would know this feeling. And no one would suggest that you peel off the insulation of the twisted pair and then poke it with your bare hands, especially when your hands are wet.

But when using PoE, due to the actual protocol itself, you still rarely have the chance to suffer an electric shock from a disconnected cable. This is because the power supply device (PSE) must first shake hands with the powered device (PD) before it can provide power. That is, if there is no handshake, there is no power. This is completely different from a standard AC power outlet, which will continue to supply power regardless of whether you plug the device into an AC power outlet or not.

In short, the answer to the question at the beginning of this article is yes-PoE is safe.

There are always exceptions

So, if PoE is safe, why should it be revised for power above 60 W? The heat generated by PoE in the cable bundle is still potentially dangerous. Over time, it may cause insertion loss and decrease in cable performance, thereby hindering normal data transmission. For life safety systems from telephones and security equipment to the network and powered by the network, signal loss will undoubtedly become a life safety issue. Therefore, the number of cables in the cable bundle (based on conductor size and rated temperature) is specified for 60W or higher power PoE, or limited power (LP) cables are required.

Discussions on PoE-related fire safety are also ongoing. Although this is possible in theory, it is definitely the worst case-in the high temperature (above 40°C) environment in the ceiling space, all cables in a large cable bundle that are tightly bundled together Provide 60W or higher power PoE from one point to another point without spreading to smaller cable bundles/separate wiring. It is also possible to use low-quality unshielded cable structures near combustible materials, such as copper-clad aluminum. The above scenarios can be avoided by following the cable bundle bundling guidelines and certification (CCA wiring cannot be certified).

The important thing is that even if there is no danger, if the balance of the cable is not good, PoE will still cause serious damage to the transmission. In four-wire pair Type 3 and Type 4 PoE, power is transmitted on all four-wire pairs through a common-mode voltage, and the common-mode voltage distributes the current evenly among each wire in the pair. To do this, the DC resistance must be balanced. Excessive imbalance will cause the high frequency signal transformer to saturate, resulting in distortion of the Ethernet data signal. Therefore, we recommend that you use the DSX CableAnalyzer™ series tester to test the DC resistance imbalance.