12 Aug 2020

​ComNet Editorial: What is Electrical Power?

I’m sure you have all heard of the term Power over Ethernet or PoE. There is really one word missing from the title, and that would be cable. In fact, to be 100% correct, that should be two words, as it is copper cable.

The power is not over the Ethernet. It is being passed over the same copper cable that we use to pass the Ethernet signals. It is electrical power, and in simple terms, it’s what makes electrically powered devices switch on. We use a device at one end of our Ethernet link to power a unit at the other end. It creates simplicity in the cabling system and the potential management of the power system. Just like everything else connected with Ethernet, it is standards-based, and it is the IEEE (Institute of Electrical & Electronic Engineers) in the USA that defines the laws. I could leave it there, but I know you don’t want that - you want more physics and daft equations, and I ‘m not here to disappoint you!

A question is always a very good place to start. What is electrical power? To answer that, I need to introduce you to one of our good old friends from the physics world, Georg Ohm. Georg was a German physicist and mathematician who discovered the direct relationship between the potential difference applied across a resistance and the electrical current that flows through it. And based on that last sentence, most of you are now wishing that you had never been introduced to Georg Ohm. But he was a good guy as he gave us Ohm’s law.

Put in more readable words, if we provide Voltage(V) in a circuit, then a current(I) will flow, and the amount of current that will flow depends on the resistance(R) in the circuit that we make. Think of a circuit as a loop or circle that connects both the transmitter (Ethernet switch) and the receiver (ACT controller) through a copper medium.

How it all works

V=IR was the fantastic equation that George Ohm created. But wait a minute, we are talking Power, so where does our Ohm’s law equation mention that? Well, more physics sadly! The power in a circuit is equal to the product of the voltage and the current, or Power = V × I. Electrical power is measured in Watts (W) named after the Scottish inventor James Watt. Before I get into trouble, I will make it clear that we are talking about dc power only and not ac. No need to expand that further!

So, we provide a voltage in a device at one end of our link. As a result, current flows through the copper to the other end, through that remote device, and back to our starting point in the original device. We have voltage, current, and resistance, and from voltage and current, we can work out the power. See, that wasn’t so bad.

The IEEE sets out the power levels, voltage levels, and minimum grade cable that can be used over our 100m compliant links. All these figures can be worked out from Ohm’s Law and our power equation. For example, a 30W PoE system at 48Vdc will create a current of 1.6Amps.


Our electrical medium for Ethernet is twisted-pair copper wire grouped into four pairs to form a cable with our RJ45 connector on the end. Remember, we create a pair to eliminate common noise in the copper cable, so the four pairs act like four individual connections. Within the four pairs, we then split those into two sets to effectively create two circuits between both ends. You can think of that as an out/return, out/return combination.

We do like acronyms in our engineering world, and guess what, PoE (no pun intended) is no different. The end of our link that provides the power is the PSE (Power Sourcing Equipment), and the recipient of the power is the PD (Powered Device). When the devices are connected, the PD starts waving at the PSE to say that it needs power. Now the PD is not powered at this time, and as you know, it also does not have arm’s, so the waving is something electronic-based. The PD presents what we call a load (a resistance) across the power lines (copper connections) to advertise itself as a PD needing power. The resistance level must fall within a certain range - if not within the range, the PSE will just ignore it. This is the first stage of negotiation, which, when complete, will have the PSE providing power to the PD.

There are various power levels (standards) of PoE, and the PSE and the PD need to be based on the same standard to ensure that the PD gets the power that it needs. Some negotiation is more complex than others, but the reason for negotiation is to ensure the correct power is supplied so as the PSE does not damage the PD.


Now, our copper cable is good, but it’s not perfect. It has resistance. Georg Ohm tells us that if we have current flowing in a resistance, that will create a voltage. Over our 100m maximum copper cable distance that resistance creates a voltage drop. At the PD, we need a certain voltage level to switch the device on, so if we take that PD minimum (switch on) voltage level and add it to the known voltage drop over 100m of copper, then we can get a minimum voltage level that we need at the PSE. (Wow that was getting heavy) By building our interfaces to the IEEE standard, then everything is good, and levels are correct. If you are non-compliant, then things get bad.

For instance, let’s use the example of long-distance Ethernet links from a compliant Ethernet switch. Well, if the voltage level is set to the minimum compliant level in the PSE, then the PD will not switch on, and our ACT controller will not power up. Interestingly, the most important thing in PoE when you are PD is the voltage level and not the Power! The story is always the same - work to the standards and you are okay.

Well done to Georg Ohm for making our PoE world simple to navigate.

By Iain Deuchars, General Manager, ComNet International.