PoE: PSE isolation requirements

Question

On the robot I'm currently designing, there will be several connectors with PoE ethernet and supply voltage (0 V, 24 V) and some other signals (referenced to 0 V).

I'm unsure if I need isolation for the PoE injectors or not.

More precisely, which of the following solutions can I use:

1. A dedicated isolated DC/DC converter generating the 0 V, 55 V for a single PoE injector
2. A shared DC/DC converter generating the 0 V, 55 V for all PoE injectors (but isolated from the rest of my electronics box)
3. No isolation at all, all signal grounds are connected together

Please not that I don't require to be officially PoE compliant (if it's easy, why not, but it's just a small nice to have), only that it works in practice.

Some elements about the electrical architecture of the robot that might be relevant (or not?):

The surface unit is powered from mains, and generates and isolated DC voltage of 350 V (and forwards protective earth). The isolation from the DC rail (OV_PWR and 350V_PWR) to protective earth is monitored continuously, and power is cut in case the isolation resistance is <200 kΩ.

Then there is a long cable (400 m max) containing 0 V, 350 V, earth and optical fiber for Ethernet communication. Nothing else.

On the robot, 0V_PWR, 350V_PWR and protective earth and fiber enter the main electronics box (metal enclosure, currently not connected to PE, not decided yet if it will be connected on the new revision).

OV_PWR, 350V_PWR and protective earth leave again the main box to power the motors (control interface is isolated). They also go to the isolated DC/DC converter(s). Those converters generate all the low voltage rails (0V_SIGNAL, 1.8 V, 3.3 V, 5 V, 12 V, 24 V, 55 V), with all the low voltage grounds connected together (but isolated from the 0V_PWR).

Those lower power voltage rails will power the ethernet switch, the internal electronics, and the external connectors for low power devices (connectors shared with PoE ethernet).

The question is: can I use a 55 V supply voltage referenced to 0V_SIGNAL to power my PoE injectors, or do I need isolation? In this case, do I need isolation between each PoE port or not?

In case it's relevant, the cables outside the electronics box are usually <1.5 m, and will never exceed 4 m.

EDIT: All external connectors are subsea connectors (i.e. not RJ45), so there is no risk of the user trying to connect to a computer or a switch without thinking about it (he would need to first manufacture a specific cable to do so).

EDIT2 :

• most devices powered by PoE will be outside the metalic box
• there will be some PoE+ and PoE++ devices
• for PoE/PoE+ on 100Mbps Ethernet, the power will be on the data lines (the whole point of using 100Mbps only on some connectors instead of 1Gbps is to need only 4 wires, freeing the other ones for other signals)
• all ethernet switching and PoE(+/++) will happen on my PCB (for the 1Gbps switching in a PCB mounted module, the 100Mbps switching and injection directly with ICs). No off the shelf Ethernet switches or PoE injectors will be used (space is quite limited, so I need tighter integration than what of the shelf products would allow)

Answer 1

I'm unsure if I need isolation for the PoE injectors or not.

POE isolation is needed for two reasons:

1. If you have a large potential between devices
2. If you need to meet regulatory and compliance, more specifically, if your POE PD (Powered Device) needs to be 803.2af or at compliant (you need isolation to certify AFAIK)

I don't think either applies to you.

Since you have both devices in a metal box, and if you have chassis grounds connected to the box the potential would be the same. Unless you have some fault current on the box there should be no issue (if you think you could have a large fault current then that could create an issue in the event of a fault with large voltages being presented on the chassis.

All that being said, If I were doing this design for myself I would choose isolation (and 802.3xx devices) for three reasons:

1. Because if there is a fault or some other event with high voltages, the current will not travel down the ethernet cable and create issues.
2. Because it isn't that much more cost to get isolation and an 802.3af or at PSE (Power Sourcing Equipment ) POE, these can be had for less than 20$. POE DC 'splitters' that simply run DC over Ethernet direct are much lower cost (maybe 2-5$) but the cost vs risk would be worth not using direct injectors and lowering any risk by using isolation

The question is: can I use a 55 V supply voltage referenced to 0V_SIGNAL to power my PoE injectors, or do I need isolation? In this case, do I need isolation between each PoE port or not?

Again you don't need isolation if the ground potential is the same and will always be the same. The hard thing is determining what the fault pathway will be if there is some issue with a short. Because of this the system will most likely be more robust if you isolate the POE.

Again, if you are confident in your ground then POE would not be needed, but if you think there could be a current fault where any large current could follow the ground, it would be worth isolating.

More precisely, which of the following solutions can I use:

• A dedicated isolated DC/DC converter generating the 0 V, 55 V for a single PoE injector

If your PD (Powered Devices are 802.3af or at (or more) then you'll want a PSE (Powered Sourcing Equipment). That being said, many devices will work if you put 48-55V on the spare pairs. The direction isn't important because most devices have a rectifier that will work with either polarity. When I do Powered Device testing, I usually just hookup a bench supply for an 802.3af device and measure the power with the bench supply. 802.3xx devices also need to negotiate power and can detect bad power conditions. So if you had multiple POE devices, you will need 1 injector per cable or a hub with many POE ports. If I were doing this myself I'd probably use a POE switch some even have remote monitoring capabilities which could be useful for your application

• A shared DC/DC converter generating the 0 V, 55 V for all PoE injectors (but isolated from the rest of my electronics box)

You could do this if you wish, and use isolated POE injectors or even passive injectors, passive injectors just take the spare pairs and split them out so you can connect a bench supply or DC/DC converter to them.

• No isolation at all, all signal grounds are connected together

Since you have a chassis and it's probably always going to be zero volts you don't have to have isolation, but isolation is nice to have. This scheme with using passive POE injectors would work for your application

EDIT:

Solution #2 would be best since you are developing your own PCB:

Figure 1.1 Microchip AN3361