Community discussions

MikroTik App
 
Malvineous
just joined
Topic Author
Posts: 10
Joined: Wed Jul 14, 2021 6:21 pm

RB5009UPr+S+IN - is PoE isolated or not?

Tue Aug 16, 2022 9:40 am

Hi all,

Does anyone know whether the PoE output on the RB5009UPr+S+IN is galvanically isolated? I believe this is optional in the PoE specs, but I've run into issues before when it's not isolated.

I had a TP-Link PoE switch years ago that ran off a -48VDC power supply, and it connected the -48V rail to GND, and the 0V rail to +PoE, to make it look like PoE was being supplied at +48V. Because the switch power supply connected 0V to earth, it meant any PoE powered device had its GND sitting at -48V referenced to the switch PSU, and every other device had its GND at 0V referenced to the switch PSU. So if you tried to connect, for example, a PoE-powered Raspberry Pi to an LCD monitor, as soon as you plugged in the HDMI cable and the two different GND connections came together, you'd short out the switch power supply and the whole switch would lose power. I managed to solve the problem at the time by adding a 5V DC-DC converter between the PoE splitter and the Pi, which didn't change the voltage but provided isolation. Later, I switched to a second-hand Cisco switch which no longer needed the DC-DC converter as it had isolated PoE output.

So I'm wondering whether the RB5009UPr+S+IN electrically isolates its PoE output from the input supply, to avoid issues like this. Does anyone know?

On a related note, are the multiple power inputs isolated, or are, for example, all the negative supply inputs connected together internally?
 
User avatar
Lifz
Frequent Visitor
Frequent Visitor
Posts: 59
Joined: Tue Feb 26, 2013 1:05 pm

Re: RB5009UPr+S+IN - is PoE isolated or not?

Tue Aug 16, 2022 1:19 pm

No, PoE is not isolated, nor is the device's powering (DC-Jack, 2Pin).
 
User avatar
jbl42
Member Candidate
Member Candidate
Posts: 161
Joined: Sun Jun 21, 2020 12:58 pm

Re: RB5009UPr+S+IN - is PoE isolated or not?

Tue Aug 16, 2022 1:47 pm

In general: If the specs of a PoE source device do not explicitly mention galvanic isolation for PoE, there is none. It is quite expensive to build in.

Devices like MikroTik, Ubnt, TP etc all miss galvanic isolation on the PoE outs. The PoE GND is directly connected to the power supply ground of the PoE source device.
As general rule, such devices should only be used for application were the Ethernet cable providing power is the only external connection of the powered device, like WiFi APs, cameras etc.
As soon as you add additional connections from the powered device to other devices having their own power, you risk to run into ground loop problems, like your RPI-HDMI example.
One workaround is to use PoE splitters with galvanic isolation at the load end of the cable. But this is expensive and defeats the purpose of PoE.

Specials care must be taken if PoE cabling goes from one building to another. Potential differences can be huge and lack of galvanic isolation easily can result in fried electronics.
 
Malvineous
just joined
Topic Author
Posts: 10
Joined: Wed Jul 14, 2021 6:21 pm

Re: RB5009UPr+S+IN - is PoE isolated or not?

Tue Aug 16, 2022 3:19 pm

Excellent, thanks for the clarification and detail!

Now that I think about it, as long as the PoE output negative is tied to the input power negative (and I find a +48V PSU rather than -48V one) then I don't think I'll run into any issues in my situation as everything is ultimately run off the same mains supply, so it shouldn't cause me any problems.

Who is online

Users browsing this forum: No registered users and 4 guests