PTP820/850 DC power + side is connected to chassis ground

I am sure I have several PTP820 radios being powered via POE using +48 (the - side is grounded). We power 450i APs and a Mikrotik router from that same +48VDC. I think this is typical, a lot of 802.3af/at equipment is actually floating and doesn’t care if you ground the - side, + side, or neither.

I am doing some sites with PTP850 using DC power + fiber, and discovered the + side of the DC power input is connected to chassis ground. It appears that the - side probably has a diode in series so getting polarity reversed doesn’t damage anything, but that’s a different question than which side is connected to chassis ground.

So why does DC power in via POE seem to be floating, but direct DC power in requires -48 (positive ground)?

This is kind of a pain in the butt when trying to power both PTP820/850 and PMP450i/450m at the same site. I realize the backhauls were designed by a different team (cough, cough, Ceragon), but it’s still a pain.

802.3af/ad/at is floating and is provided via common mode across the data pairs. You can not “ground” this or the data will also stop flowing. Surge protectors use TVS diodes in a crowbar arrangement to short the pair to ground if the transient voltage goes past the surge rating for that device.

Having a + to chassis means that a telecom power source is expected. You can still power from a floating dc source and from a battery bank without issue. Just make sure your dc source is truly floating and not diode shunted to chassis.

You should be able to use the same floating source to power both the 820/850 ptp and the 450i/m. I would double check that the 450i/m doesnt have a power lead to chassis connection other than + to chassis though or you will have power flowing from one radio through the tower to the other radio and this could kill one or both radios or just blow fuses.

I opened a ticket with Packetflux to ask about this, and they confirmed you can connect -48V to the +PowerA terminal of a Powerinjector Plus Cambium Sync, also that 480i and 480m APs have isolated power and can be powered this way.

Packetflux can accept any polarity to either input as long as both voltage sources are the same polarity. So no mixing -48 and +48 unless you want 96v across the box! But you CAN have a -48 and a isolated source at the same time without issues.

The problem will be your radios, if they have the + connection connected to chassis on one and the - connection connected to chassis on the other, you will get a ground fault current that will cause problems.

If you can bench test your setup then do so then add a wire from one radio chassis with an inline 0.25A fuse to the other and see if the fuse pops. A multimeter wont work as there should be a static voltage difference caused by the RF leakage to the chassis. The connecting of grounds to the tower bleeds this off and is mostly harmless, what isnt is if there is current flowing between radios and there shouldnt be!