Last night, I had a short-haul link go out (SXT lite5 ac, powered by a PowerBox Pro) that's been running fine for about a year. I found that it came back if I forced POE on, but I don't understand why.

It is, in fact, on the last interface on the PowerBox, suggesting power budget triage, but I can't find any portion of the budget that's even close to limitation. Lowering (numerically) its POE priority did not bring it back up, it had to be forced on. Once running, turning the interface back to auto-on kills it again. When set to auto-on, the interface reports "waiting for load." When running, a cable test at both ends indicates no faults. (When waiting for load, it shows normal at 0m on the first two pairs, and shorted at 5m on the other two, which I could believe makes electrical sense).


The units on 2-4 are two 912s and an LHG ac. All of these are very reasonable numbers. Nothing here exceeds 1A per interface or 2A for the box total. The voltage number matches that shown in /system health.

My own intuition suggests the culprit is bad sensing circuitry in the PowerBox interface, but I suppose it might also be a bad resistance indicated at the SXT.

Any advice?

How do you power power-box pro? Poe-in or jack?

That's a good question. I think it's via port 1, but there were some cabling changes about five months ago and it's possible we switched to the jack. I'm about 75% sure it's still the ethernet POE in.

I'm not sure why this would make a difference, so please enlighten me as to why you asked.

With low voltage poe there is a limit - 1A (1000mA) thru ethernet port/cable. If you power your power-box from another poe-out device, than this limit clearly apply to your case.

Also I see that you have only 20.6 volts on poe-out-voltage. Do you use stock power adapter (24v @ 2.5A, you should see 23-24v on poe-out-voltage)? Or maybe you have stock adapter and power-injector but with long ethernet cable?

If you power multiple devices with PowerBox Pro then I highly recommend to connect stock power adapter using jack.

With low voltage poe there is a limit - 1A (1000mA) thru ethernet port/cable. If you power your power-box from another poe-out device, than this limit clearly apply to your case.

Also I see that you have only 20.6 volts on poe-out-voltage. Do you use stock power adapter (24v @ 2.5A, you should see 23-24v on poe-out-voltage)? Or maybe you have stock adapter and power-injector but with long ethernet cable?

If you power multiple devices with PowerBox Pro then I highly recommend to connect stock power adapter using jack.

I power the PowerBox over a long armored ethernet cable (power only, no data) that runs overground from a power source to a tower. I have looked into transitioning to an armored two-conductor cable with larger gauge wire and barrel connectors, but have not yet scheduled this job, mainly due to the extreme seasonal heat. I would move this schedule ahead if I were confident it would affect this problem, but I don't see how the voltage really figures in here. These devices have worked fine for quite a long time, so to have one of them suddenly get touchy like this for no apparent reason concerns me, especially when things continue to run fine if I force the POE-out on instead of let it run automatic. If I am wrong and the voltage really is a factor, please let me know. I wouldn't be averse to running 48V out there instead but I have a pair of APs running RB912s that would get a little crispy.

Addendum: now that I've had a chance to do the math, I see that POE out is accounting for only about 600mA here, so even adding in the PowerBox's own, I don't think I'm even close to courting 1A.

Resolution: I replaced the 100' armored CAT5 POE cable carrying 24VDC with 12ga armored "parking deck" cable fed into the barrel power connector. System health now reports 23.8V and port 5 problem is now gone.