Author Topic: Strange results in PoE measurements  (Read 437 times)

sbgservices

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Strange results in PoE measurements
« on: August 07, 2018, 12:25:29 PM »
I've purchased a PockE mainly for doing PoE checks on our installation, since we had the one or two weird device failures recently.
However, the results I get back from the PockE are inconsistent at best. Perhaps someone here can shed a light on the problem.

Switches used: HP 2530 PoE+
When I measure PoE voltage with the cable shipped with the PockE, it reads around -74V. :o
I thought I found the reason for the dying devices.. but I don't like to go from the first diagnosis, so..
When I measure with a different 0,5m cheapo Cat5 cable, it reads around -52V  :-\
The switch CLI management reports 51.5V  8)
When I measure with a good 0,5m Cat6 cable, it reads around -74V  :o
I then noticed that the Cat6 and the cable included with PockE are both shielded, the cheap Cat5 is not, so...
When I measure with the good 0,5m Cat6 cable after cutting the shielding open, it reads -52V
And since I already destroyed the good cable, I scraped off some wire insulation and clamped a multimeter in there.. which also showed 52V.

So.. what makes the PockE's PoE voltage reading about 22 Volts off when using a shielded cable?!

snowmanjax

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Re: Strange results in PoE measurements
« Reply #1 on: August 08, 2018, 12:10:32 PM »
Have you tried putting a multimeter on the shielding coming from the switch? I've seen weird issues when equipment breaks down. For example, at a past work facility, I was getting up on a desk, to put a cable in the NIC of a label printer, then I got shocked when I touched the electrical conduit at the same time. The wire shielding failed somewhere along the pipe, and had to be repaired.

Find a good ground, not on the switch, and then put the other lead on the shielding around the switch ports. Try multiple grounds if you can. Report back if possible, so I know if I'm the only one to get shocked with grounding or not! :)

Snowman

sbgservices

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Re: Strange results in PoE measurements
« Reply #2 on: August 09, 2018, 08:18:22 AM »
There's no significant voltage between the shielding of the network wall sockets and the earth ground.
This issue also doesn't just affect one port, I'm getting these weird voltage readings in the whole facility, and I tested several ports on each switch we have.
That's what got me to the conclusion that there's something weird with the PockE instead.
This might even be a design defect, if the PockE is measuring PoE voltage referenced to the socket ground,
or some unexpected ground return path exists in the PockE that puts the measuring ADC off by these 22V.