I'd like to measure the amount of power drawn by a PoE client device (PD) and would like to hook up a multimeter inline to do so. However, power is supplied to the PoE device over two pairs of wires (i.e. 2 +ve, 2 -ve) instead of the traditional one pair for a DC circuit (e.g. a light bulb powered by a battery has 1 leading to +ve and one leading to -ve). See an example here:
https://duncansonline.ca/FAQs/WhatisPoEModeAModeB.htm
Let's assume I am dealing with Mode-A PoE supply – where would I intervene and connect the multimeter probes in the the Cat5E cable? One suggestion I was given was to not mess around with the Cat5E cable but instead hook up the leads into a PoE injector just before the power is split up into the two pairs of wires. I'd like to refrain from doing this if possible since it is not a safe procedure and because we want to use a particular model of a PoE switch as the PSE (power sourcing equipment).
Note:
– it is not possible to measure power on the client device end. It is a closed box.
– I do have an inline power measurement device (Byte brothers POE1000IL) but it does not show instantaneous values and spikes. We have an expensive multimeter that does measure instantaneous spikes and this is what I would like to use.
Best Answer
If you don't want to measure at the source or at the sink, your only choice is to break the cable. Cut both +ve wires. Connect the two ends from the source together; connect the two ends to the sink together; put your meter between. You will have to break both wires to get an accurate measurement, but you must also allow current flow through both wires.