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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.

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  • \$\begingroup\$ make a "test jig" out of a short piece of lan cable with a male connector at one end and a female connector on the other end .... separate out the +ve wires and insert an ammeter \$\endgroup\$
    – jsotola
    Jul 17, 2018 at 20:27
  • \$\begingroup\$ measure the current in one of the pairs ( the total current of two wires ) that's your load current, multiply it by the supply voltage ( 48 V) and that's your load power. example : measure current in line 1 and 2 , do the sum ( one will be negative ) and thats your load current \$\endgroup\$
    – Chebhou
    Jul 17, 2018 at 21:07

2 Answers 2

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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.

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  • \$\begingroup\$ if using ModeA PoE, wires 1 and 2 would be +ve. So, you mean like this? Wouldn't that affect the data lines that are shared (along with power) in ModeA? \$\endgroup\$
    – inquisitive
    Jul 17, 2018 at 22:10
  • \$\begingroup\$ What do you mean by "affect"? Anything you do to the cable may have an affect on the data lines. If you won't measure at the source or sink, what choice do you have? There is no such thing as a free lunch. \$\endgroup\$
    – Elliot Alderson
    Jul 17, 2018 at 22:38
  • \$\begingroup\$ Not like the image it is wrong , you should have two multimeter s one at each wire and do the sum \$\endgroup\$
    – Chebhou
    Jul 17, 2018 at 22:49
  • \$\begingroup\$ If you have clamp ammeter then put the pair of wire inside the clamp and read directly the measured value \$\endgroup\$
    – Chebhou
    Jul 17, 2018 at 22:51
  • \$\begingroup\$ @Chebhou Can you explain why two multimeters are necessary? Also, the OP has a specific meter they want to use, not a clamp ammeter...they need to measure spikes. \$\endgroup\$
    – Elliot Alderson
    Jul 18, 2018 at 0:23
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There are plenty of ways measuring a current. See https://www.analog.com/media/en/technical-documentation/application-notes/an105fa.pdf for examples.

On your case, you want to look at current spikes so you probably need to use a scope to display the measurement result. To get current information, you will have to break both +ve and -ve lines and insert a small resistor in series in each one and then measure the voltage across the resistors (you need two differential probes to do that).

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