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Re: [802.3_PDCC] Changing resistance due to temperature rise

Peter, power delivery is a quadratic dependent on three variables: the loop resistance, the voltage of the PSE, and the power of the PD. For us to have a problem, all three need to be worst case. The easiest is the PSE voltage: it’s rare to find a PSE that is the minimum voltage. Raising that gives headroom for the current. Second is the PD power. It’s very hard to make a PD that draw the exact max power (unless it’s a static load, something like a light). This would also give headroom to the current. 

But let’s assume that we’re at the min PSE voltage and we have a resistor load at Max Ppd. If the loop resistance is too high, the PSE will supply more than the allowed power, in other words we exceed Ppse max. In this case, the PSE MAY shut down the link. I don’t know the clause 104 specs by heart, but in the 4P realm, the PSE isn’t required to shut down until we’ve exceeded Ppse by 11%. I’d assume Clause 104 is similar (why fix what isn’t broken). 

This is a long way of saying that you’d have to exceed the max allow Ppse by something like 10% before the PD would shut down. There is no renegotiation scheme for a cable that causes the PSE power to exceed Ppse. The PD can use LLDP to renegotiate, but the PD would have to know that it needed to renegotiate. I don’t see how the PD can know that the cable has exceeded the allowed loop resistance. 


Regards,


Chad 


From: Peter Fischer <000042efa5d19355-dmarc-request@xxxxxxxxxxxxxxxxx>
Date: Tuesday, April 28, 2026 at 3:56 AM
To: STDS-802-3-PDCC@xxxxxxxxxxxxxxxxx <STDS-802-3-PDCC@xxxxxxxxxxxxxxxxx>
Subject: [802.3_PDCC] Changing resistance due to temperature rise

Dear all
 
I have a question I would like to discuss during one of the next PDCC meetings:
 
Let us assume a procedure as follows:
 
Initiale situation: SPE-Link has 9 Ohm DC loop resistance
SPE device, which requires power class 15 is connected over the cabling to the switch and therefore to the power source.
Due to relativ resistance (ohm per meter) and installation environment conditions the cabling is getting hotter and therefore the DC loop resistance is rising to 10 Ohm.
 
What will happen?
These points are important as it might impact the way cabling has to be defined to avoid certain unwanted behaviour.
 
Best regards
 
Peter

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