I understand all that but... let's imagine I run 60W 802.3bt over 100m of cat5. The voltage drop will be bad. So what actually happens? Does the device detect voltage droop and shut off? Or does the cable just catch on fire?
Looping the cable or putting it in a confined space could cause issues. The cable could then catch fire even though it appeared to be operating normally to the PoE controller.
The device needs a certain amount of power to keep itself alive. Depending on how the device is designed and if actually adhering to standards, the device should simply not have enough power to start at say 80m, or let's say they pushed the install from the get-go (happens all the time) and it's actually 110m on poor / underspec'd cable.
And let's say the device has enough power to start, but you're using indoor cat5 and it's been outdoor for 7 years, and you don't know this but it's CCA. If it's in a bundle with other similar devices drawing high power and there is enough heat concentrated at a bend, then yes, the cable could catch fire without the device having a problem. As long as the device has enough power it's going to keep doing its thing until the cable has degraded enough to cause signal drop and assuming it's using one of the more modern 4pair PoE standards, would just shut off. But that could be after the drapes or that amazon box in the corner of the room caught fire.
We're just lucky in the residential space that PoE hasn't been as "mass market" as an iphone, and we've been slowly working into higher power delivery as demands have increased.
IMO? It's all silly though. We should just go optical and direct-DC whenever possible ;)
Longer wires don’t increase the overheating risk because the additional heat is divided over the additional length.