Limited Power Source
Finally, looking at the lowermost row of Fig. 10.1, we have the LPS requirement. This stands for limited power source. What this basically asks from us is that we place a fuse in series with either Port_P or Port_N to limit the total power to 100 W (VA) in case of failure of the PSE (like shorted FET). So most people calculate: 100 VA/57-V = 1.75 A, and put either a 1.75-A (but preferably a 1.5-A) fuse in series with the port.
The IEC safety standard also places a limit of 250 VA on the power coming into the PSE chip, but typically, this is open to multiple interpretations today and is being somewhat ignored. However to be unequivocal, we need to use either a limited energy AC-DC power supply (called a SELV-EL power supply, ...
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