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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Practical Voltage and Current Limits during Detection
Besides ensuring detection, there are safety concerns too. What are the worst-case PSE-side voltages and currents allowed by IEEE on the line during the detection process? These are the key limits during detection.
1.  Maximum (open-circuit) voltage Voc = 30 V.
2.  Short-circuit current Isc = 5 mA
Open circuit is defined by the standard as any measured port resistance over 500 k.
Note that there is an odd/contradictory situation here. Can we use the maximum safe-current source of 5 mA to probe a valid PD? A valid PD would by definition present 25 k on the port. However, 5 mA through it gives a voltage drop of 5 m × 25 k = 125 V! But obviously that is not only unsafe (above 30 V), but ...

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