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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Reasons for Protection
Here are the basic reasons we want to implement short-circuit protection:
1.  Protect the cabling infrastructure
2.  Protect the PSE controller—but in fact the dissipation during “short-circuit” may be less than during an overload, because of current foldback (lowering of port current; an optional feature)
3.  Limit peak transient currents to control EMI and its effect on data integrity
4.  Protect PCB traces of legacy equipment
5.  Protect power supply operation in a multiport system
6.  Allow reasonable and cost-effective support for port-to-port cross-regulation effects
As indicated, a short-circuit is not defined as 0 Ω, or even close to that. Because that, in fact, is not the most severe condition. Possible ...

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