Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

I have a switch with Power over Ethernet capabilities. Is it allowed to connect a PC to a PoE switch?

share|improve this question

2 Answers 2

up vote 4 down vote accepted

Yes, of course. The PC won't use the power.

share|improve this answer
3  
Not sure about the of course bit here. It'd be bad if there was excessive voltage or current in the cable which would damage the PC. –  Richard Jul 30 '12 at 22:47

The following is drawn from this site, the last paragraph is the relevant bit.

Network cables, such as Cat 5e and Cat 6, comprise eight wires arranged as four twisted pairs. In 10 and 100BASE-T Ethernet, two of these pairs are used for sending information, and these are known as the data pairs. The other two pairs are unused and are referred to as the spare pairs (Gigabit Ethernet uses all four pairs).

Because electrical currents flow in a loop, two conductors are required to deliver power over a cable. PoE treats each pair as a single conductor, and can use either the two data pairs or the two spare pairs to carry electrical current.

Power over Ethernet is injected onto the cable at a voltage between 44 and 57 volts DC, and typically 48 volts is used. This relatively high voltage allows efficient power transfer along the cable, while still being low enough to be regarded as safe.

This voltage is safe for users, but it can still damage equipment that has not been designed to receive PoE. Therefore, before a PoE switch or midspan (known as a PSE, for power sourcing equipment) can enable power to a connected IP camera or other equipment (known as a PD, for powered device), it must perform a signature detection process.

Signature detection uses a lower voltage to detect a characteristic signature of IEEE-compatible PDs (a 25kOhm resistance). Once this signature has been detected, the PSE knows that higher voltages can be safely applied.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.