802.3bt - High power ( 90W ) Power over ethernet standard. Cable protection

[mrpackethead]

Hey folks, I'm very curiously following development of the high power POE systems.    90W over a hunk of UTP is a wonderfully useful solution for all sorts of things.  I've been having a discusison with the folks at TI about how they achieve protection across four pairs of cable.     

there is only current sence in two of the four pairs, and two of the pairs are run in parrallel, yet they seem to claim that the circuit is protected.    What have i missed.

In this picture they say that both cables are protected. because it all flows via the two FETS ( the diagram does not show the currnet sence but it is there, its simplified )


I have posed this situation where one pair is broken.   and suggest there is no way for the system to know there is a broken pair based on cable currents alone.



Heres the link to the TI forum, for some context.

https://e2e.ti.com/support/power_management/power_interface/f/204/t/666870#pi320098=2


Am i missing somethign.

[jbb]

Seems like a reasonable question.

Possibly the answer is 'oops,' or maybe 'the expected temperature rise from overload has been determined to present an acceptable fire risk.'

[mrpackethead]

Seems like a reasonable question.

Possibly the answer is 'oops,' or maybe 'the expected temperature rise from overload has been determined to present an acceptable fire risk.'

1.8A through a 23 AWG cable is pushing your luck.   

[tszaboo]

Probably it has the same answer to this question, as all Ethernet layer 0 related problems. No way to detect it, no way to avoid it, and it will just not work. I think you also broke a complete pair, not just 1 cable.

[mrpackethead]

Probably it has the same answer to this question, as all Ethernet layer 0 related problems. No way to detect it, no way to avoid it, and it will just not work. I think you also broke a complete pair, not just 1 cable.

one wire or one pair broken you still are pushign current down a path that has not enough copper.

[tszaboo]

Probably it has the same answer to this question, as all Ethernet layer 0 related problems. No way to detect it, no way to avoid it, and it will just not work. I think you also broke a complete pair, not just 1 cable.

one wire or one pair broken you still are pushign current down a path that has not enough copper.

CAT5 is rated to 0.6A.
3x54Vx0.6A = 97W

[David Hess]

If one pair if broken, then one current sense will indicate high current and one will indicate low current which can be considered a fault.  On the hardware side, a window comparator could detect the current imbalance to indicate a fault.