Re: [802.3af] Late comment
"Jack Andresen" <jandresen@xxxxxxxxxx> wrote:
> Any common point needs an impedance to ground.
I would agree with this comment, and add:
It is unusual practice to distribute a commoned
conductor to a number of physically seperate
locations without providing an earth return, somewhere.
If a fault should happen that causes a hazardous
voltage to be applied to the commoned conductor,
that hazardous voltage will then be present in a
number of physically separate locations. This could
create a hazard for a person handling a connector,
or a hazard for the electronics in certain circumstances.
If the common point were grounded, the hazardous voltage
would normlly only affect a smaller number of devices:
Particularly, just the devices on the cable to
which the hazardous voltage was applied.
In the situation where a fault causes a hazardous voltage
to be applied to the commoned (but ungrounded) conductor
for an extended period of time, and the electronics is
not affected, the fault is likely to remain undetected
until such time as it causes a failure or a person comes
into contact with a conductor somewhere in the system.
A danger of electrocution might exist in this circumstance,
the issue being that the fault may remain undiscovered until
such time as it harms a person. In point-to-point data wiring
(eg a LAN Hub and a PC) such a fault condition would really
only affect the two endpoints. However with commoned
(but ungrounded) POE wiring, the hazardous voltage could
well be present at many physically seperated locations.