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RE: Static Discharge

Title: RE: Static Discharge

Unfortunately, that does not work.

The problem is not that of a connected cable charging up, but that of having a pre-charged cable (with no built-in resistors) suddenly being plugged into the RJ-45 connector.

What you get is a sub-nanosecond rise time pulse with several hundred millijoules of stored energy. This pulse is coming from a resonant cable so what you see is an exponentially decaying damped oscillatory discharge. 5 kV on a 25-foot cable, 700 to 900 V on a 100-m cable.

If you are lucky, both connector pins mate at the same time and you get the common-mode rejection of the magnetics helping you by not passing some of this through to the PHYs. A worse case is if only one pin of a pair mates, and you get differential transfer through the magnetics.

We (and PHY vendors here) have seen as much as 60 volts appear on the PHY pins.

In any case a bleeder resistor that would absorb this would have to be such a low value that it would interfere with normal signals. We have had good luck with diode clamping networks that have a dynamic (clamping) impedance on the order of 1 ohm.

This is, fortunately, a comparatively rare phenomenon that usually only occurs when a cable is being plugged in in a new installation. Careful network installers would make sure that they discharged all cables BEFORE plugging them in, dont'cha fellas?

We have had a few Environments from Hell where a dry climate, combined with moving belts on assembly lines (opportunistic Van der Graaf generators!), have caused constant, repetitive failures. You know who you are out there....(grin)