🚀 go-pugleaf

  Incoming and outgoing currents on two AC wires is a
differential mode transient.  First such transients are easily
stopped or made irrelevant by computer power supply. Power
supply contains layers of protection including isolation
transformers, optoisolators, and shunt devices that make
differential transients irrelevant.  Second differential mode
are typically not the destructive transient.

  Defined is a typically destructive transient - lightning - 
that would be common mode.  Incoming on any one or all AC
electric wires.  And yes, one incoming AC wire even bypasses
power supply to connect directly to motherboard and
peripherals.  Look for outgoing path elsewhere, such as mouse
cord hanging on baseboard heater, via network cable, or a most
common outgoing path - modem and phone line to earth ground.

  Again, this is common mode.  But to appreciate it (and what
ineffective protectors would have you not understand to make
their sales), the analyst's perspective must include cloud,
building, earth, and computer.  Analysis only inside computer
is too myopic.  A destructive transient seeks earth ground. 
One path to earth ground is via computer because transient was
not earthed before entering the building.

  Only now are we ready to identify that common mode transient
path, inside computer, to earth ground.  There is no such
outgoing path through CPU or memory.  One path may exist via
printer port - depending on how and where printer connects. 
But to find those outgoing paths, analyst must include other
electrically conductive materials such as baseboard heat
pipes, vinyl or concrete flooring, etc.

  Some may talk about external modem damaged today.  Then
serial port fails tomorrow.  It is called overstress.  Expect
an overstressed semiconductor to fail days or even months
later.  This is why we identify the path that a common mode
transient would seek to earth ground.  This is why any or all
three incoming AC wires are typically only the incoming
current path.

  Big difference between differential and common mode
transients.  Since common mode transients are a typically
destructive type, then effective protection must be located at
the building service entrance; to earth that transient before
it can enter the building.  Once inside a building, well, one
must spend significant time locating numerous potential paths
to earth ground through that Dell computer.

Clint wrote:
> I'm not sure I'm following your incoming/outgoing path statements.
> AFAIK, every electrical device has to have both, otherwise you'd 
> have no current flowing through the device, rendering it useless.  
> Kind of like sticking a lightbulb on one end of a battery.  Until 
> you complete the circuit, you don't get a light.