w_tom wrote:
First, to have a common mode noise (leakage), the computer
must have separate incoming and outgoing electrical paths.
Incoming is AC electric. What is the outgoing path?


Well, that's why I lack enthusiasm for the common mode noise
explanation. But it can and does happen, of course. The question is, as
you suggested, what are the numbers. Simply imagine some capacitance to
ground on the AC lines. This forms the return circuit for the
high-frequency noise that trips the GFCI. The SMPS power bricks are
supposed to contain filtering to prevent this kind of "interference" but
the filter caps might have dried out.


Second, leakage through a resistance is rare. Leakage
occurs more often through reactive devices. That means the
ohm meter will not measure leakage through components whose
conductivity increases with frequency and voltage. IOW these
leaks would appear as high resistance (notice I did not say
impedance) to the meter.


I think the leakage resistance is easier to understand if you consider
that all real-world capacitors and inductors contain some resistance.
You can imagine a real capacitor as a perfect capacitor (no resistance
or inductance) in parallel with a perfect resistor. That resistance is
called series resistance or leakage resistance. Sometimes it is
frequency-sensitive, but that is not what I believe we are seeing here.

The ohmmeter will readily measure this resistance (if it is not too
high) because it is quite real.

Third, all appliances have leakage. GFCI trip is not just
from one device. Sometimes it is leakage from numerous devices
combined. And yet the meter would test every device and see
no leakage from any of them.

Yes, I agree fully. However, it was established in an earlier post that
nothing but the computer was connected to this GFCI receptacle. And that
nothing was connected to the computer.


To put numbers to what was posted - appliances typically
leak less than 150 microamps when working normally. Show me
the meter that will measure this 800 Kilohm or 1 Megohm
resistor? Meter will declare infinite resistance (more than
tens of megohms) when leakage says 'impedance' (not
resistance) is lower.

Not sure what you are saying here. Almost any meter will measure 1
megohm of resistance. I don't believe the leakage causing the problem is
due to a low-reactance condition between the hot wire and the grounding
conductor. I believe the problem may be due to a (relatively) low
resistance between the hot wire and the grounding conductor. I believe
that resistance may be leakage resistance in a capacitor in the power
brick. It is a lot more common, I think, for appliances to develop "low"
resistance leakages to ground and relatively uncommon for them to
develop "low" reactances to ground.


Not mentioned is the brand name and model of that laptop.
Not mentioned is whether that power brick is from the
manufacturer or from a third party. Not mentioned is whether
a useless power strip protector is being used. I am not sure
whether the power brick is two prong or three - another
critical fact.

It appears a 3-prong plug is being used.

But this is certain. Whether the laptop starts up or is
powered constantly, the current coming in one wire will always
equal the current going out the other - if hardware is working
properly. For those two currents to be different, then
current must have another (third) path out of the laptop /
power brick system. So where is that third path? And how
much current in that third path.

Yes. And the only path I can imagine is as described earlier.

Notice - without numbers then one can only speculate. Get
numbers. How much current is in the safety ground wire for
the entire GFCI circuit?

That shouldn't matter. The GFCI will work as well without a ground wire
as with one carrying 20 amps! It is not even connected to the ground
wire. As long as there is no net current in the hot and grounded
conductor pair, the GFCI shouldn't trip.

Does the laptop cause this problem
when it is the only item on that GFCI circuit?

Yes.

Contrary to
what that meter will say - all appliances have some leakage -
which is why the ohm meter cannot report 'real' amount of
leakage.


If we measure a leakage on the power brick of 20,000 ohms, then
regardless of what is happening with other appliances, we have
established that the power brick is capable of tripping the GFCI all by
itself and we need look no further. An alternative to using the ohmmeter
is to use a sensitive clamp-meter (or even an AC ammeter) on the
grounding conductor. Keeping in mind, of course, that these instruments
are not likely to give a meaningful measurement of the high-frequency,
common-mode noise that is the other candidate explanation for the
tripping. An oscilloscope or spectrum analyzer would be more useful.

Since there are no other appliances on that GFCI circuit, then other
appliances are irrelevant to this particular problem.

Hope I haven't added to the confusion.

Chuck