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Laptop trips GFI
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? 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. 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. 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 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. 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. Notice - without numbers then one can only speculate. Get numbers. How much current is in the safety ground wire for the entire GFCI circuit? Does the laptop cause this problem when it is the only item on that GFCI circuit? Contrary to what that meter will say - all appliances have some leakage - which is why the ohm meter cannot report 'real' amount of leakage. chuck wrote: It is especially troublesome that the GFCI trips only when the laptop is plugged into the power supply. There is supposed to be substantial electrical isolation between the so-called "hot" and "cold" grounds (i.e., between the AC and DC grounds). Unless that isolation (usually an opto-isolator) is compromised, I can't imagine a mechanism by which the laptop itself could trip a GFCI, even if it were hanging overboard and immersed in seawater while connected! Not so for the power supply, of course. Larry's theory of a monster-sized common-mode switching noise signal propagating through the AC lines to trip both GFCI outlets is looking more attractive. At least that theory supports a mechanism by which plugging in the laptop causes a GFCI trip: going from no-load to full-load would cause a change in switching characteristics that might produce more noise on the line. 20,000 ohms or less of leakage can easily be measured on a DMM and leaking RF filter capacitors are common. It would be a good idea to measure leakage with and without the laptop connected to the power supply. Chuck w_tom wrote: All appliances have leakage. Leakage that should be so low as to not add up to a problem. It is doubtful that laptop is leaking milliamps on startup. IOW there would be something else on the circuit leaking so much that just a little laptop leakage could trip the GFCI. Unlikely that you will find leakage with the ohm meter. Try. But eliminating other items from the circuit, then powering on the laptop will probably provide better information. Informative may be a current measurement of that circuit's ground wire. How much current (in AC milliamps and DC milliamps) is flowing down that ground wire for various powered on appliances on that circuit? |