popping GFIs [was Re: Anyone with Roger's Lite accounts?]

[phiscock-g851W1bGYuGnS0EtXVNi6w at public.gmane.org]

> It has 3 prongs.  If I remember correctly, when I use a "cheater"
> (leaving ground unconnected), the GFI does not pop.  This is probably
> not a safety concern since the power connector on the laptop itself
> appears to have only has two terminals.
>
> The poping is sometimes (always?) many minutes after the notebook is
> plugged in.
>
The GFI works by detecting any unbalance in current between line and
neutral. (In the GFI, the line and neutral conductors thread through a
magnetic toroid. If the currents are equal, then there is no net induction
of magnetic flux in the core. If the currents are unbalanced, there is an
AC flux. Another 'detection' coil turns that AC flux into a voltage that
is used to trip the GFI breaker.)  There is a time delay built into the
GFI, but the delay may also be a function of the power supply warming up.
In any case, when you use a three-prong plug the unbalanced current must
be going to the ground connection. So there is probably some leakage
current to ground in that power supply. You can be adventuresome and use a
two-prong plug or conservative and replace the power supply.

Just don't use the laptop when you're in the bathtub. ;) Or in other
situations where your body is well and truly connected to earth ground. In
those situations, the leakage current may find a path from some metallic
point on the laptop, through you, and to the earth ground.

Peter



-- 
Peter Hiscocks
Syscomp Electronic Design Limited, Toronto
http://www.syscompdesign.com
USB Oscilloscope and Waveform Generator
647-839-0325

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