Mailinglist Archive: opensuse (3666 mails)

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Re: [SLE] PC Crash: Hard Disk Problems
  • From: "Carlos E. R." <robin1.listas@xxxxxxxxxx>
  • Date: Mon, 21 Mar 2005 21:40:54 +0100 (CET)
  • Message-id: <Pine.LNX.4.58.0503212130130.9807@xxxxxxxxxxxxxxxx>

The Monday 2005-03-21 at 06:56 -0800, Randall R Schulz wrote:

> GFCIs essentially _infer_ that current is traveling to ground because
> there's unequal current flow through the two power wires. (Kirchoff's
> laws to the rescue). The _assumption_ (a conservative one) is that the
> current is flowing through a human. Humans are notoriously ill-suited
> for use as electrical conductors.

English is a nice language for some kind of humor :-p

> Hmmm... I wonder, how do GFCIs deal with reactive loads? Do they time
> average the current measurement over several cycles?

The kind that are in use in Spain (they are mandatory here, as I
mentioned) are simple electromagnetic devices. Both wires, live and
neutral, are coiled with the same number of turns around the same nucleous
of a relay - manual on, automatic off. If both currents are equal, the
magnetic fields that both coils produce are equal, but oposite, with a
combined nil efect. Of course, as soon as there is a current leak there is
a current diference, and one of the fields is stronger than the other. As
soon as the difference is enough (20mA for home use, perhaps 50 for
industry, I'm unsure) it trips over.

Thus, they are current sensitive devices. They don't sense voltage, only
instantaneous current (really average), and thus know nothing about
reactive load (ie, they can not measure the I·A phase difference), they
are all the same to it. They would trigger.

More or less :-)

--
Cheers,
Carlos Robinson


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