00_DotNetWarrior ) said...

Now, here is something interesting. My bedroom is equipped with a special
fuse called "AFCI" (by Siemens) 15A.

New electrical code requires bedroom outlets to be AFCI protected.

An AFCI breaker detects arc faults and will trip. Arcing (sparking) is
believed to be a common cause of house fires and would occur if you did
something like pinch an extension cord with something metal or hit a
cable when hammering a nail into a wall.

The AFCI breaker also protects for overcurrent situations, just like a
non-AFCI breaker.

1. What causes the fuse to tripped all of a sudden???? doesn't make sense
when I was sleeping.

It depends on what is on the circuit, or maybe it doesn't. When the
inspector was in for our final inspection, we were talking about AFCIs.
I was under the impression that AFCIs only protected against "parallel"
arcs, like the shorts I mentioned above. I thought they did not detect
"series" arcs, like what can happen when you turn a light switch on or
off. He told me that one problem with the current technology is that it
cannot tell the difference, so our code only requires them on the outlet
circuts and recommends that they not be used on lighting as false trips
may occur when a light is turned on or off.

So, one explanation might be that something that cycles on an off on its
own may have caused the trip. I would not likely suspect this unless you
were seeing it a little more often, though.

Another possible explanation may be along the lines of why a GFCI can
sometimes trip for no reason. AFCIs sense arcs between line and neutral
and between line and ground. The sensing between line and ground is
actually done using the same technique as GFCIs (see below), except that
many AFCI breakers have a 60 mA sensitivity -- meaning that they do not
provide the 5 mA sensitivity needed for true GFCI protection. We have two
bedrooms that have sinks in them, and though the near-the-sink outlets are
AFCI protected, they had to be GFCI outlets as the AFCI breaker was not
sufficient. I understand that some manufacturers are now producing
AFCI breakers that also provide true GFCI protection.

Back to the explanation: GFCI protection is provided by measuring the
current between the line and neutral. If they differ by more than some
threshold (the 5 mA and 60 mA levels mentioned above), then it trips.
The theory behind this is that ALL the current passing through the line
should be returning through the neutral. If it isn't, then you have a
ground fault providing an alternative return path.

One problem with GFCI detection (though this appears to be much less of
a problem these days than 20 years ago), is that transients on the power
line can cause them to trip. Transients are narrow spikes of high voltage
and are the reason why we buy surge suppressors (both power bar, as well
as whole service - the prevalance of these these days may be one reason
why I see far less false GFCI trips than 20 years ago!).

Transients are high frequency (the narrower the width, the higher the
frequency) and their movement along a power line is not unlike a signal
on a transmission line. Their movement is relatively slow (to the speed
of light) and this means that current flow due to a transient will not
appear on the line and neutral at the same moment. Depending on the size
(width and amplitude) of a transient, various wiring characteristics, and
the sensitivity of a GFCI, a transient could cause it to trip for what
appears to be no reason.

Unless you have a whole-house surge suppressor, any surge suppression in
or at your UPS would not stop the effects that could trip an AFCI or GFCI.
I suspect that this was the cause and if it is not seen regularly, it may
be because the one transient that caused it was unusually large, especially
in duration.

2. Why is this special fuse in my bedroom?

I covered this at the top.


3. Does that mean my bathroom is not GFCI protected! That's not good..
should I do something?

A quick check would be to plug in an outlet tester that has a GFCI test
button on it and press it to see if it trips the power. The electrical
inspector SHOULD have done this when the final inspection was done.

--
Calvin Henry-Cotnam
"Never ascribe to malice what can equally be explained by incompetence."
- Napoleon
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