got a weird issue. In an upstairs bedroom, I have a computer that is
running off a UPS. After investigating the wiring in the house, I have
found a significant number of grounds "bootlegged." Little by little I
am picking away at them, but until spring, the upstairs will remain as
is because it's darn cold in the attic at the moment.
So here's the deal. I have determined that there is one properly
grounded outlet in the room where the computer is, because that outlet
is fed directly from the breaker panel, and that homerun is in BX not
the cloth covered Romex that the rest of the concealed wiring is. So as
a stopgap until I get around to fixing everything correctly I wanted to
make sure that the computer's UPS was plugged into that particular
outlet for proper surge protection. I bought a heavy extension cord,
unplugged the UPS from the outlet that it was plugged into, plugged the
extension cord into the correct outlet. Then I plugged the UPS into the
extension cord, and the lights went out. Reset the breaker, figuring
that I just made an arc, if I plugged it in faster it would be OK. Same
effect. Reset the breaker, plugged the UPS back into the outlet that it
was using before, and everything is fine. This is a less than year old
Is there some fundamental incompatibility between this UPS and the AFCI?
I'm wondering if the UPS tests for the presence of an earth ground
(there is a "building wiring fault" light on it) and if so does that
cause enough current to trip the AFCI? (I wouldn't have found this
before, because the outlet into which it's currently plugged is not
grounded but has the ground bootlegged to the neutral.) I've heard that
some AFCIs are also GFCIs is why I'm asking. The AFCI is a Siemens
Q115AF breaker in the breaker panel.
I'll replace the receptacle in the morning (once it's light out) and see
if that solves the problem, but I'm not sure what kind of fault could be
in a receptacle that would cause a breaker, AFCI or not, to trip only
when a load is connected to it but not when a plug is inserted.
Does this mean also that I run the risk of having the AFCI trip if the
surge protection of the UPS kicks in?
Should I just give up the idea of having everything "to code" and ditch
the AFCI and/or investigate the possibility of running a dedicated
circuit to that bedroom to feed an outlet solely for the computer, not
protected by the AFCI?
any thoughts greatly appreciated...
replace "fly" with "com" to reply.
No, I put the AFCI in myself after purchasing the house. However, a lot
of the wiring is late-40's vintage. That was a good enough reason in my
mind to think that an AFCI would be if anything more important here than
in new construction.
replace "fly" with "com" to reply.
First, break the problem into parts; then diagnose those parts. For
example, do you have other GFCIs in the bathroom or kitchen? If so,
then good. Run that computer extension cord to that GFCI; see
computer or extension cord causes the other GFCI to trip.
Have you run anything significant on that tripping AGFI circuit?
For example, if the AFGI circuit is somehow wired so that its neutral
(white) wire is started with another circuit, then AGFI trip will
occur. For example, an iron on that newly AGFIed circuit would be a
If any newly modified safety ground has somehow shorted to a neutral
wire, then that also will trip the AGFI. But long before taking
anything apart, first verify which suspect (computer and extension
cord, or AGFI circuit) causes GFCI trip.
Second, that plug-in protector is too far from earth AND is hoping
to earth via a wire bundled with other wires. Just two in a long list
of reasons why a power strip protector does not even claim (in
numerical specifications) protection from surges that would cause
damage. The length of that earthing wire (a short distance to earth
and a significantly long distance between computer and surge
protector) has long been necessary for effective protection. That
also means the 1940s mains box must be upgraded so that earthing both
meets and exceeds post 1990 National Electrical Code requirements.
Manufacturer specs for protection are readily available for plug-in
Both the IEEE guide on surges and surge protection at:
and the NIST guide at:
say that plug-in suppressors are effective.
Plug-in suppressors, as explained in the IEEE guide, work primarily by
clamping the voltage on all wires (power and signal) to the common
ground at the suppressor. They do not work primarily by earthing.
Unless the authorities "know" about what you are doing, I suggest you
replace the AFCI with either a normal breaker or jus a GFCI breaker.
The AFCI is quickly developing a reputation among the "pros" as something
that will cause the homeowner to make a lot of trips between the bedroom and
If the NEC really, really considered arc faults to be a special danger in
bedrooms, it should have considered making arc detection set off an alarm
rather than cutting off the power.
This scheme (AFCI for bedroom circuits) is the equivalent of having a smoke
detector turn off the lights rather than just make a loud noise.
I was a little too hard on the NEC. The AFCI will cut off a circuit well
before a loose connection gets hot enough to actually be a safety problem.
It does happen. I found a loose wire when I realized that a plug for a
lamp was WARM. Years ago the fire department responded to a small fire
from a "hot" outlet.
I hope that withing a few years the AFCI manufacturers/NEC will sort things
out. In the past the NEC as paid attention to the problems of false
For the time being, I would suggest that folks don't volunteer to get them
The AFCIs out there now detect parallel arcs - line to neutral or ground
(a fault). They don't detect a loose connection - a series arc. Series
arc protection is required starting Jan 2008. Far as I know these
devices are not yet on the market.
Parallel arcs can draw high current and I have read AFCIs detect pulses
around 60A. Series arcs are limited by the load (will usually be lower
than the breaker amp rating) and should be much harder to detect and
separate from "normal" arcs.
Starting in the 2008 code AFCIs will be required on all 15 & 20A 120V
residential branch circuits. Coincidentally these will be the new AFCIs
that include series arc detection that aren't yet on the market - large
numbers of devices with inadequate field experience will be installed.
In this instance perhaps you aren't hard enough on the NEC. It
will be interesting if all the panel manufacturers even have the new
AFCIs by 2008.
At least the AFCIs out there now have a track record. Advice about not
volunteering sounds much better for the new AFCIs.
I guess I don't understand WTF is going on.
A "series" fault is, exactly, what?
My hope is that the gadgets detect a loose connection that might result in
What, exactly, DOES the gadget detect?
Often bedrooms have devices plugged in using a two wire (zip cord)
extension wire. The bed continuously rolls over that wire until
eventually the wire arcs - causes a fire. AFCI will detect that short
and cut off power before an arc can cause a fire. This is why AFCIs
are required for all bedroom circuits.
However, if you use a real Christmas tree, then the outlet that
powers lights on that tree should also have an AFCI so that a short
and resulting arc in Christmas tree lights do not create a fire. A
Christmas tree fire created when his wife only turned on lights took
out the entire house - killed almost all pets - in but 5 minutes.
These are arcs that kill and that an AFCI quashes.
You can view arcs as being of 2 kinds.
One is from a hot wire to a neutral or ground wire. This is called a
parallel arc. The possible arc current is the available fault current
which can be very high. w_tom describes a fire source from a parallel
arc. AFCIs now detect only parallel arcs.
The other kind of arc is from a loose connection, like a worn out
receptacle that loosly grips a plug. This is a series arc; it is in
series with the circuit. It is not a "fault" as it is not across the source.
AFCIs now look for high current pulses, maybe 60A. The current in a
series arc is limited to the load current, and that is far too low to be
detected by AFCIs now sold. As I said previously, detecting a series arc
at load current levels is probably not easy and differentiating from
"normal" arcs (switches, brush motors) is probably very difficult.
Waveform "signatures" are used now, and analysis should be much more
sophisticated in series AFCIs.
I was surprised when I learned AFCIs detect only parallel arcs, but
fault is the F in AFCI. Starting 1-1-08 the NEC requires AFCIs installed
(produced?sold?) to detect both series and parallel arcs. I heard
SquareD just announced them on its web site - don't know if they are
And when the 2008 NEC comes to your turf, AFCIs (the new ones) will be
required on all new 15 & 20A 120V residential branch circuits (except
The NEC (and UL) refer to the AFCIs used now that are parallel only as
"branch/feeder AFCIs" and the new series-parallel ones as "combination
type AFCIs". I'm sure it was obvious from the name which ones detected
parallel and which detected series arcs.
Far as I know, AFCIs sold now are also required to have 30mA
ground-fault detection. (This is not the same level as the 5mA detection
of a GFCI.) I think the logic is arcs may go from H-N to include ground
if a ground is present. There is an interesting paper at:
on "glowing connections" (series arcs) at receptacle binding screws that
may (or may not) eventually cause a trip with a parallel AFCI on the
ground fault function.
Devices can obviously be made with both AFCI and 5mA GFCI detection. I
believe they will have 2 test buttons.
AFCIs came out of work done at UL sponsored by the Consumer Product
Safety Commission, which was interested in reducing the number of fires.
An interesting paper from the Consumer Product Safety Commission on
AFCIs is at:
It explains the rationalle for using AFCIs, and why normal circuit
breakers aren't adequate. It also describes how AFCIs work. It is
technical enough you may like it.
If that were true, then the Christmas tree fire would not have taken
out the entire house in only five minutes. A conventional circuit
breaker trips after the fire has started. This fire is why AFCIs are
now required for bedrooms - where an sleeping occupant has the least
warning time; where such a fire can be most fatal.
Huh? Unless there is something to limit the current, arcs tend to ramp up
in current demands.
Perhaps I am missing something here but I would be hard pressed to create an
"arc" between HOT and NEUTRAL (or ground) that would NOT trip a conventional
breaker. A "direct" arc would not have anything to limit the current.
Remember another parameter - time. Conventional circuit breakers
trip after significant energy has been dissipated at the arc. To
better appreciate how circuit breakers and fuses work, lean about a
famous 'I squared t' rule. Does a 20 amp circuit breaker trip or fuse
blow immediately when current is 25 amps? No. Visit application notes
from fuse manufacturers such as Littelfuse to learn that 'I^2t" rule
and to appreciate why fuses / circuit breakers don't trip fast enough
to quash an arc. Notice how long it takes a 20 amp circuit breaker to
trip when conducting 25 amps.
1) Why would anyone want a 20 amp circuit in a bedroom?
2) All this ASSumes that suddenly, in the middle of the night, a fault
occurs that draws more than 20 amps but less than, what, 30 amps. A "magic
short" as it were. But this "magic short" is supposed to have the
"signature" of an arc. What we have is a 4 kW heat source. In the
several minutes it takes to trip the breaker, this "magic short" is supposed
to set the house of fire without setting off any of the smoke detectors.
Frankly, it "reads" like Bull Sh*t.
I suspect that some "wise guy" is making BIG money from this nonsense.
Slight OT: Some 30 years ago I used the then relatively new SOFT contact
lenses. Every evening I took them out and cleaned them and they boilding
them in a salt solution. I had no problem for over a year. I made the
salt solution from little tablets given by the optician and mixed it with
bottled distilled water in a small bottle than held the correct amount of
BUT, the federal government decided that commercial "distilled" water mixed
with salt pills (which cost about $.01 each) wasn't "safe." It ordered the
"salt" pills (of a premeasured size) off the market and essentially forced
contact lens wearers to use a "commercial" solution that, amoung other
things, user a Mercury compound as a preservative.
My eyes were quite sensitive to the Mercury so I gave up on contacts.
BUT the "rest of the story" is that the bureaucrat responsible for the silly
rule had relatives who were in the business of "packaging" the mercury
preserved contact lens solution. The bureaucrat ended up doing some hard
time but that didn't do me any good.
When "they" decided that R-12 was bad for the ozone hole, the makers of R-12
quickly had EXPENSIVE replacements. IOW: it was the BIG chemical
companies who are behind the "ozone hole" nonsense.
A short circuit will trip the breaker but this is an arc. Ideally the
breaker would trip "instantaneously", but that takes a lot of current.
The available fault current in an extension cord may not be high enough
to trip a breaker on "instantaneous". If it is high enough, the arc is
not necessarily continuous and the breaker still may not trip on
"instantaneous". And an arc is not a short circuit - the current will be
less than the available fault current. That often leaves the breaker in
its inverse-time mode. With a constant load of 30A on a 15A breaker, for
example, the breaker may not trip for 2 minutes. The time delay likely
with an arc leaves opportunity for the arc to start a fire. AFCIs were
developed to detect an arc and provide a fast trip.
The NEC requirement for AFCIs was based on research done by UL for the
CPSC. When the research was done AFCIs didn't exist. The CPSC, UL and
the NFPA (author of the NEC) think parallel AFCIs can prevent a
significant percentage of fires with electrical causes, estimated at
40,000 per year.
The information above is a part of that contained at
The link contains a more detailed explanation of why circuit breakers
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