# AFCI and UPS?

A lot? With a 15 amp breaker, a 30 amp arc should cause a trip within a few seconds.

Oh!
Now we are speaking the same language. Is the NEC worried that some of that crappy lamp cord has enough resistance to limit current flow to less than, say, 30 amps when subject to essentially a direct short?

An arc is quite close to being a short circuit. To a good approximate, an arc can be modeled as a "short" with a fixed voltage drop that stays about the same regardless of current. That voltage drop is only a few volts but even if it were, say, 20 volts, the line cord would have to soak at 100 volts and to get a 100 volt drop in a line cord would take enough amps to trip a breaker.

That long?
OK, then why not require breakers that "magnetic trip" at a current closer the the rating for bedroom circuits? Or require a shorter "heat" trip time?

Just WHAT is that "significant percentage?"
If it's only, say, 10% then I say the AFCI is a WASTE.
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John Gilmer wrote:

Might help if you read http://www.cpsc.gov/volstd/afci/AFCIFireTechnology.pdf hereafter called "the paper".
The paper quotes the UL standard as only requiring a trip in 2 minutes at 200% of rated current.
Looking at the time-current curve for SquareD breakers, at 200% a trip can take 40 seconds.
Breakers work on a thermal element to trip and it takes longer the lower the overload - hence "inverse-time element". They also have a magnetic trip for "instantaneous".

From the paper: UL field tested 1,590 receptacles in 80 dwellings to determine the available fault current at the receptacle. 16% of the receptacles would likely not trip on "instantaneous" with a short at the receptacle. 44% of the receptacles would not trip on "instantaneous" with a short at the end of a 6' of #18 cord.
For a SquareD breaker, a 15A breaker may require up to 11 times the rated current to trip "instantaneously". That is 165A for a 15A breaker. More than 16% of the surveyed receptacles would not supply 165A.
Not tripping on "instantaneous" puts the trip on the inverse-time curve. And this was for a short. An arc reduces the current further, slowing the trip. "Crappy" #18 is widely used for extension cords and appliances.

An arc may involve material that has been carbonized - which can run on a lower current. And "parallel arcing faults have erratic current flow" which reduces the current value.

Yes - see above.

Lower "instantaneuos" ratings can cause nuisance trips for motors and lights which draw about 6x full load when they start. This is covered in the paper - it was considered.

I would say 10%, 4,000 fires a year, would be well worth it. Consider deaths and injuries. And the cost of medical for burns and cost of building loss, which of course we all pay for.
There was a "cost-benefit" analysis done - so the requirement for AFCIs in bedrooms had some grounding, if you'll excuse the expression.
I think the extension to all 15 & 20A circuits is a lot more questionable. Particularly since the new series/parallel devices are barely on the market (if at all) 10 months before they are required for widespread use.
Incidentally - parallel arcs were considered more dangerous than series because the current availble was much higher than a series arc.
-- bud--
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a
Frankly, that's "gud enuf."
Your AFCI causes a lot of problems to protect us from "magic shorts" which draw from 100 to 200% of the breaker rating and have the correct "signature.

So? The NEC would have specified a class of breakers with the "magnetic" trip point closed to the rating and a shorter time constant for the thermal trip. That would have solved the problem cheaply....But NO!

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John Gilmer wrote:

Fires can easily start in 40 seconds of parallel arcing.
And parallel arc doesn't necessarily cause a trip in 40 seconds.

"Lower 'instantaneuos' ratings can cause nuisance trips for motors and lights which draw about 6x full load when they start. This is covered in the paper - it was considered." Faster thermal trip would probably be an ever bigger problem with nuisance trips.
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"Bud--" <

"magnetic"
thermal
This is getting circular.
The AFCIs are required in "NEW" construction. So far as I know, there isn't any requirement that they be retrofitted in existing homes.
Yet the justification for them (over a more effective solution) is that a "new" bedroom would have a room air conditioner and an electric heater, neither of which (so the claim) could work on a 15 amp circuit.
No matter how you slice it, it's still ....
Again, I bet is 2 or 3 jails the "real reason" will become known. Odds are that when someone checks our who benefited from the rule someone might end up in jail.
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Is that circuit part of a three wire cable (Black, red, white) used for two circuits or is it just a two wire bx with the armor as the grounding conductor?
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John Grabowski wrote:

It's two conductor BX with the armor grounded.
nate
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two
AFCI circuit breakers are not required here in New Jersey, so I don't have much experience diagnosing problems with them. My thoughts are that your AFCI does not like the BX armor ground, there is something else going on with the BX cable, or it does not like your UPS. Have you tried plugging in another three prong appliance into the outlet?
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John Grabowski wrote:

No, that outlet has been unused since we've moved in. It is however the first one in the circuit so the only possibilities I can see are that the AFCI is actually a GFCI as well (anyone know?) and there's enough current flowing through the real ground due to the UPS's fault sensing circuit, OR that there is a physical problem with that receptacle, which I'll be attempting to rule out shortly, now that the sun is up. (I've been on a program of replacing all the receptacles as soon as wiring "issues" are cleared, because a lot of the old receptacles were loose as well.)
nate
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AFCI's do have GFCI functionality, but they trip at 60 ma instead of the 5 ma of a GFCI

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Nate Nagel wrote:

Update:
I replaced the receptacle. when I turned the breaker back on it tripped immediately. I popped the cover on the panel and ohmed everything out, seemed OK although the resistance hot to ground was lower than I thought (~1 meg; nothing plugged in) although granted my meter is very old and possibly inaccurate. Everything seemed OK so I tried pulling the wires *leaving* the receptacle I'd just replaced. Then reset the breaker. Fine. Plugged in UPS. Fine. Reattached wires and replaced breaker with regular 15A breaker. Fine. Tried another AFCI breaker (I just so happened to have one laying around because I wanted to split the upstairs into two circuits eventually.) Tripped immediately. Removed the wires leaving the receptacle again, new AFCI did not trip.
So what I apparently have is something is causing the AFCI to electronically trip but not because of overcurrent. It's got to be a wiring fault because I have everything unplugged. and all I know is that it is somewhere upstairs - one circuit serves the whole second floor, except for one outlet in the hallway (apparently intended for an air conditioner.)
I'm not sure why it tripped only when I plugged the UPS in before, coincidence, or just reached some kind of threshold? who knows?
F'ing great. Of course it's about 10 degrees outside, and I assume most of this wiring is in the attic. I'm a little too paranoid to just leave the regular 15A breaker in... or are the Siemens AFCI breakers known to be problematic?
nate
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Nate Nagel wrote:

Heh... all right, I feel dumb now.
When I started to replace the receptacle, I plugged the UPS and the associated extension cord into a receptacle in the hallway which was on a different circuit.
I just went upstairs to start pulling receptacles out of the wall to inspect the wiring/try to track down the presumed ground fault. Despite the fact that I thought I'd unplugged every appliance upstairs, what did I see at the first receptacle I came to, but a printer plugged in to a wall receptacle. It, of course, was connected (by a USB cable) to the computer that was connected to the UPS. I feel obligated to point out at this point that I did *not* connect it this way, and in fact, the reason that I feel particularly stupid is I was just explaining yesterday why it was important to connect all peripherals of a computer to either the same power strip or the same UPS so there's no issues with floating grounds etc. and voltage imbalances that end up going through USB or other ports. (was actually on my list of things to rectify.)
I dug out another extension cord (the reason the printer was plugged into a different receptacle is that it is on the other side of the room from the computer desk) and plugged the printer into the back of the UPS on one of the "surge only" receptacles. All appears to be good now.
I'm still not sure why the breaker tripped when I simply tried to move the UPS' power feed from one receptacle to another, unless the fact that the UPS now had a proper ground while the printer still had a bootlegged ground was enough to cause a problem.
I ASSume that this little exercise does indeed confirm that the Siemens AFCI is also a GFCI...
nate
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Yes the AFCI is a GFCI (plus). What the USB printer is plugged into should make no difference. Implied is that something is leaking via the printer. Printer on another circuit is not desireable only because backup protection is missing. But if working properly, powering from both circuits should not have contributed to AGFI tripping.
Meanwhile, temporarily disconnect neutral wire and safety ground wire of the AGFI circuit in mains box. Infinite ohms should exist between those two wires. Also infinite ohms should exist between that disconnected neutral and mains box. All tests best performed with AC electric off so that leakage and other problems do not distort that ohm meter reading nor harm meter.
Disconnected safety ground might or might not conduct to mains box - that is acceptable. But disconnected neutral must not measure conductivity to disconnected safety ground nor to mains box (test obviously done with appliances unplugged).
One final test. Ohm meter test (use extension cord to measure this) should measure conductivity from receptacle safety ground prong to disconnected safety ground wire in mains box. If this conductivity does not exist, then that test explains everything seen previously and suggests problem still exists.
Testing appliances (computer and printer) on another GFCI circuit, and tested with computer and printer split between GFCI and non-GFCI circuits should still be performed as posted earlier. Information from that test is necessary - and would provide more useful information for many reasons including because GFCI trips at only 5 ma.
AGFIs have been required for years now in New Jersey on bedroom circuits.
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w_tom wrote:

all I can say is that it apparently did, because moving the power source to the UPS solved the problem. I printed a test page after hooking everything back up, so there appears to be no damage (thank you AFCI)

I can't disconnect the "safety ground" because it grounds to the breaker box through the cable clamp... there's no separate ground conductor in the BX. (the wiring is cloth covered as well, and appears to be tinned, to give you an idea of the age of the stuff I'm dealing with.) I will test hot and neutral just to satisfy my curiosity however now that I have located *all* the appliances that were plugged in and can disconnect them all... When I get around to rewiring the second floor, I may see if I can drop a new homerun to the box, but I'm guessing that the BX has at least one hidden staple somewhere along the way. It'd be nice if I could do that though, because then I'd just pull some 12/2WG Romex and switch to a 20A breaker (assuming that everything else on that circuit is accessable from the attic.)

I could try that... I'd have to actually wire in a GFCI receptacle upstairs though, would be easier than moving all the equipment to the basement or kitchen :) (I actually do have a couple laying around though for future use in garage etc. so it's not a huge deal.)

They've probably been required here for years as well, I don't think this house has had any electrical upgrades in the last 20 years however (previous owners were not really DIY types, and were here 18 years - while the owner before that was apparently a contractor, so I'm guessing he is responsible for a lot of the work) and I'm guessing that the work wasn't inspected then, ref: bootlegged grounds mentioned above.
nate
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