AFCI and UPS?

[Bud--]

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 actually available.

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 alarm panels).

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:

"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:

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.

-- bud--

[John Gilmer]

Why bother? That type of arc will also trip the breaker.

Seems to me that NEC has "outsmarted" itself.

on "glowing connections" (series arcs) at receptacle binding screws that

[w_tom]

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.

[Bud--]

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

link contains a more detailed explanation of why circuit breakers aren't adequate.

-- bud--

[John Gilmer]

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.

[John Gilmer]

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.

[w_tom]

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.

[Bud--]

Might help if you read

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--

[John Gilmer]

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 water.

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.

[John Gilmer]

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!

>

[Mark Lloyd]

The resistance of the conductors would limit the current.

[Mark Lloyd]

[snip]

Electric heater as well as lamp, TV, fan etc... could be too much for

15A. [snip]

[Bud--]

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.

-- bud--

[John Gilmer]

"Bud--" <

This is getting circular.

The AFCIs are required in "NEW" construction. So far as I know, there isn&#39;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&#39;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.