*
AD;
Here is an excerpt of an article in the General Electric web site on arc fault…
i “During the December 1997 meeting of National Electrical Code Panel 2, the 1999 NEC was amended to require the use of Arc Fault Circuit Interrupters in 15 and 20 Ampere residential bedroom circuits beginning in the year 2002. The addition of Arc Fault Technology to existing circuit protection represents a major step forward in the evolution of residential safety devices. The technology and the products that are now coming available are an important part of the future of fire protection.”
http://www.ge.com/industrialsystems/components/moreAFCI.htm
“Arc fault” is the new article for circuit breakers in the 99 NEC. Arc faults are very dangerous situations as the steady current draw is below the thermal trip rating & the in rush current is below magnetic trip rating of the breaker. (Most breakers are of the thermal/magnetic variety.) The arc is actually an air resistor.
If a wire comes loose in dryer or range circuit and contacts grounded metal it will strike the arc and very quickly burn holes.
Arc is one of the reasons we use AC power distribution instead of DC. The alternating of the voltage & current flow means we have 0 volts twice per cycle or 120 times per second in 60 hz. This hastens the extinguishing of the arc when opening a switch. DC is very efficient, but breaking the arc was the biggest danger to people & difficult for equipment. Some DC switches would use compressed air to literally “blow it out” as in the arc. A few companies used contacts in a sealed capsule containing SF^6 to cool the arc & help extinguish it.
cheers;
JE
Replies
*
Hey guys--
a common MCCB (molded case circuit breaker) is good at clearing a dead short. It'll pass it's rated current for quite a while (minutes to hours) before opening. That's why you de-rate the breaker ampacity when you're designing circuits. You don't want to be bumping up against the breaker/conductor ampacity; the conductor will heat up.
AFCIs will save a lot of lives. Put 'em in now if you have a choice.
*
The 80% loading limit is for "continous duty", meaning 3 hours or more steadily, such as store lighting or electric heat. It doesn't refer to loads that cycle on and off excepting certain motor loads. My experience has been that breakers loaded above the 80% which serve continous loads don't last as long, though they don't necessarily trip. The bimetal action tripping time is inversely proportional to the load, so a 21 amp load might run for an hour before tripping a 20a breaker. It's unusual to find breakers tripping below their rating unless the ambient temp is pretty high.
I've been looking into using AFCI's. My local supplier has quoted SquareD QO and HOM 115's and 120's at $78.00 each, my cost. I've read that they will be required for use in residential bedrooms as of NEC 2002.
Lee
*A rule of thumb in industry is that a breaker will kick after 6 continuous cycles of over current. A fuse will blow with 1/6 cycle of over current. Overloads on motor starters fall into the 6 cycle segment as well. There is one exception - Telemchanique makes the "integral" series of motor starters that are "powered trip" (my term) where there is a mechanism that uses a small electric coil to trip it as opposed to a bi-metal spring or bar.No one has yet explained where this "arc" is happening, at the box or at the panel. The graphic shows it at the breaker, I am not yet sure why the NEC thinks this breaker will prevent fires in bedrooms. But then again, I have other issues with supposed "safety" issues and the NEC code.-Rob
*I think the arc in the graphic is spurious -- though i was interested in the difference in switching AC & DC loads described above. The GE brochure explains that AFCI detect arcing faults in the circuit that fuses & breakers don't.
*Rob:The arc in the graphic is produced when the circuit breaker contacts open. The larger the current being opened, the larger the arc. If you have ever closed a circuit breaker that was feeding a short circuit ( say a hot wire touching a metal box that is grounded) you know how much fire can fly. The arc channel in the breaker is designed to cool the flame front as it escapes so as to minimize damage inside the breaker panel.These arc fault breakers are trying to detect arcs occouring down line from the breaker. I do not know if they are detecting arcing from loose connections ( like the typical aluminum wire failures ) or hot to ground arcing ( such as frayed wires touching ) or both. But they seem to be detecting arcs that do not draw enough current to be recognized as an overload or short circuit, but will generate enough heat to start fires.Also, there are many different styles of fuses with different time vs current graphs. In the old plug (edison base ) fuses, a type W was the quick blow style, and the type T was the time delay style. The type T was the quick answer to furnace motor and room air conditioner fuse blowing with out overloading the conductors. Of course those who used 30 amp type T wanted to get to know their fire department better.Anybody remember renewable cartridge fuses?Frank
*The real rocket scientist was the one who made the screw-in sockets for all fuse ampacities the same size -- or large enough to admit a penny for that matter.The arc fault could be an air gap or faulty insulation just resistive enough to permit a spark only at the voltage peaks. So you'd get a 60Hz high-temp sparking machine, but the power dissipated in an RMS sense would slip under the thermomagnetic breaker's radar. Whether this is the scourge of the nation or not, i don't know. Must be interesting to do post-fire investigations -- I knew a guy who tried to figure out what went wrong when an airplane hit the ground, fascinating stuff...
*
i I tried logging this message yesterday, but the site went down.
ALL, repeat ALL, circuit breakers have an integral design (arc chutes) for dealing with arcs. The arc is created whenever the contacts are opened. The opening can either be a fault trip or a manual trip. Both AC & DC power devices will see arcs. DC is much more serious as the arc must be extinguished either by a large distance between the contacts, cooling the arc with air or physically interrupting the arc by placing a dielectric material between the contacts. AC arca are usually extinguished by minimal distance between the contacts & the twice per cycle 0 volt (current) benefit.
I'll restate my description of arc faults, which is a totally different condition:
>Arc faults are very dangerous situations as the
b steady current draw is below the thermal trip rating & the in rush current is below magnetic trip rating of the breaker.
(Most breakers are of the thermal/magnetic variety.) The arc is actually an air resistor load in the circuit.
An example of arc fault is to take a lamp cord and strip the insulator and separate both conductors. Hold on to the insulation and strike the bare conductors to each other. The copper will vaporize or splatter and stop the arc (fault) long before a circuit breaker will trip. Another example is to reform a paper clip into a "U" shape. Push each end into a receptacle. Use pliers or you'll burn your fingers. Again, the paper clip will usually melt away before the breaker will trip. I saw this demo in 1982. The only breaker that would trip in either condition was Square D QO.
Most molded case breakers are HACR rated. This means it will not trip under motor starting conditions. In order to achieve HACR, breakers are greatly desensitized to avoid a (magnetic) trip in high inrush conditions of motor starting. Resistance loads also start as a dead short. As the load heats, the resistance increases and the current drops quickly. This happens in milliseconds. Again the bimetallic element has enough thermal inertia so it will not heat up & trip in the few cycles it takes to for the current to drop off. All of these design elements do not favor detecting & tripping arc fault conditions.
Why does the code call for arc fault in bedrooms? What do you find in bedrooms? Electric blankets & lamp cords & children playing with metallic objects (paper clips?)! Go figure.
Hope this helps.
Cheers; JE
*
JE:
Your post reminds me of when I worked ( oh great, here he goes on a long story...) at the City of Richmond Wastewater Treatment plant. My electricians had their "method" of finding which breaker in which panel of the many to chose from would be the appropriate one to turn off the power so they could work on a dead circuit. They would short the hot wire to ground with what ever Klein tool was in their hand. I had to replace many screwdrivers and needlenose pliers due to being too short. They did complain that sometimes the breakers took forever to open!
Andrew:
Edison was the guy the base is named after. Have you ever worked on houses that had Edison base screw outlets for receptacles? You needed to screw in an adapter to plug in our now common two prong plug. I have seen advertisements of appliances with Edison screw plugs on their cords. Imagine how twisted those cords could get.
One advantage to the Edison base fuses: If you were tracing a short circuit, you could just screw in a 25 to 100 watt light bulb to power up the circuit without drawing excessive current. Also you could tell by the relative light output if you had a short circuit or an overload. This trick still comes in handy on automobiles and intermittents. Just shake the wire till the bulb glows bright.
The other name for these fuses are plug fuses, because they were the same sizes as the plugs- see above.
I was looking at a house with knob & tube wiring the other day....
Frank
*Was it Edison & Westinghouse who had the great DC/AC war? I read a bizarre story that they secretly tried to manipulated which would be used for the electric chair, thus giving the impression that the other's method = death.Imagine electrocution by DC.
*Andrew:Sorry for the delay, but yes! It was Edison vs' Westinghouse in the AC/DC wars. Edison would electrocute dogs using AC and claim to have Westinghoused them. Telsa was the brains behind AC ( transformers, induction motors, 3 phase, etc.), but Westinghouse took the idea and ran with it to promote AC as the way to transport electricity.Electrucution by DC. Well, at least you won't shake to death. Ever play with a 225 volt photoflash battery? It can get your attention real quick. I have also been bitten by the B+ in tube equipment. But DC does not have that added 60 cps tingle. I guess that is why Edison thought ( or thought he could tell the public) DC was safer.Frank
*If anyone has an historical bent, the Richmond Times has a nice article tying together the fates of Tesla, Edison, and Westinghouse with some questionable geometry ("Tesla was at the center of a highly charged triangle of electrical geniuses, with lightning bolts flashing between himself, Thomas Edison and George Westinghouse.") Center of a triangle? The author made a $6 Tesla coil and used it for a variety of mischief.I found one Web source, the Morbid Fact du Jour (read: not authoritative) agreeing with my recollection, and crediting Edison with the electrocuted cats and the electric chair(?):>October 13, 1996Thomas Edison helped to develop the electric chair in order to prove the deadly dangers of alternating Current electrical systems.You see, he was in direct competition with Westinghouse, whose AC system was becoming the preferred method of electrical production, thus threatening Edison's Direct Current (DC) system with irrelevance. Realizing that he was losing the war, Edison began holding demonstrations in which he would electrocute large numbers of cats and dogs by luring the animals onto a metal plate wired to a 1,000 volt AC generator. Fortunately, this act of cruelty did not work to sway the public to his side and the more economical and efficient AC became the electricity standard. However, the legacy of Edison's Machiavellian maneuver was the development of the electric chair.I also encountered some lovely detail on the biology and ... side effects ... of electrocution.No tour of the 'net would be complete without visiting theelectricchair.com. Includes (gulp) pictures. Tons of information....This rocket (political) scientist has a different version: "1890 Thomas Edison attempts to scare the public into rejecting the electric chair by spreading terrible stories about Westinghouse's Alternating Current (AC). The NY Legislature chooses Westinghouse over Edison's Direct Current. First electrocution."P.S. before one of my online surrogate parents again tells me I have too much free time, i did the research and writing here while eating lunch. total time = 23 minutes (digestion may take a bit longer) efficient and edifying, no?
*
A "new generation" circuit breaker?
*I stumbled upon a description of these at the Leviton Web site but could not find a satisfying technical discussion. Best I did was: http://www.zlan.com/ Pretty interesting, and a neat-o graphic of conventional circuit breaker operation: Any experience with AFCI's out there with the electron nerds? Always interested in the latest doohicky.BTW, I'm still trying to prove to Frank DuVal that an electromechanical breaker will trip well below its rated current given enough time. I think the design of the breaker supports this -- if a 20 amp breaker is expected to trip within a "reasonable time" (how long?) under 20.1 amps, it seems a logical inference that it would have to trip after a longer time at 19.9 amps -- what does the bimetal know? Where the "never trip" line is I don't know and, alas, I have found no graphs to support this in manufacturers' technical specifications.Stubbornly yours, Andrew
*Andrew:I am home because of the snow in our area also. 8" on the ground now on the news and still snowing. Not bad for a predicted 3", and it rarely snows as deep as predicted.I am going to read section 240 of the NEC and get back to you. As far as what does the bimetal know, the bi metal can be adjusted quite accurately, although ambient temperature will affect its operating point. The magnetic portion will be more accurate.In a magnetic starter for motors, a device called a heater is sized for the amp range of the motor at load. This heater is designed to produce heat in proportion to current, and is calibrated to produce a certain temperature at its rated current. This certain temperature is the point where the eutectic alloy in the starter will cease to be a solid, thus allowing the spring to open the control circuit contacts. After the alloy cools down, you can push the reset button which resets the spring action against the now solid alloy, ready to go again. I know that these heaters are sold to carry the current specified and trip with excessive current. You do not need to oversize the heaters to avoid nuisance tripping on motors that run continuously.Also, isn't this arc fault circuit interrupter an ordinary breaker? But I do recall seeing Leviton breaker panels in this area.Back soon,Frank
*AD;Here is an excerpt of an article in the General Electric web site on arc fault...i "During the December 1997 meeting of National Electrical Code Panel 2, the 1999 NEC was amended to require the use of Arc Fault Circuit Interrupters in 15 and 20 Ampere residential bedroom circuits beginning in the year 2002. The addition of Arc Fault Technology to existing circuit protection represents a major step forward in the evolution of residential safety devices. The technology and the products that are now coming available are an important part of the future of fire protection."http://www.ge.com/industrialsystems/components/moreAFCI.htm"Arc fault" is the new article for circuit breakers in the 99 NEC. Arc faults are very dangerous situations as the steady current draw is below the thermal trip rating & the in rush current is below magnetic trip rating of the breaker. (Most breakers are of the thermal/magnetic variety.) The arc is actually an air resistor.If a wire comes loose in dryer or range circuit and contacts grounded metal it will strike the arc and very quickly burn holes.Arc is one of the reasons we use AC power distribution instead of DC. The alternating of the voltage & current flow means we have 0 volts twice per cycle or 120 times per second in 60 hz. This hastens the extinguishing of the arc when opening a switch. DC is very efficient, but breaking the arc was the biggest danger to people & difficult for equipment. Some DC switches would use compressed air to literally "blow it out" as in the arc. A few companies used contacts in a sealed capsule containing SF^6 to cool the arc & help extinguish it.cheers;JE
*Neat stuff. The AFCI is like a next-generation circuit breaker -- see below. Zlan sells one, Leviton does too. Never seen it.I lack the honor of owning the NEC, or using words like "eutectic alloy" (you got me there!), but I do recall the NEC wants circuits to be loaded no more than 80% of rated capacity? Not that this has any relevance. I'm just sure I'v read more than once that a breaker will eventually trip at over 80%. The magnetic tripper, according to the Zlan summary anyway, only kicks in when there is a massive current surge.P.S. unbelievable conincidence -- we have 8" too. I love it. Hardlyh a blizzard, but a good start. Virginia drivers in the snow was pretty funny... I was weaving up I-395 at 25 mph through cars going 7-15 mph -- like a turtle race. I am now a believer in ABS.
*Andrew:Not too much coincidence, I'm just off the beltway in Alexandria/Fairfax County. When I went to the store at 10am there was hardly anyone there. I figure they were all stuck on the beltway or I-395. I couldn't beleive Fairfax hadn't closed or 2 hr delay the schools by the time my daughter left. The snow was comming down real good by then. Usually the schools have a delayed opening and the weather here is clear, just bad in the far west section of Fairfax.NEC 1996 had nothing in article 240 about arc-fault breakers or time/current relationships.The bi-metal part of the breaker appears to be the set point for the trip. The magnetic part is the short circuit protection. So if you have a fault current of 1000 amps, the magnetic section trips, and if you start a motor at 30 amps, the bimetal part will have to heat up first, thereby providing the time-delay feature.I'm sticking by a breaker should never trip at 80 percent of capacity when operating continuously in normal ambient temperatures ( say up to 120 degrees F.) If it is set to trip at 20 amps, then due to manufacturing tolerances some may trip at 19.9 amps.I have an Amprobe, do you have a variable load?Say, does this arc fault breaker mean that Jack will be able to use 20 amp circuits in bedrooms again?Frank