Arc Fault Circuit Interrupters

Residential fires caused by electricity occur more than 40,000 times each year in the U.S. alone. 40% are due to arcing faults!

Yearly statistics for U.S. residential electrical fires (necdigest.org) 700 deaths 3,000 injuries $700 million property damage

Division of residential wiring into zones Zone 0Zone 1Zone 2Zone 3 2 %36 %12 %50 % % of residential fires in each zone AFCI protection

bedrooms living rooms kitchens closet/storage areas garages bathrooms laundries halls dining rooms Fire locations (in descending order):

The National Electrical Code requires arc fault protection for the circuits supplying receptacle outlets in the bedrooms of new homes. The AFCI has been called the first major advance in home electrical system protection since the ground fault circuit interrupter was introduced in the late 1960's. Arc heat can exceed 6000 degrees Celsius and ignite building components such as framing, flooring, or paneling. Frayed line cords can ignite furniture, rugs or draperies. Common causes of electrical arcing faults include broken down insulation due to age or lightning strikes, poor electrical workmanship, and home owner abuse.

Circuit Breaker Review A circuit breaker can be designed to trip on:  Current faults (oldest technology)  Ground faults  Arc faults (newest technology)  All of the above

20A CoilSpring Armature Contact points A magnetic circuit breaker trips when the current in the circuit (and coil) is large enough to move the armature.

20A CoilSpring Armature Contact points The trip time is small and the trip current is much higher than the rated current. Residential breakers trip magnetically in the range of 125 to 200 A rms.

Amperes % of circuits Percentage of circuits having short circuit current available versus short circuit current Fault at the end of 6 feet of #18 appliance wire Conclusions: 1.All circuits can supply 75 A. 2.Half can supply 255 A. 3.Half can supply 203 A with appliance cord. 4.Magnetic tripping will not occur for all dead shorts. UL data for “bolted faults” (hot to neutral dead short)

20A Spring Latch Bi-metal element Contact points A thermal circuit breaker trips when the current in the circuit is high enough to heat and warp (bend) the bi-metal element.

20A Spring Latch Bi-metal element Contact points The trip time is long and the trip current is only a little more than the rated current. The time associated with the thermal tripping of breakers is specified in UL 489.

A thermal-magnetic breaker provides better protection: fast circuit interruption for severe overloads (magnetic action) and eventual circuit interruption for modest overloads (thermal action). This allows loads like motors to come on-line without tripping the breaker. Running current: 18 A Starting current: 50 A

The thermal-magnetic breaker provides adequate protection from many kinds of faults; even some caused by a lack of common sense!

However, a thermal-magnetic breaker cannot prevent fires in all cases. Here, a frayed line cord is arcing at an average current of 15 amperes. The breaker does not trip. The heat is intense! 15 A x 120 V = 1.8 kW

Arc current Time (20 ms/div) 0 A 20 A 40 A 60 A Typical arc waveform AFCIs will trip on this waveform. Other breakers will not because the rms value of the fault current is too low to produce a thermal trip and the peak value is too low to produce a magnetic trip. AFCI trip

AFCIs use an electronic circuit that looks for an “arc signature.” The trick is to differentiate the arc signature from the high-frequency noise associated with thyristor light dimmers, sparking motor commutators, and other high-frequency sources. Also, the peak starting current demanded by motors must not cause tripping. Arc signature

Test Requirements Definition Examples Operational Denotes condition that theThe AFCI device Efficacy device trips when arc fault istrips when arc fault detected.detected. Parallel Arcing Denotes an arc fault between Damaged insulation Faults the line and neutralon fixed wiring conductors in parallel with or extension cords. the loads in the circuit. Series Arcing An arc fault in series with A corroded Faults the load. Such arcs resultor loose terminal. from loose connections, or severed wires. Time and heat can cause a series fault to become a parallel fault.

Arcing to GroundDenotes an unintentional If a wall plug or arcing fault between a part switch was not operating normally at some installed properly or potential-to-ground and where connections ground. became loose. Operation Denotes the concealment of The device should Inhibition an arc fault by the normal trip even if the arc operation of circuit fault occurs on a components such as circuit that has lamp switching devices, lamp dimmers, RF line dimmers, RF filters, long filters, or long wire lengths, etc.; use of wire lengths. Faults equipment such as drills should not be fans, or other devices. masked by drills or fans.

Unwanted Tripping Denotes a tripping function Device should not in response to an arcingtrip when switching condition that is not an arcdevices, drills, fault but a condition thathairdryers, lamp occurs as part of the normal dimmers, or motors operation of circuitare operated. components. Currently UL489 allows for a product to be labeled as "mitigating the effects of arcing faults." A UL Standard (UL1699) specifically defines an arc fault circuit interrupter.