.................... Arc-fault circuit Breaker


An arc fault circuit breaker or interrupter (arc fault circuit breakers) is a circuit breaker designed to stop fires by sensing non-functional electrical arcs and disconnect power before the arc starts a fire. The arc fault circuit breakers should distinguish between a working arc that may occur in the brushes of a vacuum sweeper, light switch, or other household devices and a non-working arc that can occur, for instance, in a lamp cord that has a broken conductor in the cord from overuse. Arc faults in a home is one of the leading causes for household fires.

Arc fault circuit breakers look like a GFCI/ circuit breaker (Ground-Fault Circuit Interrupter in that they both have a test button, although it is important to  distinguish the difference between the two. GFCIs are designed to protect people against electrical shock, while arc fault circuit breakers are primarily designed to protect against fire.

Electrical Code Requirements
Starting with the 1999 version of the National Electrical Code in the United States and the 2002 version of the Canadian Electrical Code in Canada, arc fault circuit breakers are now required in all circuits that feed receptacles in bedrooms of dwelling units. The National Electrical Code is an industry consensus document adopted by many U.S. municipalities. This requirement of the NEC is typically accomplished by using a kind of circuit-breaker (defined by UL 1699) in the breaker panel that provides combined arc-fault, ground-fault, and over-current protection.

The arc fault circuit breakers is intended to prevent fire from arcs to ground and works at a higher threshold (30 mA) than the GFCI/RCD (Ground-Fault Circuit Interrupt/Residual-Current Device) implementations protecting against the safety hazard of electric shock (which operate at 6 mA). However, advanced electronics inside an arc fault circuit breakers breaker detect sudden bursts of electrical current in milliseconds, long before they would trip a regular overcurrent circuit breaker or fuse. Combined devices are available which trip at the lower, 6 mA threshold of a true GFCI/RCD.

In 2002, the NEC removed the word "receptacle" leaving "outlets", in effect adding lights within dwelling bedrooms to the requirement [debated interpretation]. The 2005 code made it more clear that all outlets must be protected, despite code making panel discussion about excluding bedroom smoke detectors from the requirement.

Beginning Jan 2008, only "combination type" arc fault circuit breakers' will meet the NEC requirement. These can protect cords as well as wiring.


Limitations
Even arc fault circuit breakers, however, do not provide protection against all of the possible circuit faults that could ignite a fire. In particular, they provide no special protection against so-called "glow faults" where a relatively low-resistance short circuit draws a modest amount of current (within the trip limits of the circuit breaker) but heats the localized area of the fault to red heat. Glow faults also can occur where a connection in series with a load suddenly develops a high resistance; this might be the result of a now-defective switch, socket, plug, or wire connection (series faults are also commonly observed in aluminum wire junctions). No practical circuit breaker could detect either such fault as there is no measurable characteristic that any circuit breaker could employ to distinguish a glow fault from the normal operation of a branch circuit.