21-06-2012, 03:33 PM
Electrical Branch Circuit Breakers
INTRODUCTION
A new generation of residential electrical branch circuit breakers that incorporates technology to detect and mitigate the effects of arcing faults is described. Fire loss estimates attributed to electrical wiring and the development of the arc-fault circuit interrupter for the prevention of residential electrical fires are discussed.
Annually, over 40,000 fires are attributed to home electrical distribution systems. These fires results in more than 300 deaths and over 1,400 injuries each year. When unwanted arcing occurs, it generates high temperatures and discharges molten metal that can ignite nearby combustibles such as paper, insulation, vapors, and some carpets. The temperature of an arc can be several thousand degrees Celsius depending on the available current, voltage, and materials involved
Evolution of Circuit Protection Devices
Circuit protection devices have evolved in capability and function from the time that fuses were introduced in the late 1800s, to the introduction of circuit breakers in the 1920s, to today with the introduction of arc-fault circuit interrupters (AFCIs).These changes evolved both for convenience and to provide enhanced protection for electrical wiring systems. Fuses were the first over current protection devices used in residential electrical systems. A fuse interrupts current when an internal metal element melts if the current exceeds its rated capacity for a specified length of time. Fuses are designed with characteristics to match the application. A fast-acting fuse opens quicker than a slow blow fuse, which has a greater thermal inertia to allow short-term overloads that may occur as a consequence of a normal load change.
ARCING
Technically, an arc is defined as “a continuous luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of the electrodes”. Some arcs are a normal consequence of device operation, such as opening a light switch or commutation from a motor. These devices are designed to contain arcs from combustible surroundings. However, other arcs are unwanted and may occur as a result of damaged or deteriorated wires and cords. For arcs in electrical distribution systems, the insulating medium is an air gap(for parting arcs), wire insulation, or any other insulator used to separate the electrodes or line and neutral conductors. An arc will not jump an air gap and sustain itself unless there is at least 350 V7across the gap. Therefore, in 120/240 Vac systems, it is difficult for arcing to cause ignition unless arc tracking occurs, or the electrodes loosely contact each other causing a sustained arcing faults.
How Arcing Faults Develop
There are two basic types of arcing faults – series and parallel. Series arcing faults occur when the current-carrying path in series with the load is unintentionally broken. Arcing may occur across the broken gap and create localized heating. The magnitude of the current in a series arc is limited by the load. The series arcing currents are typically well below the typical circuit breaker’s ampacity rating (often referred to as handle rating) therefore, would never trip the conventional circuit breaker either thermally or magnetically. Series arcing can lead to overheating that can be hazardous. Examples of conditions that may result in series arcing faults include loose connections to a receptacle or a wire splice, a worn conductor from over flexing of a cable, or a pinched cable in which the conductor has been severed.
INTRODUCTION
A new generation of residential electrical branch circuit breakers that incorporates technology to detect and mitigate the effects of arcing faults is described. Fire loss estimates attributed to electrical wiring and the development of the arc-fault circuit interrupter for the prevention of residential electrical fires are discussed.
Annually, over 40,000 fires are attributed to home electrical distribution systems. These fires results in more than 300 deaths and over 1,400 injuries each year. When unwanted arcing occurs, it generates high temperatures and discharges molten metal that can ignite nearby combustibles such as paper, insulation, vapors, and some carpets. The temperature of an arc can be several thousand degrees Celsius depending on the available current, voltage, and materials involved
Evolution of Circuit Protection Devices
Circuit protection devices have evolved in capability and function from the time that fuses were introduced in the late 1800s, to the introduction of circuit breakers in the 1920s, to today with the introduction of arc-fault circuit interrupters (AFCIs).These changes evolved both for convenience and to provide enhanced protection for electrical wiring systems. Fuses were the first over current protection devices used in residential electrical systems. A fuse interrupts current when an internal metal element melts if the current exceeds its rated capacity for a specified length of time. Fuses are designed with characteristics to match the application. A fast-acting fuse opens quicker than a slow blow fuse, which has a greater thermal inertia to allow short-term overloads that may occur as a consequence of a normal load change.
ARCING
Technically, an arc is defined as “a continuous luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of the electrodes”. Some arcs are a normal consequence of device operation, such as opening a light switch or commutation from a motor. These devices are designed to contain arcs from combustible surroundings. However, other arcs are unwanted and may occur as a result of damaged or deteriorated wires and cords. For arcs in electrical distribution systems, the insulating medium is an air gap(for parting arcs), wire insulation, or any other insulator used to separate the electrodes or line and neutral conductors. An arc will not jump an air gap and sustain itself unless there is at least 350 V7across the gap. Therefore, in 120/240 Vac systems, it is difficult for arcing to cause ignition unless arc tracking occurs, or the electrodes loosely contact each other causing a sustained arcing faults.
How Arcing Faults Develop
There are two basic types of arcing faults – series and parallel. Series arcing faults occur when the current-carrying path in series with the load is unintentionally broken. Arcing may occur across the broken gap and create localized heating. The magnitude of the current in a series arc is limited by the load. The series arcing currents are typically well below the typical circuit breaker’s ampacity rating (often referred to as handle rating) therefore, would never trip the conventional circuit breaker either thermally or magnetically. Series arcing can lead to overheating that can be hazardous. Examples of conditions that may result in series arcing faults include loose connections to a receptacle or a wire splice, a worn conductor from over flexing of a cable, or a pinched cable in which the conductor has been severed.