ELECTRIC.IE • The Magazine & Website for the Irish Electrical Industry • 7 principally an arc fault is a function of the voltage and the distance between the two points of contact. Initially, an arc is composed of metallic vapours (for example, carbon) and current can flow through the damaged section (Fig 1). As the current flows through a material that is a relatively poor conductor, heat is built up due to the I^2 R heating effect. Series arc Whilst it is possible for a stable arc to be formed with low currents, say less than 2 A, there is insufficient energy to ignite the cable and hence, the probability of ignition is severely limited. Most arcing faults detected by an AFDD exist in currents ranging from 3 A to 10 A, and these values reflect normal operating currents in domestic installation circuits. Causes of series arc faults vary but include damaged conductor cores and loose connections. Parallel arc In a parallel arc, insulation breaks down between two or more conductive parts that are normally separated: line to neutral, line to earth and neutral to earth. It is more likely that a parallel arc fault will operate a typical overcurrent protective device, such as a circuit-breaker or fuse than a series arc. Parallel arcs may be formed by tight bending radius of cables, twisting and bending of flex (perhaps by being trapped under door), crushing, other mechanical damage and tracking. Means of detection of arc faults Manufacturers differ in how they choose to conform to the requirements of the Standard, however, Scolmore apply three characteristics in complex algorithms which, when taken together, will cause their AFDD to trip. These are: • Noise – HF noise in the high kHz to the MHz region. This ‘noise’ is generated by the movement of the arc roots at the cathode. Whilst there are other sources of ‘noise’ which are naturally generated by electrical equipment operating normally, these are of a different nature and the algorithm filters them out. • Continuous – ‘noise’ must not be of a brief duration or very intermittent. It is only a continuous process that will lead to the creation of a fire risk and, therefore, where ‘noise’ is intermittent this will not lead to the operation of the AFDD. • Current variation – erratic variations in current and waveform distorting the typical sinusoidal waveform of the normal supply. A series arc will have an impact on the nature of the load current within a final circuit. At the point of arcing there will be an increase in heating with a corresponding variation in current. The AFDD algorithm will sense this changing current. Unless all three elements are present the AFDD will not operate as it is unlikely a true series or parallel arc will be present. Conclusion It is well recognised that arcing can, and will, lead to fire. An AFDD provides additional protection against fire and is particularly beneficial when protecting final circuits supplying socket-outlets. LCEl iDc kV AL iNt eChEo u s e www.clicklitehouse.ie/products/plastic-consumer-units
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