New-Tech Europe | December 2016 | Didital Edition

be selected for each individual application. Circuit breakers can also provide other functions like emergency stop and mains switching via a modular accessory assortment. Residual or leakage currents Residual or leakage currents are not as large or energetic as short- circuits, but if a leakage current as low as 30 mA is allowed to flow through a human being for more than a fraction of a second, it can cause cardiac arrest or serious harm. Accordingly, power distribution systems must include residual current devices (RCDs) that open when they detect an imbalance between energised line and neutral conductor currents. Any such imbalance normally indicates a short circuit or other electrical anomaly. Apart from electric shock risk, there is also the danger of fire arising from excessive residual currents. However, machine systems often contain variable speed drives and these generate operational earth leakage currents. Therefore it is essential that the RCD reacts adequately to fault currents that are actually dangerous, without ‘nuisance tripping’ in response to normal drive system earth leakage currents, or allowing reduced protection of the operator. Machine builders should be concerned with Type B RCDs to meet the protection requirements in machinery equipment. The challenge is to keep a high system up-time combined with a high protection level for the equipment and the operator wherever the machine is located. Therefore it is

essential to consider the compliance with all standards and regulations. Digital RCDs are now available that offer several advantages to machine users. With real-time measurement of the residual current, they can provide notification both locally via LEDs and remotely via potential-free contacts. Faults can be recognized before tripping occurs, which reduces the need for unscheduled maintenance, therefore increasing system uptime. Arcing faults Arcing faults can occur from insulation faults or loose contacts on wiring, and is the main cause of damage in electrical installations. As well as any such electrical installation damage, arc faults can easily ignite fires which may have severe impacts on operators, machinery and infrastructure. A typical cause for such an arc would be damage to a machine cable by a mechanical lifter. Insurance companies estimate that 25% of all fires caused by electrical failure have at one stage been an arc. Initially Arc Fault Detection Devices (AFDDs) were designed to protect people from fire hazards in residential buildings, but since the technology has proven to be reliable and affordable, they are now becoming increasingly attractive to machine builders. The detection of an arc is handled by complex electronic circuitry that senses high frequency signals on the power line. Arcs have a noise pattern on a wider bandwidth, different to other high frequency noise. Once an arc is detected, a connected miniature circuit breaker (MCB) or residual current circuit breaker (RCBO) will trip and cut the

supply power to the arc. The most important quality differentiator for an AFDD is low nuisance tripping. This is a challenge for a machine builder as there are many signals on the line that might be misinterpreted as arcs, for example, relay switching. A quick and safe detection (and mitigation) of an arc is extensively tested during the approval period. Accordingly, arc fault protection strategies centre on detection. AFDDs work in partnership with circuit breakers or RCBOs. The AFDD should trip on detection of any arc with the energy of 100 joules or greater, with the allowable trip time reducing as the energy of the arc increases. Overall, protection success depends critically on fast response to minimise arc energy. AFDDs are essential even in systems that already have over-current protection. Circuit breakers and RCDs cannot detect arc faults, which typically cause neither overcurrent nor residual currents. AFDDs combined with miniature circuit breakers protect from serial arcing faults as well as phase-neutral or phase-phase parallel faults. AFDDs combined with RCDs provide protection from phase-protective conductor faults. Surge Protection The need for surge protection across the distribution network has grown steadily with the ever-increasing use of electronics in machinery. Computers, PLCs, displays and communication components are becoming increasingly common as Industry 4.0 is adopted. Surges can wreak havoc on electronics, causing catastrophic failures, process

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