FEATURE SURGE & CIRCUIT PROTECTION
CIRCUIT PROTECTION: A vital consideration for
the machine builder Dr Peter Terhoeven, R&D group manager at Eaton, discusses how circuit protection ensures operator safety, machine uptime and manufacturing OEM profitability
M
anufacturers in all types of industries insist on high throughput machines,
reliable performance, reduced costs, and increased operator safety. As machine builders work to meet these demands, they face ever- greater challenges from an increasingly global marketplace. To provide optimum performance, a machine must have suitable electrical circuit protection against four possible fault conditions. The fault conditions can be broadly classified as over-currents, residual or leakage currents, arcing faults, and electrical surges. All four can affect operator safety, as well as a risk of equipment damage and extensive downtime.
OVER-CURRENTS (OVERLOAD OR SHORT- CIRCUIT CURRENT) Over-currents are caused by harsh environments, deterioration, natural or accidental damage, or overloading of the distribution system. They can be either in the form of overload or short-circuit currents. An overload current is one that exceeds the normal operating parameters of the conductors, but is confined to the electrical distribution system, whereas short-circuit currents flow outside these normal conducting paths. A temporary overload, usually caused by a
electrical surge that occurs when equipment is energised, are harmless and brief in duration. It is imperative that protection devices should not react to them. However, continuous overloading, caused by defective motors, worn bearings, equipment working beyond its normal operating parameters or too many loads connected to one circuit, are destructive and must be removed in a timely manner. If not isolated within a few milliseconds, a short circuit can result in severe insulation damage, melting of conductors, metal vaporisation, arcing and fires. Two forms of protection are used - circuit
breakers and fuses. Although the circuit breaker is considered a replacement for the fuse, both have their applications. The key advantage of the fuse is the response time, opening within 4-5ms compared to that of a circuit breaker. High fault currents are therefore prevented. Help with fuse selection is essential, as the breadth of application, together with the depth
1 MAY 2017 | ELECTRICAL ENGINEERING 0
of choice available, is vast. Circuit breakers are resettable after a fault, in
some cases even remotely. The ability to reset a circuit breaker from another location, rather than sending a technician, can improve machine up-time. Circuit breakers also perform better than fuses in circuits with inductive loads that draw heavy transient start-up currents. They can more easily be set to open on genuine faults, without ‘nuisance tripping’ during the inductive transients. Additionally, they have adjustable protection characteristics, suitable for many different applications, whereas a fuse must be selected for each individual application.
RESIDUAL OR LEAKAGE CURRENTS If a leakage current as low as 30mA 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. 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. As machine systems often contain variable speed drives, which generate operational earth leakage currents, it is essential that the RCD reacts to fault currents that are actually dangerous, without ‘nuisance tripping’ in response to a normal drive system’s earth leakage currents. Digital RCDs are now available and with real-
time measurement of the residual current, they can provide notification locally via LEDs and remotely via potential-free contacts. Faults can be recognised before tripping occurs, reducing the need for unscheduled maintenance.
ARCING FAULTS Arcing faults can occur from insulation faults or loose contacts on wiring and are the main cause of damage in electrical installations. Arc fault detection devices (AFDDs) have proven to be reliable and affordable and are 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. 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. There are many signals on the line that might be misinterpreted as arcs, for example, relay switching. Arc fault protection strategies centre on detection and their success depends critically on fast response to minimise arc energy. AFDDs work in partnership with circuit
breakers or RCBOs. AFDDs combined with miniature circuit breakers protect from serial arcing faults, and phase-neutral/phase-phase parallel faults, while combined with RCDs, they protect from phase-protective conductor faults.
SURGE PROTECTION The need for surge protection across the distribution network has grown steadily with the increasing use of electronics in machinery. Computers, PLCs, displays and communication components are becoming increasingly common and surges can wreak havoc on these electronics, causing catastrophic failures, process interruptions and failure due to repetitive damage. Causes can be external events such as lightning or grid switching or internally located motor and relay switching. Surges rise to dangerous voltage levels very
quickly, often within nanoseconds. Fuses and circuit breakers cannot react quickly enough, so alternative approaches are often necessary to add to any overcurrent measures already in place. The most widely used components are spark gaps and varistors. Spark gaps, which have a long lifetime and can absorb high amounts of energy, typically require some level of activation energy, whereas varistors are very fast without the need for energy to trigger. It is recommended to install at least one surge
protection device (SPD) per distribution cabinet, one per sensitive device and one per the sensor line that leaves a building. The cost of this is usually a tiny fraction compared to the damage it helps to prevent.
Eaton
www.eaton.eu/en/cp/gen
/ ELECTRICALENGINEERING
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