Circuit protection – a vital consideration for the machine builder
09 September 2016
Machine builders must be aware of their manufacturing customers’ priorities to gain a competitive edge – and these priorities centre on maximising uptime and productivity while protecting both personnel and equipment from harm.
Achieving these objectives depends heavily on a successful circuit protection strategy, however choosing and sourcing the right protection components and balancing them against one another across an entire power distribution network is complex and challenging. This article reviews the electrical problems that can occur, and their possible consequences, outlining that with the help of the component partner, solutions that are successful for both the machine builders and their end user clients can be realised.
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 induced by lightning strikes or other installed equipment. All four represent a hazard to operator safety as well as a risk of equipment damage leading to extensive down-time.
Over-currents (overload or short-circuit current)
Over-currents are caused by harsh environments, general deterioration, damage from accidents or natural causes, 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 a short-circuit current flows outside these normal conducting paths.
A temporary overload, frequently between one and six times the normal current level, is usually caused by a harmless electrical surge that occurs when motors start up or equipment is energised. Brief in duration, any conductor temperature rise is trivial, with no harmful effect, and it is imperative that protection devices should not react to them. Continuous overloading though can be caused by defective motors, worn bearings, equipment working beyond its normal operating parameters or too many loads connected to one circuit. These overloads are destructive and must be removed by protection equipment in a timely manner to prevent damage.
Unlike overload currents, a short-circuit current can be many hundred times larger than normal operating current levels, rising to in excess of 50,000A. If not isolated within a few milliseconds, damage and destruction can become rampant, resulting 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 that can damage machine power electronics are therefore prevented. The fuse’s voltage and current rating for both continuous operation as well as interruption must be carefully considered to provide the correct protection. Help with fuse selection is often useful, if not essential, as the breadth of applications where fuses can be used, together with the depth of choice available, is vast. Eaton, for example, catalogues 8,500 different fuse types.
Circuit breakers, however, are resettable – in some cases even remotely – after a fault. For some applications, 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 such as motors or transformers 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, circuit breakers have adjustable protection characteristics suitable for many different applications, whereas a fuse with exactly the right parameters must 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 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. 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 recognised before tripping occurs, which reduces the need for unscheduled maintenance, therefore increasing system uptime.
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. 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.
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 interruptions and repetitive damage leading eventually to failure. Causes can be external events such as lightning or grid switching or internally located motor and relay switching.
Surges rise to dangerous voltage levels quickly, often within nanoseconds. Fuses and circuit breakers cannot react quickly enough to prevent damage. Alternative approaches are therefore necessary to add surge protection 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 sensor line that leaves a building. The cost of SPDs is usually a tiny fraction compared to the damage it helps to prevent.
It is clear to see that there are many different types of fault conditions and suitable circuit protection methods. The ultimate success of any circuit protection strategy to increase uptime and improve operator safety depends very much on choosing the right partner. Circuit breakers, fuses and other devices rarely work best as standalone items. It therefore makes sense to source all these elements from a single partner that can guarantee their efficacy in working together, and can advise on building a balanced solution. The issue of circuit protection can become a sales advantage instead of a design problem.
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