New-Tech Europe | December 2016 | Didital Edition

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

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 50,000 A. 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

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.

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