Arcing periods in cartridge fuselinks
When an Arc or arcs have been established they must persist until the current reaches zero, at which time extinction will occur and it is clearly desirable that the arc or arcs should not then restrike. This process must always occur and it is desirable that satisfactory clearance should be obtained at all current levels. At relative zero currents, the duration of arcing is very short relative to the pre-arcing period and the effect of arcing on the energy let through to the protective equipment is not very significant. This is not the case at very high current levels, however, when the arcing durations may be comparable to or even greater than the pre-arcing periods.
During the pre-arcing period associated with short-circuit conditions that are high currents, the current varies with time in a manner determined by the circuit itself and the resistance of the fuse. It will be marginally lower than it would have been had the fuse not been present. The current which would have flowed in these latter circumstances is known as the prospective current and typical variations which would be obtained in a circuit containing significant inductance and some resistance are illustrated in figure1.7.
At the instant when arcs are initiated in a fuselink there is a rapid increase in the voltage drop across it. This voltage then rises as the arcs lengthen due to more material being vaporized from the element.
consideration of the circuit shown in figure 1.8, assumed to apply for a fault condition, shows that the following relationship applies:
e = iR+d/dt(L.i) +Vf
in which is the source of e.m.f R and L are the resistance and inductance of the circuit, Vf is the voltage across the fuselink and i is the current.
When the current is positive as arcing commences, it is necessary that the rate of change of current di/dt should become negative so that the current falls to zero.
Clearly, this situation will obtain more quickly and more rapid extinction will be obtained, the greater the voltage across the fuse arcs. These conditions are illustrated in figure 1.9 from which it can be seen that the current which flows during fault conditions is lower than that which would have flowed had the fuse not been present.
Fuses, therefore, have the beneficial effect of providing current limiting, which assists in reducing damage to the circuits they protect.
Fuselinks containing notched strip elements may be made to reduce the working period by increasing the number of restrictions and thus the number of arcs in series. Care must be taken however to ensure that the rate of current reduction di/dt is not so great that excessive voltages will be induced in inductive components. permissible upper limits for fuselink voltages are included in specifications.
The behavior of fuselinks with cylindrical wire elements is less well controlled because such elements do not have well-defined restrictions at which arcs occur during operation. In practice, however, conditions are not uniform throughout a wire element and distortions occur during the pre-arcing period. As a result, the cross-sectional area does vary along the element length, producing thinner and fatter sections, known as unduloids.
Gaps ultimately form at the centers of thin sections, their number not being fixed however as in notched elements. In some cases, sufficient arcs may be formed to cause excessive voltages to be produced in the protected circuits.
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