By George E. Happell
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Thinner and fatter sections, known as unduloids. Gaps ultimately form at the centres of the thin sections and their number is not fixed, as with notched-strip elements. In some cases, the number of arcs may be great enough to cause excessive fuselink voltages to be produced. Because of the insignificant heat movements which occur during the clearance of very high currents, the thermal properties of the body, end caps and terminals of a fuselink have no significant effect on the performance or arcing, provided that they are not affected by thermal or mechanical shock.
Those which comply with IEC 60269-2-1), close agreement was only obtained for currents at and above those which give pre-arcing times of about 3 ms, and the situation was even more restricted for fuselinks used to protect semiconductors, satisfactory agreement only being obtained for currents which gave pre-arcing times of 0·3 ms or less. This led to the interesting conclusion that, although in the past the simple treatment, which is possible when heat movement is neglected, was satisfactory for a significant part of the time/current characteristic, this is no longer so, and for many present-day fuselinks it is only applicable for severe short-circuit currents.
The associated power input would then cause rapid vaporisation of the whole of each notch. As an alternative, it could be taken that there are temperature gradients within an element and that it will be hottest at the centres of the restricted sections, causing vaporisation to commence at these points. The vapour escaping through the surrounding liquid could produce a hairline crack or gap across the notch. The vapour in the gaps is not ionised initially and capacitance is present across them. A similar situation must arise also with wire elements.