Switchgear-The electrical behaviour of cartridge fuselinks

              

The electrical behaviour of cartridge fuselinks

These cartridge  fuselinks incorporate one or more elements which melt and then vaporize when currents above a certain level flow through them for a certain time. Thereafter the arc or arcs which result have to be extinguished to complete the interruption process. The operating time is therefore made up of two periods, designated the pre-arcing and arcing periods. The behavior during this period is considered in the following sections.

The pre-arcing period

The element or elements and the other conducting parts of a cartridge fuselink using posses resistance and as a result a fuselink must absorb electrical power when it carries current. While the current is constant and below a particular level, the temperatures of the various parts of the fuse must be at levels above the ambient value so that Power, equal to the input power is dissipated to the surroundings and a state of equilibrium exists.

Should the current be increased and maintained above a certain level, a new state of equilibrium will not be achieved because, although the temperatures of the  fuselink parts will rise, the power dissipated from the fuselink will not become equal to the power input by the time that parts of the element or elements melt. Disruption of the element or elements will result and circuit interruption will take place after a period of arcing. The time from the instant when the current exceeds the critical value until the melting and initial vaporization of the elements occurs is known as the pre-arcing period. The time taken thereafter to achieve interruption is known as the arcing period.

The more the current through of using exceeds the maximum value at which equilibrium can be achieved, the shorter is the time taken before melting of the element or elements occurs. This is because the power available to cause the temperatures to rise is equal to the difference between the input power, which is proportional to the square of the current, and the power dissipated from the cartridge fuselink. The latter quantity is limited because the temperatures of the  fuselink parts cannot exceed the melting point of the element material.

As a result, all cartridge fuses have inverse processing time / current characteristics in the range of currents above that at which thermal equilibrium can be established. A current marginally above this latter level would theoretically likely cause operation after an infinite time.

Clearly, this current could not be determined experimentally and therefore in practice tests are done on a number of similar cartridge fuselinks to determine a current level at which each of them will operate in a particular time, typically 1-4 hours depending on the fuse rating. This current is termed the 'conventional fusing current'. Further, the tests are done on the same number of fuselinks to determine the current level, termed the 'conventional non-fusing current at which none of them will operate in the same time as that used in the earlier tests. A current between the above two current values is done designated as the minimum fusing current of the fuselink and for simplicity, only this value is referred to in the remainder of this chapter.

It will be appreciated that a cartridge fuselink cannot be operated near the minimum fusing current continuously and that the rated current of the circuit and the fuse must be at a lower level. The ratio of the minimum fusing current to the rated current of a fuse link is called the fusing factor.

After a part or parts of a fuse element have melted, the current must continue to flow in the liquid metal. The situation which then arises is not fully understood but there are a number of conditions which may exist during the initial stages of vaporization. In cartridge fuselinks during the transition period when current levels are very high, it is possible that the material in the sections of reduced cross-sectional area boils. There would then be bubbles present in the liquid metal which would cause the resistance to rise, thus increasing the power input. This would lead to rapid vaporization of the notched sections. Alternatively, it could be postulated that the temperatures must be highest at the centers of restricted sections, causing vaporization to commence at these points. The vapour escaping through the surrounding liquid could produce hairline cracks or a gap across the notch.

Whatever process takes place, the vapour in the gaps will not be ionized initially and capacitance will be present across them.

Because of the small cross-sectional areas of fuselink elements, these capacitances must be small. They will charge rapidly because of the current flow which will be maintained through them by the circuit inductance. The resulting voltage will cause the gaps to break down anthers initiate arcs. This transition period between melting and arcing must always be of very short duration in a cartridge fuselink.