Low-voltage Fuses and Circuit Breakers

By: Mitja KoprivÅ?ek - Eti d.d.

The paper is attempting to describe in a simple way the physical background of low-voltage fuses and circuit breakers operation in conditions of high short-circuit currents. Further, it attempts to draw some conclusions from their operational peculiarities, that substantially influence the practical application of fuses and circuit breakers.

Introduction

Under the slogan "Safety First" are presented in the following discussion low-voltage fuses and circuit breakers as the most important and most frequent protection elements in electrical engineering, viz. electrical installations.

The basic physical and electrical processes in short-circuit conditions are shown. The paper attempts, on the one hand, to demonstrate the advantages and drawbacks of both protection elements, and, on the other hand, to indicate the way of thier interaction, with particular emphasis on their operation in the domain of high short-circuit currents.

First of all, the following criteria have to be observed when selecting the protection elements:

- Safety

- Reliability

- Quality

- Diversity

- Economy

An electrical installation has to be designed and manufactured in such a manner to allow the protection device to cut an electric circuit only when it is really necessary. But at that time with an absolute reliability! When a fault arises, the protection device, whether a fuse or a circuit breaker, must disconnect an electric circuit before a damage on installation, power source or consumer occurs.

Advantages and drawbacks of fusible cut-outs - fuses

As said above, when evaluating advantages and drawbacks of an individual protection element, the above mentioned criteria have to be observed:

Safety:

* High degree of current limitation,

* Low values of transient integrals,

* No leaking out ionizing gases,

* Low power losses.

Reliability:

* After the cut-out the fuse cartridge is replaced, the properties of the new one are fully available,

* Aging resistance,

* Simple use.

Quality:

* Today's production is highly automated, which ensures small deviations in technical characteristics of the whole quantity of products from the regular production.

Diversity:

* Use in low- and high-voltage networks,

* Protection of electronic apparatuses and devices.

Economy:

* In comparison with its capabilities and dimensions as well as price, this is the most economic solution.

The drawbacks of the fusible cut-outs are mainly due to the following facts:

* They have to be replaced when a fault occurs, which usually means a specific time without power;

* More difficult setting of the I-t overload characteristic, which depends on the physical processes in the fusible element.

Advantages and drawbacks of the circuit breakers

Comparison of circuit breakers by the same criteria:

Safety:

* Possibility of exact setting of the current-time characteristic;

* Otherwise large enough degree of the current limitation, still on account of economy.

Reliability:

* The truth is that modern design solutions ensure a high degree of reliability, but with time, or with the number of cut-offs the characteristics of each circuit breaker are gradually being deteriorated.

Quality:

* The same applies to quality.

Diversity

* Very wide possibility of use, at NN, SN and VN levels;

* The use of electronic in tripping circuits increases the applicability.

Economy:

* The economy appears primarily in short reconnecting times;

* The power circuit breakers involve a big difference in the product price.

The weakness of circuit breakers appears primarily through the following facts:

* At disconnection an exhaust of ionizing gases appears, that can cause various consequences in the switch cabinet;

* With the number of short-circuit disconnections a circuit breaker gradually loses its technical properties.

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