PROTECTION - General questions

Is the peak current (cut-off) important?
Short circuits are dangerous for several reasons:
-The destructive electrodynamics effects increase as the square of the peak current value, during the short circuit.

-The destructive thermal effects increase in proportion with the I2t reached during this short circuit.

Fuselinks are designed to drastically limit these effects.
For the prospective short circuit current of 10,000 Amp rms with a totally asymmetrical current, the maximum value could reach 25,000 Amp peak. A 100A gG fuselink limits the first peak to 8,000 Amp, less than the third part of the prospective maximum value. The destructive electrodynamic effects are reduced by 90% (8,000/25,000)2.

Why is energy limitation so important?
If a protective device cuts off a short circuit current in less than one half cycle, before it reaches its total available (and highly destructive) peak value, the device is "current limiting". It restricts fault currents to such low values that a high degree of protection is given to circuit components against even very high short circuit currents. This minimises the needs of other components to have high short circuit current "withstand" ratings.

If not limited, short circuit currents in industrial applications can reach tens of thousands of amperes in the first half cycle after the start of a fault. The heat that can be produced in circuit components by the immense energy of short circuit currents can cause severe insulation damage or even explosion. At the same time, huge magnetic forces developed between conductors can crack insulators and distort and destroy bracing structures. It is therefore important that a protective device limits fault currents before they reach their full potential level.
A fuse interrupts a short circuit current very quickly. Consequently the energy let-through is very low.

Why is overcurrent protection required?
There are many factors that contribute to the occurrence of overcurrents, such as general deterioration and accidental damage. The effect of overcurrent on electrical equipment can be dramatic if no appropriate protection is provided:
- Distortion of conductors or busbars.
- Vaporisation of metal.
- Ionisation of gases.
- Arcing, fire and explosion.
- Insulation damage.

Apart from being hazardous to personnel, significant economic losses can result from downtime and repairs required to restore damaged equipment. The most widely used and respected overcurrent protective device is the fuselink.

Protection - What Specific Advantages Do Fuselinks Have?

Fuselinks have many unique performance characteristics. These include:

a. Optimum Component Protection
They reduce short circuit currents that flow to a low value by "current limitation". There is no need for complex short circuit calculations and no concerns about costly future upgrades due to system expansion with increased fault currents. Their compact size offers low cost overcurrent protection for the highest short circuit levels.

b. Safety
No emission of gas, flames, arcs or other materials when clearing any value of overcurrent up to the highest level of short circuit. In addition, the speed of operation on high short circuit currents limits significantly the flash hazard at the fault location.

c. Reliability
No moving parts to wear out or become contaminated by dust, oil or corrosion and no nuisance tripping.
If a fault occurs, the fuselink immediately operates in its predetermined manner or co-ordinates with other circuit components. The cause of the fault is then ascertained, corrected and a new fuselink fitted. Fuse replacement ensures protection is restored to its original state of integrity. It should be stressed that the time taken for the replacement is very small in relation to the fault correction.
Assured 100% protection after operation
Economical cost effective protection

d. Simple Co-ordination
Standardised fuselink characteristics and a high degree of current limitation ensure that there is simple and effective co-ordination between fuselinks and with other devices.