Electrical installations have over the years demonstrated the need for protection from the hazardous effects of the over-current condition. This condition includes both overload currents and short circuit currents. Overload current flows for example, as a result of too many devices connected to the same energy source at the same time. Short circuit current flows when there is a zero resistance path to the ground. It proved to be very useful during the formulation of Norton’s theorem; hence it also became to be known as the Norton current. The values of these currents are both larger than the rated current value of the device to be protected.
Fuses find applications in systems where the load does not vary much above the normal value (overload protection). They also find applications in systems where the loads vary considerably (short-circuit protection).
Advantages of fuses
Fuses offer a combination of exceptional features such as:
1. High breaking capacity and speed
Fuses have high breaking capacities (current interrupting ratings). The speed and the high breaking capacity give the insurance of better safety factors against equipment damages and people injury.
In the case of large short circuit currents, nothing is faster than the fuses. The consequence is that the peak current is limited down to low values by the fuse. When the application must protect sensitive power electronics downstream (inverters, UPS systems, VSDs, etc. ) the fast tripping time (2-5ms) under over current (short circuit) conditions offers the best protection.
2. Easy short circuit calculations
Fuses do not need complex short-circuit calculations. The fuse installed to protect against such current must be able to break this current as I²t₁.
3. Inexpensive solution
Cost, size, and performance are chief aspects considered by the end user. Low cost is by far the most important criteria. Fuses are still the most economical protection. They can be expanded easily where increased fault currents are concerned.
4. Mandatory fault elimination before resetting
Unlike other short-circuit protective devices, fuses cannot be reset, thus forcing the user to identify and correct the overcurrent condition before re-energizing the circuit.
No moving parts wear out or become contaminated by dust, oil, or corrosion. Fuse replacement ensures protection is restored to its original state of integrity.
6. Compact size
Compact size offers low-cost over-current protection at high short-circuit levels. In this way, a low-voltage distribution board can be made more compact.
7. Suitability for type 2 protection
By limiting short-circuit energy and peak currents to extremely low levels, fuses are particularly suitable for type 2 protection without damage to components in motor circuits.
8. Safe and silent operation
Fuses emit no gas, flames, arcs, or other materials when clearing the highest levels of short-circuit currents. In addition, the speed of operation at high short-circuit currents significantly limits the arc flash hazard at the fault location. The arc extinction is totally enclosed so that no hot or ionized gases can go outside.
9. Easy co-ordination
Standardization of fuse characteristics and a high degree of current limitation ensure effective coordination and discrimination between fuses and other devices. Current-limiting fuses make achieving selective coordination easy and simple by maintaining a minimum amp ratio between upstream and downstream fuses. Using published ratio tables eliminates the need for selective coordination studies.
10. Standard performance
Fuse links according to IEC 60269 ensure the availability of replacements with standardized characteristics throughout the world.
11. Improved power supply quality
Voltage sags caused by power system faults can cause serious problems for computer systems, drives, and other industrial equipment. Current limiting fuses interrupt high fault currents in a few milliseconds, minimizing dips in system supply voltage.
Once installed, fuses cannot be modified or adjusted in order to change their level of performance, thus malfunction is avoided.
13. Future system growth
In many plants, the total power increases with time. Consequently, the total short circuit currents increase as well. Fuses still comply with the new requirement owing to their initial large breaking capacities. Adding new fuses to an existing system helps to upgrade the breaking capacity of the protection system.
14. Low power consumption
Fuses have a low power loss compared to circuit breakers. Thus, reducing operation costs.