Functions and Selection Criteria of Fuses
1. Functions of Fuses
Commonly known as safety fuses, fuses serve to protect circuits against overload and short-circuit conditions. When the current flowing through a fuse exceeds its specified rated value, the internal melt element melts to automatically disconnect the circuit. Fuses are categorized into general-purpose fuses and special fuses for semiconductor device protection.
2. Main Factors to Consider When Selecting Fuses
2.1 Selection of General-Purpose Fuses
1. Cable and Wire Protection
Overload and short-circuit currents in circuits will overheat wires and cables, damaging insulation or even burning them off. Fuses for cable overload protection shall be selected properly according to cable ampacity, laying method, cable type and insulation temperature class. Overload protection fuses can be installed at both inlet and outlet ends of cables, while short-circuit protection fuses must be fitted at cable inlet terminals.
2. Motor Protection
A typical motor circuit consists of a fuse, contactor, thermal relay and motor. Empirically, the rated current of the fuse shall be 1.2 to 1.5 times the rated current of the motor.
3. Overload and Short-Circuit Protection for Mining Circuits
Fuses that meet mining electrical equipment standards can be used for overload and short-circuit protection of mining power lines.
4. Protection for Capacitor Switchgear
Fuses are recommended for short-circuit protection of capacitor switchgear. The rated current of the selected fuse shall not be less than 1.6 times the rated current of the capacitor.
2.2 Selection of Fuses for Semiconductor Device Protection
During rectifier operation, internal or external faults of semiconductor devices will generate fault currents that need to be quickly interrupted by fuses. Appropriate fuse selection can reliably protect semiconductor components and converter equipment. The selection principles are as follows:
1. Rated Voltage
The rated voltage of a fuse is determined by the operating voltage at its installation position and shall be greater than or equal to the circuit operating voltage.
If the circuit operating voltage exceeds the rated voltage of a single fuse, two fuses can be connected in series. In series connection, the short-circuit current at the installation point shall reach at least 10 times the rated current of the fuse.
2. Rated Current
The rated current of the fuse is determined by the RMS current at the installation point, which varies with different circuit topologies of converters. If there is no fuse with matching current rating, two identical fuses can be connected in parallel. A current imbalance of around ±5% exists between parallel fuses.
3. Breaking Overvoltage
Overvoltage will be generated in the circuit during fuse arc extinction. Excessively high overvoltage may cause reverse breakdown of semiconductor devices. The breaking overvoltage value shall be checked against the curve in product samples, and it shall not exceed the allowable reverse peak voltage of semiconductor devices.
4. Rated Breaking Capacity
The rated breaking capacity of the fuse shall be higher than the maximum possible short-circuit current of the circuit.
5. Protection Characteristics
The time-current characteristic curve of a fuse can be obtained from product samples. Other switching devices such as DC high-speed circuit breakers can be adopted for overload protection of rectifier units.
Supplementary Instructions on Fuse Matching for Fused Isolating Switches
In practical application, fuses frequently need to be configured for fused isolating switches. Fuses are classified by shape into knife-type, cylindrical and screw-type. Knife-type fuses are divided by dimensional specifications: 00C (000), 00, 0, 1, 2, 3, 4. Knife-type fuses are the mainstream matching type for fused isolating switches. During configuration, the dimensional specification of the fuse must match the requirements of the fused isolating switch base.