General Principles for the Selection of Low-Voltage Electrical Appliances:
When selecting low-voltage electrical equipment, it's crucial to follow certain key principles to ensure safety, efficiency, and proper functionality in electrical systems. These guidelines help in choosing the right components that can handle the operational demands of the circuit.
1. The rated voltage of the low-voltage appliance should be equal to or higher than the circuit’s operating voltage, i.e., Ue ≥ Ug.
2. The rated current of the device must be at least as high as the calculated working current of the circuit, i.e., Ie ≥ Ig.
3. The breaking capacity of the equipment should be sufficient to handle the short-circuit current, i.e., Izh ≥ Ich.
4. The thermal stability value must be greater than or equal to the calculated value to prevent overheating during faults.
5. Choose the electrical devices based on the starting conditions of the circuit. For example, fuses and automatic switches should be selected with motor starting currents in mind.
First, Selection of Circuit Breakers
Circuit breakers are essential for protecting circuits from overloads, short circuits, and under-voltage conditions. They provide reliable protection while ensuring safe operation of the system.
General Selection Criteria:
1. The circuit breaker’s rated voltage should be equal to or higher than the line voltage.
2. Its rated current must be greater than or equal to the line’s calculated load current.
3. The release unit’s rated current should also meet or exceed the line’s load current.
4. The interrupting capacity of the circuit breaker must be sufficient to handle the maximum short-circuit current in the line.
5. The tripping setting for short delay or instantaneous trip should be at least 1.25 times the short-circuit current at the end of the line.
6. The undervoltage release’s rated voltage should match the line’s rated voltage.
Selection of Power Distribution Circuit Breakers
1. The long-time delay setting should be set between 0.8 and 1 times the wire’s allowable current rating.
2. The return time for a 3x long-time delay setting should be longer than the motor with the highest starting current in the line.
3. The short-time delay setting should not be less than 1.1 × (Ijx + 1.35kIedm), where Ijx is the line’s calculated load current, k is the motor starting current multiplier, and Iedm is the maximum motor rated current.
4. The short-time delay duration should be based on the thermal stability of the protected component.
5. If there is no short-time delay, the instantaneous setting should be at least 1.1 × (Ijx + 1.35k1kIedm), where k1 is the impact coefficient of the motor starting current (typically 1.7–2).
Motor Protection Circuit Breaker Selection
1. The long-time delay setting should be equal to the motor’s rated current.
2. The return time for a 6x long-time delay setting should be greater than the motor’s starting time.
3. The instantaneous setting for a squirrel-cage motor is typically 8–15 times the trip unit’s rated current, while for a wound rotor motor, it is 3–6 times.
Lighting Circuit Breaker Selection
1. The long-time delay setting should not exceed the line’s calculated load current.
2. The instantaneous setting should be 6 times the line’s calculated load current.
Second, Selection of Knife Switches
Knife switches are primarily used as isolating switches and are not designed to cut fault currents. However, they must withstand the thermal and mechanical effects caused by such currents.
Selection Criteria:
1. The knife switch’s rated voltage should be equal to or greater than the circuit’s operating voltage.
2. Its rated current should be at least the circuit’s operating current. If motors are present, the starting current should be considered.
3. Check for thermal and dynamic stability: Imax ≥ Ich, where Imax is the maximum allowable current, and Ich is the three-phase short-circuit inrush current.
Third, Fuse Selection
Fuses are used for short-circuit protection and have limited overload protection capabilities. Their reliability is generally lower compared to other protective devices.
1. Selection of Fuse Element:
(1) Select based on normal operating current: Ier ≥ Ig.
(2) Verify sensitivity using Idmin/Ier ≥ Kr, where Idmin is the minimum short-circuit current, and Kr is the fuse action coefficient (usually 4).
2. Selection for Different Equipment:
(1) For single motor circuits: Ier ≥ Iqd/α, where α depends on the fuse characteristics and starting conditions.
(2) For distribution lines: Ier ≥ (Iqd1 + Ig(n-1))/α, where Iqd1 is the highest motor starting current, and Ig(n-1) is the line current excluding that motor.
(3) For lighting circuits: Ier ≥ Ig/αm, where αm is the calculation coefficient.
(4) For transformer fuses: For transformers below 160KVA, use 2–3 times the rated current; for above 160KVA, use 1.5–2 times. For low-voltage side, select based on rated current or 20% overload.
(5) For capacitor banks: Ier ≥ (1.5–2) × Iec, where Iec is the capacitor bank’s rated current.
3. Fast Fuse Selection
Fast-acting fuses are used for protecting silicon rectifiers, thyristors, and complete systems.
(1) For small rectifiers: Ier = 1.57 × IF, where IF is the thyristor’s average forward current.
(2) For large converters: Ier = (m × Ki × Ar)/Ak, where m is the number of parallel paths, Ki is the current sharing coefficient, Ar is the fuse’s maximum blow current, and Ak is the surge current of the silicon component.
Fourth, Thermal Relay Selection
Thermal relays are mainly used for overload protection in motors.
1. Selection for Continuous or Intermittent Motor Operation:
(1) Select based on motor starting time: Tf = (0.5–0.7) × Td, where Tf is the relay’s return time at 6Ie, and Td is its operating time at 6Ie.
(2) Select based on motor rated current: Iz = (0.95–1.05) × Ied, where Iz is the relay’s setting current, and Ied is the motor’s rated current.
(3) Consider phase failure protection: Use a three-pole relay for star-connected motors, and a relay with phase failure protection for delta-connected motors.
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