General Principles for the Selection of Low-Voltage Electrical Appliances:
1. The rated voltage of low-voltage electrical equipment should not be less than the working voltage of the circuit, i.e., Ue ≥ Ug.
2. The rated current of the equipment must be greater than or equal to the calculated load current of the circuit, i.e., Ie ≥ Ig.
3. The breaking capacity of the device should be sufficient to handle the short-circuit current, i.e., Izh ≥ Ich.
4. Thermal stability must be ensured, meaning the equipment's thermal withstand capability should meet or exceed the calculated value.
5. Select low-voltage devices based on the starting conditions of the circuit. For example, fuses and automatic circuit breakers must be chosen with these conditions in mind.
First, Circuit Breaker Selection
Protection: Overload, short circuit, under-voltage.
General Selection Criteria:
1. The circuit breaker’s rated voltage must be higher than or equal to the line’s rated voltage.
2. Its rated current should be at least equal to the line’s calculated load current.
3. The release unit’s rated current should match or exceed the line’s load current.
4. The circuit breaker’s breaking capacity must be sufficient to handle the maximum short-circuit current in the circuit.
5. The instantaneous (or short-delay) tripping setting should be at least 1.25 times the short-circuit current at the end of the line.
6. The undervoltage release’s rated voltage must match the line’s rated voltage.
Power Distribution Circuit Breaker Selection:
1. Long-time delay current setting should be between 0.8 and 1 times the wire’s allowable current carrying capacity.
2. The return time for a 3x long-time setting should be longer than the motor’s starting time.
3. Short-time delay current setting should be at least 1.1 × (Ijx + 1.35kIedm), where Ijx is the line’s load current, k is the motor starting current multiple, and Iedm is the largest motor’s rated current.
4. Short delay time should be set according to the thermal stability of the protected equipment.
5. If no short delay is used, the instantaneous setting should be at least 1.1 × (Ijx + 1.35k1kIedm), where k1 is the impact coefficient (1.7–2).
Motor Protection Automatic Switch Selection:
1. Long-time current setting = motor rated current.
2. The return time for a 6x long-time setting should be greater than the motor’s starting time.
3. Instantaneous setting for squirrel-cage motors is 8–15 times the trip unit’s rated current; for wound-rotor motors, it is 3–6 times.
Lighting Circuit Automatic Switch Selection:
1. Long-time delay current setting should not exceed the line’s calculated load current.
2. Instantaneous setting should be 6 times the line’s calculated load current.
Second, Knife Switch Selection
Protection: Primarily used as an isolating switch. It does not interrupt fault currents but can withstand the thermal and electrodynamic effects caused by them.
Selection Criteria:
1. Rated voltage of the knife switch must be higher than or equal to the circuit’s working voltage.
2. Rated current must be greater than or equal to the circuit’s operating current. If there are motors, calculate based on their starting current.
3. Thermal and dynamic stability must be checked: Imax ≥ ich, where Imax is the maximum allowable current, and ich is the three-phase short-circuit inrush current.
Third, Fuse Selection
Protection: Mainly for short circuits. It offers limited overload protection and is not highly reliable.
1. Selection of Fuse Element:
(1) Choose 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 (typically 4).
2. Various Equipment Fuse Selection:
(1) Single Motor Circuit: Ier ≥ Iqd / α, where Iqd is the motor starting current, and α depends on the fuse characteristics.
(2) Distribution Line: Ier ≥ (Iqd1 + Ig(n-1)) / α, where Iqd1 is the highest motor starting current, and Ig(n-1) is the remaining load current.
(3) Lighting Circuit: Ier ≥ Ig / αm, where αm is the calculation coefficient.
(4) Transformer Fuses: For transformers below 160 KVA, select high-side fuses at 2–3 times rated current; above 160 KVA, use 1.5–2 times. Low-side fuses should be selected based on rated current or 20% overload.
(5) Capacitor Bank Fuses: Ier ≥ (1.5–2) × Iec, where Iec is the capacitor bank’s rated current.
3. Fast-Acting Fuse Selection:
Used primarily for protecting silicon rectifiers, thyristors, and complete systems.
(1) Small Rectifier: Ier = 1.57 × IF, where IF is the thyristor’s average forward current.
(2) Large Converter: Ier = m × Ki × Ak, where m is the number of parallel paths, Ki is the dynamic current sharing coefficient (0.5–0.6), and Ak is the component surge current.
Fourth, Thermal Relay Selection
Protection: Overload only.
1. Selection for Continuous or Intermittent Motor Operation:
(1) Based on motor starting time: Tf = (0.5–0.7) × Td, where Tf is the return time at 6Ie, and Td is the operating time at 6Ie.
(2) Based on motor rated current: Iz = (0.95–1.05) × Ied, where Iz is the relay setting current, and Ied is the motor’s rated current.
(3) Phase failure protection: Use a three-pole thermal relay for star-connected motors, and one with phase failure protection for delta-connected motors.
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