In the electrical field, low-voltage circuit breakers are crucial, as most people have heard of. However, don't underestimate them; there's a lot to it! Low-voltage circuit breakers cover a wide range of applications, with diverse product types and a wide variety of protection functions.
Generally, ordinary molded case circuit breakers mostly provide short-circuit and overload protection. Residual current circuit breakers, on the other hand, provide residual circuit protection in addition to short-circuit and overload protection. Universal circuit breakers typically provide short-circuit, overload, and grounding protection. Today, we'll delve into the setting principles and selection considerations for low-voltage circuit breaker protection.
I. Differences Between Low-Voltage Circuit Breakers and Ordinary Circuit Breakers and Considerations for Protection Setting Coefficients
Compared to ordinary circuit breakers, low-voltage circuit breakers generally offer both long-delay overload protection and instantaneous short-circuit protection. Ordinary circuit breakers rely on thermally actuated bimetallic strips for overload protection and electromagnetic trip units for short-circuit protection. However, due to the difficulty in achieving high precision in mechanical coordination, the trip unit's error is relatively large. Typically, the overload protection error is no greater than 10%, and the short-circuit protection error is no greater than 20%.
With the rapid development of electronic technology and the improvement of product reliability, many mid-to-high-end new circuit breakers now use transistors and single-chip microcomputers as electronic trip units, significantly improving accuracy and performance. Some manufacturers' products can control overload protection errors to ≤5% and short-circuit protection errors to ≤10%.
Therefore, when setting and verifying circuit breaker protection, we cannot simply copy the unchanging coefficients from design manuals; we must select appropriate coefficients based on the specific type of circuit breaker.
II. Setting the Operating Current of Long-Delay Overcurrent Trip Units
The general formula for setting the operating current of long-delay overcurrent trip units is: In ≥ Kzd1lb. Here, b is the calculated current of the line, and Kzd1 is the reliability coefficient of the long-delay trip unit for low-voltage circuit breakers, usually recommended to be 1.1 in manuals. However, in practice, this coefficient mainly considers the circuit breaker's error.
For general circuit breakers, such as CM1 and DZ20, the error of the long-delay overcurrent trip unit is 10%, so a Kzd1 of 1.1 is more appropriate. However, for cases like Moller's IZM switches using digital trip units or Schneider's NS switches with STR53 trip units, the trip unit error is ≤5%, in which case Kzd1 can be set to 1.05. This value is more accurate and can better protect the feeder cables.
III. Verification of Circuit Breaker Trip Unit Sensitivity
To ensure that low-voltage circuit breakers can reliably interrupt ground faults, we need to verify the sensitivity of the circuit breaker trip unit using the following formula: Kilkmin/In. Where, Izd is the instantaneous or short-delay trip unit setting current of the circuit breaker, lkmin is the minimum short-circuit current at the end of the protected line (generally taken as the single-phase ground fault current), and Ki is the reliability coefficient of the circuit breaker trip unit; manuals generally recommend a value of 1.3.
In fact, Ki mainly considers the error of the circuit breaker trip unit. For general electromagnetic trip units, such as CM1 and DZ20, the error is generally 20%. To ensure reliable circuit breaker operation, Ki should be greater than 1.2, so it is usually taken as 1.3. However, if using switches with high-precision electronic trip units, such as Schneider Electric's NS switches with STR trip units, the short-circuit short-delay and instantaneous trip unit errors only need to be greater than 1.15, and 1.2 is sufficient. This avoids many situations that would otherwise require additional residual current devices (RCDs) or increased cable cross-sections, saving considerable investment.
IV. Key Selection Points for Low-Voltage Circuit Breakers
1. General Principles for Selecting Low-Voltage Electrical Appliances:
The rated voltage of the low-voltage electrical appliance must not be less than the operating voltage of the circuit, i.e., Ue ≥ Ug.
The rated current of the low-voltage electrical appliance must be at least equal to the calculated operating current of the circuit, i.e., Ie ≥ lg.
The breaking current of the equipment must be greater than or equal to the short-circuit current, i.e., Izh ≥ Ich.
The guaranteed thermal stability value must not be less than the calculated value.
Low-voltage electrical appliances must also be selected based on the circuit's starting conditions. For example, fuses and automatic air switches must be selected according to their starting conditions.
2. General Selection of Circuit Breakers for Protection Against Overload, Short Circuit, and Undervoltage:
The rated voltage of the circuit breaker must be greater than or equal to the rated voltage of the line.
The rated current of the circuit breaker must be greater than or equal to the calculated load current of the line.
The rated current of the circuit breaker's trip unit must be greater than or equal to the calculated load current of the line.
The ultimate breaking capacity of the circuit breaker must be greater than or equal to the maximum short-circuit current in the line.
The single-phase-to-ground short-circuit current at the end of the line must not be less than 1.25 times the instantaneous (or short-delay) tripping setting current of the automatic switch.
The rated voltage of the circuit breaker's undervoltage release mechanism must be equal to the rated voltage of the line.
3. Selection of Circuit Breakers for Distribution:
The long-time delay operating current setting should be 0.8 to 1 times the allowable current carrying capacity of the conductor.
The reset time of 3 times the long-time delay operating current setting should not be less than the starting time of the motor with the largest starting current in the line.
The short-time delay operating current setting should not be less than 1.1 (Ijx 1.35kledm). Here, ljx is the calculated load current of the line, k is the multiple of the motor starting current, and Iedm is the rated current of the largest motor.
The short-time delay time should be verified according to the thermal stability of the protected object.
If there is no short-time delay, the instantaneous current setting should not be less than 1.1 (Tjx 1.35klkledm), where k1 is the impact coefficient of the motor starting current, taken as 1.7 to 2. If there is a short-time delay, the instantaneous current setting should not be less than 1.1 times the calculated short-circuit current value at the incoming terminal of the downstream switch.
4. Selection of Automatic Switches for Motor Protection:
The long-time delay current setting should equal the motor's rated current.
The reset time of 6 times the long-time delay current setting must meet the motor's starting time requirements.
The instantaneous setting current for squirrel-cage circuit breakers is 8-15 times the rated current of the trip unit; the instantaneous setting current for wound-rotor circuit breakers is 3-6 times the rated current of the trip unit.
5. Selection of Automatic Switches for Lighting:
The long-time delay current setting should not exceed the calculated load current of the circuit.
The instantaneous current setting should equal 6 times the calculated load current of the circuit.
The protection setting and selection of low-voltage circuit breakers is a complex but crucial process. Understanding the characteristics and error ranges of different circuit breakers, rationally determining the protection setting coefficients, and accurately selecting the appropriate type based on specific application scenarios and requirements are essential to ensuring that low-voltage circuit breakers provide reliable protection in electrical systems and guarantee the safe and stable operation of electrical equipment.
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In complex power supply systems, improper circuit breaker selection or incorrect settings can lead to anything from unexpected power outages to serious safety accidents. Choosing a reliable circuit breaker is like choosing a "smart safety guardian" for your electrical system.
Shaanxi Huadian, with years of experience in the electrical field, understands that the value of an excellent low-voltage circuit breaker lies not only in its breaking capacity listed on the parameter sheet, but also in its precise matching with your system to achieve comprehensive, safe, reliable, and intelligent protection. Using high-quality materials and rigorous processes, we ensure our circuit breakers have industry-leading mechanical and electrical lifespans, stable performance, and durability. Precise protection settings prevent cascading tripping and failure to operate, building a solid safety barrier. For inquiries, please contact us.
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