Complete Guide to MCCB and DC MCCB: From Principles & Selection

Many people mistakenly assume that AC and DC circuit breakers differ only in voltage type. In reality, there are fundamental differences in arc extinction principles, structural design, and application scenarios — they must never be used interchangeably.

Alternating current naturally crosses zero every half-cycle (100 times per second at 50Hz). When the current passes through zero, the arc briefly extinguishes. The breaker takes this opportunity to rapidly stretch the arc and cool the medium, preventing it from re-igniting before the current increases again. This makes AC arc extinction relatively straightforward, and the arc chute structure is simpler.

Direct current has no zero-crossing points — the current is constant. Once an arc forms, it burns continuously and steadily at extremely high temperatures and will not extinguish on its own. DC circuit breakers must rely on specialized reinforced arc extinction systems to forcibly stretch, cool, and interrupt the arc.

⚠️ Safety Warning: AC circuit breakers must NEVER be used in DC circuits! Due to insufficient arc extinction capability, the arc cannot be extinguished during breaking, which can destroy equipment and even cause explosions and fires.

3. MCCB Selection Guide: Key Parameters Explained

3.1 Rated Current (In)

• Determined based on the actual load current of the circuit, typically 1.1–1.3 times the calculated load current
• Consider ambient temperature: derating required in high-temperature environments
• Must match cable ampacity to prevent cable overheating

• Single-phase circuit: I = P / (V × PF)
• Three-phase circuit: I = P / (√3 × V × PF)

3.2 Rated Operational Voltage (Ue)

• Must be ≥ the maximum system operating voltage
• Pay attention to distinguishing between AC/DC systems; DC circuit breakers require specialized design
• DC systems require special attention to system voltage levels (e.g., DC500V, DC1000V, DC1500V)

3.3 Breaking Capacity (Icu / Ics)

• Icu must be greater than the prospective short-circuit current at the installation point
• For critical loads or scenarios requiring rapid power restoration after faults, select the highest possible Ics (100% Icu recommended)
• For locations with high short-circuit currents such as transformer outlet cabinets, prioritize high breaking capacity ratings

3.4 Frame Size

3.5 Trip Unit Types

• Simple structure, high reliability, low cost
• Overload protection (thermal) short circuit protection (magnetic)
• Relatively fixed setting range
• Suitable for most conventional distribution scenarios

• Richer protection functions (L-long time delay, S-short time delay, I-instantaneous, G-ground fault)
• High setting accuracy, wide adjustable parameter range
• Enables selective protection coordination
• Suitable for critical applications requiring high protection precision

3.6 Pole Selection

• 1P: Single pole, used for phase protection in single-phase circuits
• 2P: Double pole, used for phase neutral in single-phase circuits, or positive negative in DC systems
• 3P: Three poles, used in three-phase three-wire circuits
• 4P: Four poles, used in three-phase four-wire circuits, or multi-pole series connection in DC systems for voltage boosting

3.7 Three-Step Selection Method

• System voltage (AC/DC, voltage level)
• Calculate load current
• Estimate short-circuit current level

• Basic protection: Thermal-magnetic type is sufficient
• Precision protection: Choose electronic trip unit
• Special requirements: Earth leakage, undervoltage, and other additional functions

• Installation method (fixed / plug-in / withdrawable)
• Installation space dimensions
• Ambient temperature and protection degree requirements

4. MCCB Installation Steps and Critical Precautions

4.1 Pre-Installation Preparation

1. Verify that the circuit breaker model and parameters match the design requirements
2. Inspect the appearance for damage, rust, and confirm all accessories are complete
3. Confirm that the installation location environmental conditions (temperature, humidity, dust, etc.) meet requirements
4. Prepare installation tools: torque wrench, screwdriver, wire stripper, multimeter, etc.

• Ambient temperature: Typically -5°C to 40°C
• Installation altitude: Generally not exceeding 2000m (derating required at high altitudes)
• Free from severe vibration, corrosive gases, and conductive dust

4.2 Detailed Installation Steps

1. Mark positions on the mounting plate or rail based on installation hole dimensions
2. For DIN rail mounting: First hook the upper part of the breaker onto the rail, then press down to lock the lower clip
3. For screw mounting: Tighten with matching bolts to ensure secure, wobble-free installation
4. When installing multiple units side by side, maintain sufficient spacing (≥10mm recommended) for heat dissipation

1. Conductor Selection: Wire cross-section must match the circuit breaker rated current to ensure sufficient ampacity
2. Stripping: Strip insulation to an appropriate length, ensuring the metal portion is fully inserted into the terminal with no exposed conductors
3. Wiring Sequence: Follow the "top-in, bottom-out" principle — power supply connects to the upper end (line side), load connects to the lower end (load side)
4. Phase Sequence: Three-phase breakers follow A, B, C phase sequence; DC systems strictly distinguish positive and negative poles
5. Torque Tightening: Use a torque wrench to tighten terminals to the specified torque for good contact

1. Install shunt releases, auxiliary contacts, alarm contacts, undervoltage releases, and other accessories per the manual
2. Pay attention to left/right accessory chamber positioning — do not install reversed
3. Connect accessory wiring correctly according to markings; control circuit voltage must match

1. Measure insulation resistance using a 500V or 1000V megohmmeter — should be ≥1MΩ
2. Check insulation between poles and between each pole and the housing
3. Confirm no short circuits, ground faults, or other hidden dangers

1. First perform no-load test: After closing, check that the breaker status indication is normal
2. Measure voltage at line and load terminals to confirm normal voltage
3. Gradually increase load and observe operation
4. If conditions permit, perform trip tests to verify protection functionality (observe safety precautions)

4.3 Key Precautions

• ⚠️ Must operate with power OFF — live wiring is strictly prohibited
• Ground wire does not pass through the breaker; connect separately to the ground busbar
• Stranded flexible wire is recommended to be crimped with terminals before connection
• Clean debris inside the cabinet after wiring to prevent foreign object short circuits

• Strictly distinguish positive and negative poles, wire correctly according to " " and "-" markings
• Polarized DC circuit breakers must not be reverse-connected
• When using multi-pole series connection for voltage boosting, ensure all poles operate synchronously

• Regularly check wiring terminals for overheating or discoloration
• Periodically clean the breaker surface and arc chute dust accumulation
• After fault tripping, identify the cause and clear the fault before reclosing
• Conduct regular operation tests to ensure flexible and reliable mechanism

5. CNLONQ LQM1/LQM3 Series DC MCCB: Product Highlight

5.1 Core Product Specifications

Parameter	LQM1/LQM3 Series Specifications
Rated Operational Voltage	Full series coverage from DC500V to DC1500V
Rated Current Range	Wide selection from 63A to 800A
Pole Configuration	Flexible options: 1P / 2P / 3P / 4P
Breaking Capacity	High rated short-circuit breaking capacity, meeting mainstream PV system requirementsOverload long-time delay protection short-circuit instantaneous protection
Protection Functions	Overload long-time delay protection short-circuit instantaneous protection
Trip Type	Thermal-magnetic trip, high reliability

Conclusion