Commonly Asked Questions About MCCB

Coordination of Cascading Protection for MCCB

Core question: Branch-end short circuit trips main breaker, causing loss of power to entire floor.

Answer: 

• Total Selectivity: Upstream Icu > max prospective short-circuit current at downstream installation point. General rule: upstream In ≥ 2.5 × downstream In, with upstream breaker being selective type with short-time delay protection.

• Partial Selectivity: This is achieved through Zone-Selective Interlocking (ZSI). Downstream fault detection sends a restraint signal upstream, forcing the upstream breaker to delay its trip until the downstream breaker has cleared the fault.

Note: Simply increasing the rated current of the upstream breaker without adjusting Im (instantaneous trip setting) is ineffective.

Why Does the Motor Operator Not Operate While Manual Closing Works?

Core question: Remote control fails, but mechanical operation Is functional.

Answer: 

• Step 1: Verify the undervoltage trip (UVT) coil voltage. An unenergized UVT will mechanically interlock the motor operating mechanism, preventing the closing lever from being engaged even when the motor is rotating.

• Step 2: For high-capacity MCCBs (630A and above), the motor charging mechanism requires  adequate time to store energy. If the storage capacitor ages and causes excessive voltage drop, the motor speed cannot compress the closing spring, resulting in failure to close.

• Step 3: Verify the positioning of the auxiliary microswitch. If the normally contact opens too early, the controller will send a false "mechanism charged" signal to the controller, which then cuts motor power early, leaving the closing spring insufficiently compressed.

Inm vs. In

Core question: How to adjust the rated current (In) within the same frame size (Inm)?

Answer: 

• Inm (Frame Size Current): The maximum continuous operating current that the physical and mechanical structure of the circuit breaker can withstand. It dictates the frame dimensions, terminal size, and short-circuit withstand capability.

• In (Rated Current): Operating current set by the the thermal-magnetic or electronic trip unit.

• In Adjustment (Same Frame, e.g., 250A): 40% – 100% of Inm; replace CT module (thermal-magnetic) or rotate setting dial (electronic); In ≤ Inm always.

Note: Long-term low-load operation can cause bimetal characteristic drift, reducing overload protection accuracy. Use the appropriate In for the actual load.

How to Select Electronic and Thermal-Magnetic MCCB？

Core question: Is it necessary to choose the more expensive electronic MCCB?

Answer: 

• TM Trip Unit (Thermal-Magnetic): Bimetal strip for thermal overload detection (overload) + solenoid instantaneous short-circuit (short circuit). Fully mechanical, chip-free, immune to EMI. Ideal for harsh environments. Accuracy affected by ambient temperature – requires temperature compensation.

• Electronic Trip Unit (ETU – Microprocessor-Based): Current is sampled via current transformers (CT) → processed and calculated by the microprocessor → trip actuation. The electronic trip unit provides three-stage protection: overload long-time delay, short-circuit short-time delay, and instantaneous protection. It supports communication protocols and zone-selective interlocking (ZSI) functionality. 

Note: 

• Choose ETU (Electronic) when: In ≥ 630A or selectivity/ZSI/communication required.

• Choose TM (Thermal-Magnetic) when: In ≤ 250A and general distribution and cost-sensitive.

Can an MCCB Be Used as an Isolator?

Core question: Is a separate isolator required for maintenance?

Answer: 

• To determine whether an MCCB may be used as an isolator means, inspect the device housing and nameplate for the isolation marking. If this marking is present, the device is approved for use as an isolator. 

  • The contacts provide a visible, identifiable air gap when the breaker is in the OFF position.

  • The device meets the required impulse withstand voltage (Uimp) rating for the installation.

• Many MCCBs are marked with an "OFF" position, but the internal contact gap may be insufficient to meet isolation requirements. These devices cannot be relied upon as isolating switches.  Always padlock the handle in OFF position and verify de-energisation with a voltage tester at the point of work.

Can DC Photovoltaic MCCB and AC MCCB be used in interchangeably?

Core question: The question is whether AC MCCBs can be cost-effectively substituted for DC MCCBs, notwithstanding the more challenging DC arc quenching requirements.

Answer: 

• DO NOT

• AC current passes through zero twice every 20 ms, enabling the arc to self-extinguish at these instants. 

• DC current has no zero-crossing points, resulting in sustained arcing. The arc-extinguishing chamber of AC MCCB is incapable of interrupting a DC arc, which can lead to direct burnout of the switch or even fire.

• Selection criteria: DC MCCBs with clear "DC" marking, polarity indication, and rated voltage (e.g., DC 1500V) are required. Installation must adopt a two-series or four-series pole configuration to ensure reliable arc extinction.

Proper Procedures for Maintenance and Periodic Testing on MCCB

Core question: No maintenance until breakdown. Replace only after failure.

Answer: 

• The internal mechanical springs and lubricating grease inside MCCB will degrade over time.

• Required annually: Loaded trip testing — verify thermal trip times at 85% and 110% of rated current (using a high-current injection tester). Pressing the "test button" alone is not effective, as it only verifies the mechanical mechanism and does not assess aging of the bimetal strip.

• Lubrication maintenance: the gearbox of the electric operating mechanism requires replenishment of low-temperature grease (non-solidifying at -40°C) every two years; otherwise, the breaker may lock up during closing in winter.

• End-of-Life replacement criteria: When the mechanical operating life approaches 80% of the manufacturer's rated value, planned replacement is recommended, regardless of its apparent physical condition. The silver-alloy contact layer will have worn away, causing contact resistance to spike and potentially resulting in dangerous overheating.