Why Does the DC MCB Trip? 5 Common Causes of DC MCB Tripping in Solar Combiner Boxes

The tripping DC Miniature Circuit Breaker (DC MCB) in a solar combiner box is an important safety signal, indicating that the device has detected a potentially electrical failure. The cause is generally attributed to one of the following five specific problems. Here are practical suggestions to diagnosing the most common causes, from a simple loose wire to a serious short-circuit.

5 Common Causes of DC MCB Tripping in Solar Combiner Boxes

1. Reverse Polarity (Fire Hazard):

Immediate tripping after new installation or wiring changes, or the DC breaker burns up with internal arcing.

• Principle: Many DC MCBs are polarity sensitive with terminals marked +/-. Reverse connection of positive and negative terminals causes internal arc extinguishing chamber malfunction. Sustained arcing during tripping may lead to contact welding or fire.

• Solution: Disconnect all power and verify that the incoming positive wire connects to the terminal marked + and the negative to the terminal marked -. This requirement does not apply to non-polarized MCBs.

2. Over-temperature (Heat & Derating）

1.1 The DC MCB housing is overheating. Tripping may occur at sunny noon or after several hours of system operation.

• Principle: Loose terminals create contact resistance. The resulting resistive heat is conducted to the internal thermal element of the MCB, causing thermal tripping. Tripping may occur even when the actual current is below the rated value.

• Solution: Disconnect all power and verify that the incoming positive wire connects to the terminal marked + and the negative to the terminal marked -.

1.2 Combiner boxes are often installed on unshaded rooftops or inside metal enclosures, where they are typically exposed to direct sunlight. If the internal temperature exceeds the DC MCB's rated value (typically around 40°C), derating may occur — meaning the breaker will trip at a current below its nominal rating.

• Principle: The thermal tripping characteristics of DC MCB are based on ambient temperature and are typically calibrated at 30～40°C. When the temperature rises, the allowable current-carrying capacity of the DC MCB will derate. For example, a 40A DC MCB may only safely carry 30A at an ambient temperature of 70°C.

• Solution: Improve the ventilation of the combiner box, for example by installing ventilation panels or enlarging the ventilation holes. Alternatively, select an DC MCB with a higher rated current and refer to its temperature derating factor table.

3. Over-current （The Actual Current Exceeds the Rated Value）

The current monitoring indicates that the measured values are approaching or have exceeded the nominal rating of the DC MCB.

• Principle: 

  Overcurrent tripping of the DC MCB may occur under either of the following conditions:

  • The total current exceeds the DC MCB's rated current as a result of an excessive number of photovoltaic (PV) strings connected in parallel;

  • A short circuit occurs in one string, the current from other strings may backfeed into the faulted string, causing its DC MCB to trip due to overcurrent.

• Solution: Calculate the string's short-circuit current Isc x 1.25, which should be less than the DC MCB's rated current. If not, replace with a higher rated DC MCB while ensuring the wire gauge is also verifed for compliance.  

4. Short-circuit

The DC MCB trips with a loud noise and cannot be reset, or trips immediately after resetting.

• Principle: 

  • Direct contact between positive and negative conductors (over-stripping or insulation damage)；

  • Short circuit to ground due to aging and failure of the surge protective device (SPD)；

  • Short circuit between positive and negative terminals caused by water ingress or metallic objects inside the combiner box.

• Solution: 

  • Visually check the combiner box for burnt wires or melted insulation;

  • Set the multimeter to resistance mode and measure the resistance between the positive and negative terminals. Normally, the reading should be infinite. A reading of 0Ω or several Ω indicates a short circuit..

5. Inrush Current (MPPT Interference)

• Principle: It's a issue specific to photovoltaic (PV) systems. High-frequency ripple current from the inverter or charge controller may interfere with the magnetic coil of standard thermal-magnetic DC MCBs, causing premature tripping at a lower threshold. This often occurs suddenly after months of normal operation.

• Solution: Upgrade to DC MCB labeled for PV applications or SMU (Switching Mode Power Supply), which are designed to withstand ripple interference.

Quick Diagnostic Procedure

Safety Notes

• Unlike AC arcs, DC arcs have no zero-crossing point. Once formed, DC arc is much harder to extinguish. Under no circumstances should a tripped DC MCB be forcibly reset or repeatedly closed while the load is connected.

• If the DC MCB housing is blackened, gives off a burning odor, or shows visible internal arcing damage, the breaker must be replaced immediately and do not continue to use it. 

• When replacing MCB: Use only DC MCB, and PV-specific breakers clearly marked with voltage (e.g., 500VDC or 1000VDC). AC breakers are NOT permitted as replacements.

Quick Reference Table

Safety Notes

• Trip immediately when switched ON: Do not repeatedly force the MCB closed. This condition typically indicates a short circuit fault. Immediately locate and clear the short circuit point before attempting to reset the MCB

• Trip after a period of operation - housing is hot: If the DC MCB housing is blackened, gives off a burning odor, or shows visible internal arcing damage, the breaker must be replaced immediately and do not continue to use it.