The miniature circuit breaker is a very important device in electrical installations. Advancements in switching and protection technology have increased the performance of the breaker. MCB offers thermal-magnetic trip protection according to tripping characteristics. But do you know why does an MCB trip? Does frequent tripping disturb you? To find out, keep reading.
Reasons for MCB frequent tripping
The reasons for MCB tripping can be the following.
An overload can trip the MCB.
Overload is a slow and small overcurrent situation that causes the ampacity and temperature of the circuit to gradually increase over time. This type of event is characterized by a slight increase in the load on the circuit and is interrupted by the thermal trip unit of the breaker.
The thermal operation protects from moderate overloads. Under overload conditions, a thermo-metallic element (bimetallic strip) deflects until it operates a latching mechanism allowing the main contacts to open.
MCB trips on a short circuit. A short circuit can trip the MCB in milliseconds.
A short circuit is a rapid and intense overcurrent situation that causes the ampacity of the circuit to increase. This type of event is characterized by a dramatic increase in the load on the circuit and is interrupted by the magnetic trip unit of the breaker.
In magnetic operation, large overloads or short circuit current actuates a solenoid causing a plunger to strike the latching mechanism rapidly opening the main contacts.
Wrong product selection – Characteristics
Using the wrong MCB in your applications can cause frequent tripping.
The tripping characteristics of an MCB can be represented by a characteristic tripping curve that plots to trip time versus the current level. The curve shows the amount of time required for a circuit breaker to trip at a given overcurrent level. Manufacturing tolerances result in a curve that is a band bound by minimum and maximum values of total clearing time. Total clearing time is the sum of the circuit breaker’s sensing time, unlatching time, mechanical operating time, and arcing time. For currents above 125% of the circuit breaker rating at an ambient of 40°C, the circuit breaker will automatically open the circuit within limits specified by the band.
An example: If you had a C type 10A breaker and the circuit was producing 30 amps of current, the breaker would trip between 2 seconds and 1 minute. But if you had a B-type breaker it would trip between 1 second and 40 seconds in 30 amps.
Tripping time changes according to the characteristic of the MCB. Choose your MCB properly and do not forget to consider the tripping characteristics. Otherwise, it will trip again and again.
If your upstream breaker trips frequently, the wrong selectivity could be the reason. Coordination between the operating characteristics of two or more over-current protection devices is extremely important. When an over-current within established limits occurs, the device designated to operate within those limits trips whereas the other does not trip. When an over-current event occurs at the branch breaker level (CB1), and the event is within the operating characteristics of the breaker, then the branch breaker should interrupt the circuit (open) and the main breaker should remain closed and energized. Consider the selectivity of the different breakers in your circuit designs.
Every device has an operating temperature. Even if you have a temperature-compensated MCB, a high ambient temperature (above 40°C) can be the reason for tripping. You should ventilate your enclosure to avoid high temperatures.
If there is abnormal heating due to the loosening of the screw at the terminal part. Re-tighten the screw according to the specified torque.
A lightning strike can trip the MCB due to greater surge currents. If there is heavy rain, damp walls together with slightly damaged cables may result in overcurrent. Remember to use a surge protection device to protect your MCB against the negative effects of the lighting. This will add extra protection to your installation.
MCBs without a trip-free mechanism can cause a nuisance trip. The switching mechanism of a high-quality MCB is independent. It means the breaker trips internally even if the operating knob is held in an ON position. If you have a low-quality product with a broken handle MCB may not work properly.
Aging and low quality
If you use a low-quality or old MCB, your device can trip frequently. High-quality products have a long lifespan. Use the products of global brands.
High inrush currents
Inrush current is what causes the lights to dim in a house when a motor, such as that on a clothes dryer or vacuum cleaner starts up. High inrush currents (e.g. those of LED lighting systems, or electrical motors) can cause the MCB to trip. To avoid nuisance trips, circuit breakers need to be sized appropriately to compensate for inrush current.
Effect of frequency variation
MCBs are designed to operate at AC frequency 50/60 Hz. However, MCBs are especially suitable for DC applications, and frequencies up to 400 Hz can be supplied on request. These can be used on different frequencies in supply from 50-60 Hz without any derating. For higher frequencies, normal MCBs can be used with a multiplication factor which shall only affect its magnetic trip current. If you do not use your MCB in 50/60 Hz, consider the multiplication factor. Check this from the catalogs of the manufacturers.
Damaged cables and small cross-section
Carry out a visual inspection. Is there any visible damage, or are there exposed cables? This could cause false tripping and this may be a fire hazard!
In case the cross-sectional area of connecting conductor is smaller than the regulation. Change the connecting conductor or change the circuit breaker’s rated current.
Damage in the mold case
Check for damage or crack on the circuit breaker’s cover and base. If there is damage replace the miniature circuit breaker immediately.
Dust and foreign substances
Check foreign substances such as dust on the miniature circuit breaker’s surface, especially the top of the live part. There must be no dust or foreign substances to secure insulation distance.
If you are using an MCB to protect star-delta starters, sometimes it trips during the star-delta transition due to peak currents.
Remember that an MCB only has thermal-magnetic protection and never trips during an earth fault. If there is an MCB with earth leakage protection in your installation, (RCBO or ELCB) earth leakage could be the reason for tripping.