MCB TRIP - What are the reasons? [Trip Curve Classes Explanation 2020]

October 2, 2019

MCB TRIP - What are the reasons? [Trip Curve Classes Explanation 2020]

MCB Trip and MCB Curves (B,C,D,K,Z)

MCB trip curves are used to show the trip current rating of the miniature circuit breakers. The trip current rating is the minimum current level at which the MCB will trip instantaneously. The trip current must persist for at least 0.1s, which is the requirement for the rating.

The trip curve can also be referred to as the I-t tripping characteristic. It consists of two sections: overload section and short circuit section. The trip duration required for the levels of overload currents is portrayed in the overload section while the instantaneous trip current level of the miniature circuit breaker is described by the short circuit section.

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Trip Curve Classes

Trip Curve Class B

The MCBs with these class characteristics experience instantaneous trips whenever the currents flowing through them is reaching between 3 to 5 times their rated current. These MCBs are used mainly for cable protection.

Trip Curve Class C

Usually, MCBs that show these class characteristics have instantaneous trips when the current flowing through them goes between 5 to 10 times its rated current. So they are suitable for domestic and residential applications and electromagnetic starting loads that require medium starting currents.

Trip Curve Class D

MCBs with these class characteristics trip instantaneously whenever the current flowing through it rates between 10.1 to 20 times its rated current. MCBs in this class is recommended for use in inductive and motor loads with high starting currents.

Trip Curve Class K

The MCBs with these class characteristics experience instantaneous trips whenever the currents flowing through them is reaching between 8 to 12 times their rated current. These MCBs can be used for inductive and motor loads with high inrush currents.

Trip Curve Class Z

The MCBs with these class characteristics experience instantaneous trips whenever the currents flowing through them is reaching between 2 to 3 times their rated current. These MCBs are usually very sensitive to short circuits and can be used for protecting highly sensitive devices such as semiconductor devices.

MCB Trip calculations

How to calculate you MCB trip settings

Look for the amperage marking on the switch of the MCB. This is usually between 15 or 20. Also look for the voltage marking also on the breaker switch, this will be between 120 or 240.
After locating the voltage and current rating, multiply the volts and the amps. The result of the multiplication is the maximum wattage load the circuit can take before tripping.

MCB Tripping reasons

What are the things that cause MCBs to trip?

Circuit Overloads

One of the main reasons why MCBs trip is as a result of circuit overload. It occurs when you try to make a circuit give more electric currents than its actual capacity. This will result in the circuit overheating, which then puts all the electrical devices connected to the circuit at risk. Take, for instance, if your desktop computer is connected to a circuit that needs 17 amps but is now using 22 amps, then the circuit of the desktop computer system will get fried and damaged. The MCB trips to prevent the overheating from happening, potentially even stopping a major fire accident. You can take care of this problem by trying to redistribute your electrical appliances and trying to keep them off the same circuits to avoid overloading the circuits. You can even put off some devices that are not currently in use to reduce the electrical Load on the circuit breaker.

Short Circuit

This is another common reason why MCBs trip. Short circuits are even more dangerous than overloaded circuits. A short circuit occurs when a ‘hot’ wire touches a ‘neutral wire’ in one of your electrical outlets. Anytime this happens, a huge amount of current will pass through the circuit, which will create a huge amount of heat, more than what the circuit can take. In this situation, the MCB will trip to shut off the circuit in order to prevent a dangerous occurrence such as a fire accident. Short circuits can occur due to a variety of reasons such as loose connection or faulty wiring. You can easily identify a case of a short circuit by a burning smell that is usually left around the circuit breaker. Also, you may notice a black or brown coloration around it.

Ground Fault Surges.

Ground fault surges are very similar to short circuits. They happen whenever a hot wire comes in contact with a ground wire made of bare copper or the body of a metal outlet box that is connected to the ground wire. When this happens, it will cause more electricity to flow through the wire, more electricity than the circuit can take. The MCB trips to protect the circuit and devices from overheating or from fire outbreaks. You can easily identify ground fault surges through black or brown discoloration around the circuit breaker. Do not overlook any of these problems whenever you notice them, because by so doing, you’ll be putting yourself and your family or roommate in great danger. If your MCB trips frequently, then it is time to notify the professionals to come and have a look at the issues. DO NOT try to do it by yourself if you’re not properly trained.