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MCB Trip Curves – B, C, D, K, and Z trip curves

MCB (Miniature circuit breaker) is a re-settable device designed to protect a circuit from short circuits and overcurrents. The trip curve of an MCB (B, C, D, K, and Z curves) tells us about the trip current rating of Miniature Circuit breakers. The trip current rating is the minimum current at which the MCB will trip instantaneously. It is required that the trip current must persist for 0.1s.

Class B trip curve

Class c trip curve, class d trip curve, class k trip curve, class z trip curve, class a trip curve, importance of mcb trip curve types, trip curves for other circuit breakers.

The MCB trip curves, also known as I-t tripping characteristic consist of two sections viz, overload section and short circuit section. Overload section describes the trip time required for various levels of overload currents and the short circuit section describes the instantaneous trip current level of MCB.

Read More: Miniature Circuit Breaker (MCB) – Principle of operation

The MCB with  class B trip characteristics trips instantaneously when the current flowing through it reaches between 3 to 5 times the rated current. These MCBs are suitable for cable protection.

MCB with class C trip characteristics trips instantaneously when the current flowing through it reaches between 5 to 10 times the rated current. Suitable Domestic and residential applications and electromagnetic starting loads with medium starting currents.

MCB with  class D  trip characteristics trips instantaneously when the current flowing through it reaches between Above 10(excluding 10) to 20 times the rated current. Suitable for inductive and motor loads with high starting currents.

MCB with class K trip characteristics trips instantaneously when the current flowing through it reaches between 8 to 12 times the rated current. Suitable for inductive and motor loads with high inrush currents.

MCB with class Z  trip characteristics trips instantaneously when the current flowing through it reaches between 2 to 3 times the rated current. These types of MCBs are highly sensitive to short circuits and are used for the protection of highly sensitive devices such as semiconductor devices.

MCB with class A  trip characteristics trips instantaneously when the current flowing through it reaches between 2 to 3 times the rated current. Like Class Z MCBs, these are also highly sensitive to short circuits and are used for the protection of semiconductor devices.

MCBs with trip curve class B and trip curve class C is the most commonly used ones. MCBs with Class C trip curves can be found in the lighting power distribution boards in residential and commercial buildings. It trips as soon as the current rises between 5 to 10 times its rated current. Class B MCBs are used in the protection of electronic devices such as PLC, DC power supplies, etc. in control panels. It trips as soon as the current rises between 3 to 5 times its rated current.

In some applications, frequent current peaks occur for a very short period (100ms to 2s). For such applications, class Z-type MCBs shall be used. Class Z-type MCBs are used in circuits with semiconductor devices.

It is important to choose an appropriate MCB current rating and trip curve in order to safeguard the circuit from damage during faults. Hence it is necessary to calculate the short circuit current and inrush current before choosing an appropriate MCB rating. If the chosen MCB rating is much higher than required, then it may not trip in the event of a fault. Similarly, if the MCB is underrated, then it may cause nuisance trips, for example even the starting currents or inrush currents may trip the MCB.

External selection tool: https://new.abb.com/low-voltage/solutions/selectivity/tools-support/curves

All circuit breakers, such as MCCB, ACB, VCB, etc have their own trip characteristics. The only thing is that may not follow the categorization as that of MCB. Also, the circuit breaker curve types are not the same for all types of circuit breakers. It varies from one circuit breaker type to the other and depends on many design factors.

Learn more about MCB:

• What is an MCB?
• Miniature Circuit Breaker (MCB) – Principle of operation
• What is kA rating of MCB and MCCB?

Related Articles: 1.  Difference between MCB and MCCB 2.  Difference between contactors and relays 3.  Difference between Soft Starters and VFDs 4.  Difference between MCCB and RCCB 5.  Difference between MCB and RCBO 6.  Difference between RCCB and RCBO 7.  Difference between MPCB and MCCB

27 thoughts on “MCB Trip Curves – B, C, D, K, and Z trip curves”

Very good explanation. I understood the concept. Thank you.

Thank you, Mr. Sanket. Kindly browse through our articles. Please subscribe or follow us on twitter/facebook for instant updates.

Thankhs google team good explace thanks again

Very good mcb make , what Amps load trip make

Very good. Nice explain.. Good job

Explanation is good but your second paragraph doesn’t match the charts. It looks like it is the B-curve that trips between 3-5 times its rated current, and C-curve that trips between 5-10 times its rated current.

Very good, thanks

very good …..thanks

Thanks very much

Very good explanation

Is this curves is applicable to Rccb ?

No. These curves are applicable for mcbs only.

Thanks for your information

The information about mcb is very useful and helpful for a technician, many many thanks for sharing your information.

Great information, I got to know a few more details out of what I wanted to know.

Which type is better choice for UPS protection?

The explanations are very good but in the video is a mistake at minute 0.38. The short circuit sections with the overload section are reversed.

Good for selection of MCB’s

On the c type Mcb on the time curves at a short circuit fault current at 220amp it shows dis connection at 6/7seconds are you saying that disconnection will be instant at this current or 6/7 seconds.

I use B-curve in my home when short circuit occured in the appliace MCB tripped but my appliance burned. My appliance lead wires were shorted by a metal piece was lying on it.I thought MCB could have protected but not. And I also headed big noise of it.

Sorry to hear that. This could be because the MCB was oversized: Much higher than the rated current of the appliance or the MCB could be faulty. We suggest you replace it with a new one. Make sure that you are choosing the right one.

Thanks for sharing such an informative article about MCB.

sir Type C is used for average current load. Type B and C are the most commonly used in DBs. Tripping of MCB Type C is 5-10 times higher than normal.  eg: if a 6A mcb put in acircuit , the rated current is 6 A , then how ever the type c mcb with stand 5 to 10 times higherr than normal .

hello, what about the CL curve mcb, because in my home installation I used the cl4 code on the mcb

Perhaps you are referring to product name of the MCB and not its trip curve.

The information is quite educative. Thank you so much

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Home > Protection > Tripping Curves of Circuit Breakers – B, C, D, K and Z Trip Curve

Tripping Curves of Circuit Breakers – B, C, D, K and Z Trip Curve

Types of circuit breaker based on its tripping curve.

A circuit breaker is a protection device employed in every electrical circuit to prevent any potential hazard. There are different types of circuit breakers used all over the world due to their various characteristics & applications. It is necessary to have a circuit breaker that offers adequate protection so that one can work safely around it without having fear of any potential hazards. That is why it is best to know about these kinds of circuit breakers & what kinds of protection do they offer before buying one.

Table of Contents

What is a Circuit Breaker?

A circuit breaker is an electrical device that provides protection against fault current. It breaks the circuit in case of overloading & short circuit. The fault currents generated due to these fault conditions can damage the electrical devices as well as cause fire in a building that can also pose danger to human life.

The circuit breaker instantly cut off the power supply to reduce further damage. A circuit breaker has two types of tripping unit i.e. thermal and magnetic tripping unit.

Thermal Tripping Unit: the thermal tripping unit is used for protection against overloading. It uses a bi-metallic contact that bends with a change in temperature. The current flowing through the bimetallic strip heats up contact & trip the circuit breaker.

The rate of bending of the bi-metallic strip depends on the amount of current. Therefore, greater the overloading current, faster the circuit breaker trips.  

Magnetic Tripping Unit: The magnetic trip unit is used for protection against short circuit current. it includes a solenoid that produced a strong magnetic field due to high short circuit current to instantly trip the circuit breaker.

Related Posts:

• MCB (Miniature Circuit Breaker) – Construction, Working, Types & Applications
• MCCB (Molded Case Circuit Breaker) – Construction, Types & Working

What is a Trip Curve?

A trip curve also known as a current time graph is a graphical representation of the response of a circuit breaker. It shows the current relationship with the tripping time of a protection device.

Why We Need Different Tripping Curves?

Circuit breakers are used for tripping the power supply as quickly as possible in case of overcurrent. But it should not trip so fast & unnecessary that it becomes a problem.

The overcurrent can happen under normal conditions such as the inrush current of a motor. Inrush current is the huge current draw during the starting of a motor that causes voltage dips in the main line. The circuit breaker should be able to tolerate the inrush current & it should provide some delay before tripping.

Therefore, the circuit breaker selected should not trip so fast that it creates a nuisance & it should not trip so late that it causes any damage. This is where the tripping characteristics of the circuit breakers come into play.

The tripping curve tells how fast a circuit breaker will trip at a specific current. The different tripping curves classify the circuit breakers into categories where each category is used for specific types of loads. It is essential to select a circuit breaker that provides the necessary overcurrent protection.

• Types of Circuit Breakers – Working and Applications
• Air Circuit Breaker (ACB): Construction, Operation, Types and Uses

How to read a Trip Curve?

The following figure shows a chart of a trip curve.

The horizontal X-axis represents the multiples of the current flowing through the circuit breaker. While the Y-axis represents the tripping time of the circuit breaker on a logarithmic scale.

The thermal region shows the response of the bimetallic contact trip unit during overcurrent. The curve shows that the circuit breaker’s tripping time reduces with an increase in the current. The first curve in the graph shows the response of a thermal trip unit.

While the magnetic region shows the response of the solenoid to fault current such as a short circuit current.

As seen from the graph, a circuit breaker does not have a fixed tripping time and we cannot predict an exact tripping point. It is because the tripping is affected by ambient conditions such as temperature. Think of it as a Schrödinger’s Cat area, we do not know when the tripping will occur unless the event happens. 

Types of Circuit Breaker Based on Tripping Curves

The circuit breakers are classified into the following five types based on their tripping curves.

Such type of circuit breaker is designed to instantly trip when the operating current is 3 to 5 times its rated current. Their tripping time falls between 0.04 to 13 seconds. They are suitable for domestic applications where surges are very low such as lighting & resistive loads.

They are sensitive and must not be used in places where the normal surges keep on tripping it unnecessarily.

Type C circuit breaker trips instantly at current surges 5 to 10 times its rated current. its tripping time lies between 0.04 to 5 seconds. As they can tolerate higher surge currents, they are used in commercial applications such as the protection of small motors, transformers, etc.

Type D circuit breaker trips instantly when operating current reaches 10 to 20 times its rated current. Its tripping time is 0.04 to 3 seconds. Such circuit breakers can tolerate the high inrush current of large motors. Therefore, they are suitable for running heavy loads in industrial applications.

Such type of circuit breakers trips at 10 to 12 times its rated current with a tripping time of 0.04 to 5 seconds. These circuit breakers are also used for heavy inductive loads in industrial applications.

Type Z circuit breakers are the most sensitive circuit breaker that instantly trips when the operating current reaches 2 to 3 times its rated current. They are used for sensitive equipment that requires very low short circuit trip settings. 

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This Post has been published by WWW.ELECTRICALTECHNOLOGY.ORG.

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Do we have to consider the tripping curves for DIY installation?

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Frequently asked questions

What is the meaning of the b, c, d, k, and z curves for c60 multi-9 circuit breakers.

19 June 2023

Issue:   What do the B curve, C curve, D curve, K curve and Z curve designations mean on Multi 9 breakers? Product Line: Circuit Breakers Resolution: For C60 Multi-9 devices, AC rated, the following is the meaning of the trip curves (in reference to instantaneous trip rating): B curve: 3.2 - 4.8 times amp rating for UL489 and UL1077 (3 - 5 times amp rating for IEC) C curve: 7 - 10 times amp rating D curve: 10 - 14 times amp rating K curve: 10 - 14 times amp rating Z curve: 2.4 to 3.6 times amp rating  

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Tripping Characteristics of MCB: B,C,D,K,Z Trip Curves

Circuit-breakers are used to protect cables in installations. They should disconnect automatically as soon as the combination of the current rise and duration causes the cable or a component to heat up excessively. Circuit-breakers are used for overload protection and short circuit protection in electrical circuits as well as protection against electric shock by automatic disconnection. In this article, we will show you the tripping characteristics (trip curves) of circuit breakers.

What are Tripping Characteristics?

Tripping characteristics describe the operational and tripping behavior of miniature circuit breakers in the event of an overload or short circuit. The tripping curves of the electromagnetic release and the thermal bi-metal release result in an overall tripping curve for overload protection.

Various tripping characteristics are available for circuit-breakers depending on the type of component or equipment to be protected according to IEC/EN 60898-1 and IEC/EN 60947-2 standards.

Comparison of tripping characteristics:

Typical loads by trip curve

Designed to protect circuits that need a very low short circuit trip setting (Ex: Semiconductors)

Designed for cable protection (Ex: Control circuits, lighting)

Designed for medium magnetic startups (Ex: Lighting panels, control panels)

D and K Curves

Designed to allow for high inrush loads (Ex: Motor or transformation circuits)

Why do circuit breakers have different tripping characteristics?

Circuit breakers must trip quickly enough to avoid equipment or wiring failure, but not so fast as to give false, or nuisance trips. To avoid nuisance trips , circuit breakers need to be sized appropriately to compensate for overcurrent. We need different trip curves to balance the right amount of overcurrent protection against optimal machine operation.

What is a trip curve?

The trip curve shows the estimated tripping time of the circuit breaker. The X-axis represents a multiple of the operating current of the circuit breaker. The Y-axis represents the tripping time. A logarithmic scale is used to show times from .001 seconds at multiples of the operating current.

The two major components of the trip curve are:

• Thermal Trip Curve: This is the trip curve for the bi-metallic strip, which is designed for slower overcurrents to allow for inrush/startup. (See below curves)
• Electromagnetic Trip Curve: This is the trip curve for the coil or solenoid. It is designed to react quickly to large overcurrents, such as a short circuit condition. (See below curves)

With lower overload currents, only the thermal trip is active. From a certain limit, the electromagnetic trip has to operate within the tolerance band.

What is curve B?

Curve B is to protect cables and low-level signal devices such as PLCs. The electromagnetic trip is three to five times the rated current of the supplementary protector (3~5 x In). The fast trip time of these devices minimizes damage to control circuit conductors from low-level faults.

What is curve C?

Curve C is developed for applications with moderate inrush currents such as lighting, control circuits, and coils, computers, and appliances. The electromagnetic trip is five to ten times the rated current of the supplementary protector (5~10 x In). The higher instantaneous trip level prevents nuisance tripping, and components being protected can typically withstand higher fault currents without being damaged.

What is curve D?

Curve D is developed for applications with high inrush currents, i.e., transformers, power supplies, and heaters. The electromagnetic trip is ten to twenty times the rated current of the supplementary protector (10~20 x In). The high instantaneous trip level prevents nuisance tripping, and components being protected can typically withstand higher fault currents without being damaged.

What is curve K?

Curve K is developed for applications with high inrush currents. The electromagnetic trip is ten to fourteen times the rated current of the supplementary protector (10~14 x In).

What is curve Z?

Curve Z is developed for applications with very low inrush currents. The electromagnetic trip is two to three times the rated current of the supplementary protector (2~3 x In). These types of miniature circuit breakers are highly sensitive to short circuits. They are used for the protection of highly sensitive devices such as semiconductor devices.

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• MERS Clinical Features
• Diagnostic Testing
• Infection Control Guidance
• Managing Cases and Contacts
• Lab Testing for MERS

About Middle East Respiratory Syndrome (MERS)

• MERS is a viral respiratory illness.
• Most MERS cases have been detected in countries in or near the Arabian Peninsula.
• MERS represents a very low risk to the general public in the United States.

More Information

Middle East Respiratory Syndrome (MERS) is a viral respiratory illness caused by a coronavirus called Middle East respiratory syndrome coronavirus (MERS-CoV). The virus can spread from camels to people through direct physical contact. Limited human-to-human transmission is possible. MERS has caused severe respiratory disease in most diagnosed cases, and many of those patients died. Typical symptoms include fever, cough, and shortness of breath.

Most MERS cases have been detected in countries in or near the Arabian Peninsula. However, a number of cases linked to travel have been detected in countries outside this region.

For more information about travel guidelines for your destination, visit CDC's Travelers' Health site .

Did you know?‎

CDC does not recommend that anyone change travel plans to, in, or near the Arabian Peninsula because of MERS.

Countries in or near the Arabian Peninsula that have reported MERS cases : Bahrain, Iran, Jordan, Kuwait, Lebanon, Oman, Qatar, Saudi Arabia, United Arab Emirates (UAE), and Yemen. Countries outside of the Arabian Peninsula with MERS cases linked to travel : Algeria, Austria, China, Egypt, France, Germany, Greece, Italy, Malaysia, Netherlands, Philippines, Republic of Korea, Thailand, Tunisia, Turkey, United Kingdom, and the United States. Only two patients in the United States have ever tested positive for MERS . Both cases were in May 2014 and were among healthcare providers who lived and worked in Saudi Arabia.

Signs and symptoms

Most people with confirmed MERS-CoV infection have had severe respiratory illness. MERS symptoms usually appear 5 or 6 days after a person is exposed. However, symptoms can appear as early as 2 days and up to 14 days after exposure.

Common symptoms of MERS include:

• Shortness of breath

Some people may also have diarrhea, nausea, or vomiting. It is also possible that someone with MERS may have only mild symptoms or no symptoms.

In most known MERS cases, the infected person developed pneumonia. Additional complications such as kidney failure have also occurred. About 3 or 4 out of every 10 people reported with a MERS infection died.

Most of the people who die from MERS had at least one preexisting medical condition. Certain conditions may increase the risk of getting severe complications from MERS. These include:

• Cardiovascular disease, including high blood pressure
• Chronic lung disease
• Chronic heart disease
• Chronic kidney disease

Who is at risk

You may be at increased risk of getting MERS if you:

• Worked in or visited a healthcare setting
• Had direct physical contact with camels (including touching or grooming)
• Had close contact with a person who was sick with fever or respiratory symptoms
• Had close contact in the past 14 days with someone who was sick with fever or respiratory symptoms of unknown cause after they had traveled in or near the Arabian Peninsula
• Had close contact in the past 14 days with someone who has tested positive for MERS

When to call your healthcare provider‎

How it spreads.

The virus that causes MERS can spread through respiratory secretions, such as saliva or mucous, of infected animals or people. The MERS virus is mostly found in camels (dromedaries), and direct physical contact with camels can lead to transmission to humans. People infected through contact with camels can then spread the virus to other people. All reported person-to-person transmissions of MERS have been linked to someone who had been in or near the Arabian Peninsula.

Large MERS outbreaks from person-to-person spread have occurred in healthcare facilities treating patients with MERS. Some spread can occur in households through close contact (like sharing a bed or caring for someone who is sick). The largest known outbreak of MERS outside the Arabian Peninsula occurred in the Republic of Korea in 2015. Multiple healthcare facilities were affected by the outbreak, which was associated with a traveler returning from the Arabian Peninsula.

Researchers continue to investigate clusters of MERS cases to better understand how the virus spreads.

No vaccine exists to protect people against MERS. CDC's Respiratory Virus Guidance provides practical recommendations and information to help people lower health risks from respiratory viral illnesses.

For travelers, the World Health Organization recommends:

• Wash your hands before and after touching camels.
• Avoid drinking raw camel milk or camel urine or eating meat (including camel meat) that has not been properly cooked.
• If you are someone at higher risk for severe MERS illness, avoid close contact with camels.

No specific antiviral treatment is recommended for MERS. People with MERS often receive supportive medical care and care to help relieve symptoms.  

• National Center for Immunization and Respiratory Diseases (NCIRD) | NCIRD | CDC

Middle East Respiratory Syndrome (MERS) is a viral respiratory illness caused by a coronavirus called Middle East respiratory syndrome coronavirus (MERS-CoV).

For Everyone

Health care providers, public health.