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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.

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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.

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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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Electrical Technology

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Understanding Miniature Circuit Breaker (MCB) Types and Tripping Curves

• by Bhekusizi Dhlodhlo
• Categories: Blog

Table of Contents

Introduction.

Miniature circuit breakers (MCB) provide protection against short circuits and are rated normally up to 125A. They can be combined with a residual current device, to provide protection against short circuits, overloaded circuits and ground faults.

MCB’s tripping characteristics are represented graphically in a trip curve.  The curve shows the response of the thermal and magnetic trip element to various overload and short circuit situations. 

Those curves are designated letters according to the circuit breaker type. The circuit breaker Types are B, C, D, K and Z corresponding to similar lettered circuit breaker curves.

Why so many different circuit breaker types?

Circuit breaker curves determine the breaker's reaction time to faults. They're the first line of defence against electrical faults. Learn how to select the correct one! 🛡️ #SafetyFirst #ElectricalEngineering #SparkyCalc Tweet

There so many reasons why we need different circuit breaker types with different trip curves and some of the main reasons are:

Selective Coordination

Different electrical components within a system might have varying levels of fault currents they can handle before tripping. 

By using different tripping curves, we can ensure that only the faulty component is disconnected during a fault, allowing the rest of the system to remain operational. 

This selective coordination can be achieved by use of circuit breakers with different tripping curves in a system. We discuss this in more detail in our blog on understanding circuit breaker co-ordination

Equipment Protection

Different types of equipment have different thermal and electromagnetic characteristics. A more sensitive trip curve might be suitable for protecting sensitive electronic equipment, while a less sensitive one may be appropriate for heavy-duty machinery. This ensures that the protection device responds appropriately to the specific needs of the equipment.

Start-Up and Inrush Currents

Some devices such as large motors  experience higher currents during startup or inrush periods. A circuit breaker that allows the flow of high inrush current without tripping can be used in this instance

Fault Types

Different fault types, such as short circuits and overloads, require distinct response times and current thresholds. Circuit breakers with different tripping curves can be used  to address these specific fault conditions appropriately.

Safety and Personnel Protection

Human safety is a critical concern. Tripping curves can be designed to quickly disconnect power in situations where personnel might be at risk, while also preventing unnecessary tripping due to minor fluctuations.

Reading Trip Curves

Reading a Trip curve is not that difficult, give me a sec and I’II explain it to you.

A typical tripping curve is shown above. 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. A log scale is used so as to fit a wider range of values on the axis.

The top part of the curve is the thermal section of the trip curve’s responds to overloads  which are sustained or long-lasting overcurrent conditions. 

Therefore, a circuit breaker with a thermal trip curve is better suited for high-inrush current applications. The thermal trip curve is typically curved, reflecting the fact that the response time of the circuit breaker increases as the level of overcurrent increases. The thermal trip unit responds relatively slowly yet consistently.

The second part of the curve is the magnetic current section of the trip curve responds to short circuits. It relies on a magnetic coil or solenoid opening when the overcurrent’s design limit is reached. 

The magnetic trip curve is typically a straight line, reflecting the fact that the response time of the circuit breaker is nearly instantaneous for high levels of current.

The bottom part of the time-current curve shows the performance of the instantaneous trip component (short circuit) of the circuit breaker. 

The maximum clearing time (time it takes for breakers to completely open) decreases as current increases. This is because of the blow-apart contact design which utilizes the magnetic field built-up around the contacts.

Breaker types and their trip curves

Now that we’ve learned how to read a trip curve, let’s explore how the type of breaker is linked to these curves and what these curves actually mean.

The first type of circuit breaker that we will look at is the Type B.

Type B circuit breakers have relatively fast tripping characteristics. They are designed to protect sensitive and low-power circuits, such as lighting circuits and some electronic devices. 

The standards state that these breakers trip at 4 times the rated current but MCB’s being mechanical devices are not that exact and will trip anywhere between 3 to 5 times the rated current. The tripping characteristics of this circuit breaker is called a B Curve.

The next one on our list is the Type C circuit breaker.

Type C circuit breakers have medium tripping characteristics. They are commonly used in applications like small motors, small transformers, and general domestic applications where a moderate level of inrush current is expected during normal operation. 

These breakers provide a balance between protecting against overcurrents and allowing for some temporary overloads. The standards state that they should trip at 7.5 times the rated current but will trip anywhere between 5 to 10 times the rated current. The tripping characteristics of this circuit breaker is called a C Curve.

Next we will discuss the Type D circuit breaker.

 Curve D circuit breakers have a delayed tripping characteristic. They are often used in applications with high inrush currents, such as large motors, industrial equipment, and power distribution systems. 

These breakers can handle significant overcurrents for a longer time before tripping, which is suitable for equipment that experiences frequent startup surges. 

The standards state that they should trip at 12.5 times the rated current but will trip anywhere between 10 to 20 times the rated current. The tripping characteristics of this circuit breaker is called a D Curve.

Lets move on to Type K. This circuit breaker is specifically designed for air conditioning and heat pump systems, which often experience high inrush currents during compressor startup. 

Type K provides a delayed response to accommodate these inrush currents while still providing protection against sustained overcurrents. These trip at 8 to 12 times the rated current. The tripping characteristics of this circuit breaker is called a K Curve.

Finally we have Type Z. This circuit breaker is used for specialized applications where the tripping time is extremely fast, even faster than Type A. 

It’s often used in situations where human safety is the primary concern, such as in some elevator systems or medical equipment. This circuit  breaker trips at 2 to 3 times the rated current. The tripping characteristics of this circuit breaker is called a Z Curve.

Rounding Up

I encourage you to reflect on your current or upcoming projects. Are you using the most appropriate type of MCB? Could a different tripping curve offer better protection or efficiency? If you’re unsure, review the technical aspects we’ve discussed, or consult with a professional. 

Your thoughts and experiences are invaluable to this discussion. Please contact me for further discussions. Let’s continue to learn and grow together in our understanding of electrical systems. For more insights, check out similar topics on our blog page and home page . And for updates, don’t forget to subscribe to our newsletter.

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My name is Bheki and I’m an Electrical Engineer. I have a passion for engineering and teaching/mentoring. Qualifications: BEng (Electronic) Hons, MIEAust, CPEng, NER, APEC Engineer IntPE(Aus), RPEQ.

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What you need to know about miniature circuit breaker trip curves

One of the critical criteria when selecting miniature circuit breakers is their trip curve. Awareness of these curves will help you select the right breakers for your applications, and diagnose potential nuisance tripping issues.

Trip curve basics Electrical protection devices, with virtually no exceptions, operate based on a simple formula: If THIS, then THIS. If this limit reaches a certain level, then the device executes the designed or programmed action. Today’s large, molded-case circuit breakers may include algorithms consisting of a complex set of If/Then parameters. Miniature circuit breakers (MCBs), on the other hand, operate based on only two parameters: overload and short-circuit.

Still, even with these two basic parameters, breaker buyers face a broad selection of miniature circuit breakers that could potentially meet their application requirements. Selecting the optimum miniature circuit breaker is critical to helping ensure proper protection, with minimal or no nuisance tripping, at the lowest possible cost. Making the right selection requires an understanding of the basics of trip curves.

Understanding trip curves Most protective devices have a defined trip curve, also referred to as a time/current curve, that describes the behavior of the device. The curve is literally a graphic representation of how the device will respond to changes in current. From a functional perspective, the curve parameters specify the high and low current thresholds that will cause the device to trip.

Selecting the appropriate trip curve achieves a good balance between overcurrent protection and optimal machine operation. A fast-acting trip curve will do an excellent job of circuit and production equipment/load protection, but at the cost of frequent and costly nuisance tripping (mostly due to inrush currents of motors and transformers). Choosing a breaker with higher trip points or thresholds will better keep the process up and running, but might cause more temperature rise in cables/conductors and connected loads.

Trip curves are defined by IEC standards 60898-1 and 60947-2. The curves actually represent two different trip functions within the miniature circuit breaker – thermal and electro-magnetic. The thermal section (top section of the chart) that responds to overloads typically consists of a bi-metallic strip. The response of the thermal trip unit is relatively slow. The thermal section, which also complies with UL standards in addition to the before-mentioned IEC standards, is similar across all trip curves.

The short-circuit section (bottom) relies on a magnetic coil or solenoid that opens if the overcurrent design limit is reached. This section of the breaker responds within milliseconds. This characteristic of the trip curve has no counterpart on the UL side.

Trip curve origins The concept of trip curves originated in the IEC world. The alphabetic code used to categorize miniature circuit breakers (B, C, D, K, and Z) carried over from IEC standards. The standard defines the lower and upper thresholds for tripping, but manufacturers have the flexibility to decide the precise specifications within those thresholds that will cause a trip in their products. The trip curve graph shows the tolerance band within which manufacturers can set the individual tripping point of their breakers.

The characteristics and applications of each curve, from most- to least-sensitive are: Z: Trips at 2 to 3 times rated current. Suitable for highly sensitive applications, e.g., semiconductor devices. B: Trips at 3 to 5 times rated current. C: Trips at 5 to 10 times rated current. Suitable for medium inrush currents. K: Trips at 10 to 14 times rated current. Suitable for loads with high inrush currents, mostly for use with motors and transformers. D: Trips at 10 to 20 times rated current. Suitable for high starting currents.

Referring back to the “Comparison of all IEC trip curves” graph, you can see that higher currents trigger more rapid trips.

The ability to tolerate inrush current is an important consideration in trip-curve selection. Certain loads, notably motors and transformers, experience a momentary change in current, the inrush current, at contact closure. Faster protective devices, like a B-trip curve, would see this inrush as a fault and open the circuit. For these types of loads, trip curves with a higher magnetic tripping point, either D or K, can “ride through” the momentary inrush of current, protecting the circuit without nuisance tripping.

Choosing the right trip curve The availability of a range of trip curves and options for different current levels within each range enables selection of an appropriate breaker to protect a variety of loads in various applications.

With large, molded case circuit breakers , you can select one of a number of breakers and then adapt it to the application by adjusting its operating parameters. The same isn’t true for miniature circuit breakers. Their operating parameters are fixed, making it critical that you select the breaker to meet the required specs, including the appropriate trip curve for the application.

Armed with the operating values for a circuit or application/load, it’s possible to make a good selection of the appropriate trip curve. However, this may require some trial and error to arrive at the optimum breaker. Recurring nuisance tripping may be a sign an alternate may be a better fit.

To help ensure the optimal balance between protection and minimal nuisance tripping, the best approach may be to consult with the device manufacturer or distributor. Provide them with the details of your application, and they should be able to recommend the right miniature circuit breakers to meet your needs.

— Thomas Weinmann Senior Product Marketing Manager

ABB Electrification Business

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