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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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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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The basics of trip curves for panel builders

A critical but often confusing characteristic of miniature circuit breakers are trip curves. Knowing the basics of trip curves can ensure selecting the lowest-cost breaker that will provide the needed protection without nuisance tripping. 

To the average consumer, circuit breakers are basic on/off devices that trip when they detect incoming current above the rated threshold. Panel builders, however, understand that breakers operate in a far more sophisticated way considering multiple variables: variables that can be represented graphically in a trip curve that shows the range of conditions that will initiate a trip. 

Selecting the optimum miniature circuit breaker (MCB) is critical to helping ensure proper protection, with minimal or no nuisance tripping, at the lowest possible cost. Matching a breaker to an application requires an understanding of the basics of trip curves. 

Molded-case versus miniature circuit breakers

Almost every electrical-protection device operates based on a simple formula: If THIS, then THAT. Some large, molded-case circuit breakers can be adjusted to adapt the breaker to the application, reacting to THIS variable with the appropriate THAT response. 

This adjustability enables panel builders to select a molded-case breaker with the required specifications – plus a “safety factor” to account for application unknowns or future equipment changes – and then “tune” the breaker once it’s in use. 

MCBs, on the other hand, operate based on only two parameters that cannot be adjusted: overload and short-circuit. Even with just these two basic parameters, though, breaker buyers have a broad selection of MCBs that could potentially meet their application requirements. Selecting the appropriate breaker includes considering not just features like amperage, voltage, and interrupting rating, but also trip curves.

Trip curve basics

Most protective devices have a defined trip curve, also referred to as a time/current curve, that describes their behavior. The curve is represented in a graph showing how the breaker will respond to changes in current, basically the high- and low-current thresholds, that will cause a trip.

The optimum trip curve balances overcurrent protection and machine operation. A fast-acting trip curve will do an excellent job of protection, but at the cost of frequent and costly nuisance tripping (mostly due to inrush currents of motors and transformers). A breaker with higher trip points will better keep the process up and running.

Trip curves are defined by IEC standards 60898-1 and 60947-2. The curves represent two different trip functions: thermal and electromagnetic. The thermal section (top/red area of the chart) that responds to overloads typically consists of a bi-metallic strip. The thermal trip unit responds relatively slowly and is consistent across all trip curves.

The short-circuit section (bottom/grey) relies on a magnetic coil or solenoid that opens when the overcurrent’s 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. 

The concept of trip curves originated in the IEC world, and the alphabetic code used to categorize MCBs carried over from IEC standards. They define the lower and upper thresholds for tripping (no-trip threshold and trip threshold). The trip-curve graph shows the tolerance band within which manufacturers can set the individual tripping point of their breakers.

The characteristics of the magnetic/short-circuit trip unit 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 to the graph above, 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

Experienced panel builders can often identify the appropriate trip curve for various applications but may require some trial and error to arrive at the optimum breaker. For many panel builders, though, the best approach may be to consult with your 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.

See related blog post “Choose enhanced breaker and controller options to build more-competitive panels” .

Thomas Weinmann

Senior Product Marketing Manager

ABB Electrification Business

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