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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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Why Is My Circuit Breaker Tripping? 4 Potential Problems and Solutions

Learn the proper way to figure out reasons for a circuit breaker tripping in a home—as well as when to let an electrician do the sleuthing..

By Glenda Taylor and Bob Vila and Evelyn Auer | Updated Dec 15, 2023 4:16 AM

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Photo: istockphoto.com

Q: Every few hours—sometimes minutes!—my living room and one side of my kitchen lose electrical power. I’ll check the breaker panel and, sure enough, a circuit breaker has tripped…again. Should I call an electrician, or is there a simple DIY fix I can try first?

A: While it’s frustrating when a circuit breaker keeps tripping, they are important safety mechanisms. Designed to shut off the electrical current when something goes wrong, circuit breakers are one of the best ways of protecting a home from an electrical fire. “When a circuit breaker trips, typically it is because we use too much electricity, which causes it to overload and turn off,” says Christopher Haas, expert electrician and owner of Haas & Sons Electric in Millersville, Maryland. For those who need an electrical panels 101 refresher course or aren’t sure how to reset circuit breakers, each breaker has an on/off switch and controls a separate electrical circuit in the home. When a breaker trips, its switch automatically flips “off,” and it must be manually turned back on to restore electricity to the circuit. For those wondering, “Is it dangerous if a circuit breaker keeps tripping?” the answer is that it can be, depending on the source of the problem. An electrician can ultimately deal with the root issue, but a little sleuthing will reveal whether it’s something that’s easily remedied.

In many cases, the cause of a circuit breaking tripping is an overloaded circuit.

A circuit overloads when more electrical current is being drawn through the wires than they can handle, tripping the circuit breaker. If this happens, there may be a few additional signs:

• Buzzing noises coming from outlets
• Devices charging slowly
• Electrical outlets not working
• Flickering lights
• Scorch marks on outlets and light switches

If a circuit breaker keeps tripping in one room, homeowners can test for circuit overload by turning off all the switches in the affected area and unplugging all appliances and devices. After the breaker is flipped back on, the devices can be turned back on one at a time, with homeowners waiting a few minutes in between to see if the circuit remains on. If the breaker trips before all the appliances are turned on, the experiment can be repeated, this time turning them on in a different order. It may be necessary to do this several times to find out how many appliances can be operated at once before the circuit overloads.

“As a short-term solution, you can unplug unnecessary appliances to prevent tripping circuit breakers. You may still get some trips, but you can limit them by unplugging devices that you don’t need to use,” advises Dan Mock, vice president of operations at Mister Sparky , an electrical company with 90 locations in the U.S. The best long-term solution, however, is to pay an electrician for the cost to rewire the house and add additional circuits. The cost to replace an electrical panel is about $1,274 on average.

Other times, the issue may be caused by a short circuit.

A “short” circuit means that two wires that should not be coming into contact are inadvertently touching, triggering a sudden surge of electricity through the wires. A short can occur in an outlet, a switch, or within an appliance if wires are loose or have been chewed through by mice or pets. Some signs of a short circuit include:

• Popping sounds
• Discolored outlets or switches
• Burning smells

Testing to see if an appliance has a short is similar to testing for an overloaded circuit. When an appliance that has a short in its wiring is turned on, it will immediately trip the circuit. Homeowners can also try plugging it into an outlet in a different room. If the breaker for that room trips, there’s a short in the appliance (if it’s unclear what breaker goes to what room, the breaker can be identified with one of the best circuit breaker finders ). Electrical shorts can be a major fire hazard, so it’s a good idea to call a licensed electrician for this circuit breaker repair. It’s wise to stop using the outlet or appliance until a pro takes care of the problem.

Another potential cause of a circuit breaker tripping is a ground fault.

A ground fault occurs when the electricity running through a home’s wiring diverts from the wiring loop and travels to the ground, usually due to faulty wiring or water infiltration in an outlet or switch box. Water is a conductor, which is why walking through puddles is often listed as something not to do in a power outage in case of downed power lines. Once water makes contact with wires, electricity can jump from the wiring loop and follow the water trail. This creates a surge in electricity leading to a tripped circuit breaker. If a person comes in contact with the electricity that is on its way to the ground, this can result in electrocution. Homeowners may notice a few signs of a ground fault, including:

• Tripped GFCI (ground fault circuit interrupter) outlets;
• A burning smell coming from an outlet; and
• Lights flickering.

Newer electrical breakers have features designed to protect against the danger of ground faults. According to Haas, “Ground fault breakers sense electricity going to earth as opposed to going through the wires of the circuit. You’ll find [these] for bathrooms, kitchens, garages, exteriors, and basements.” GFCI outlets are another safety feature that shut off the electric current within a fraction of a second of sensing a ground fault.

If a ground fault is the problem, the cause of the errant water must be discovered and repaired, and any damaged wiring must also be replaced. It’s also a good idea to install GFCI outlets in rooms where water is commonly used. A GFCI outlet costs $210 on average.

Sometimes a bad or worn-out circuit breaker can be the culprit.

In some cases, the circuit breaker itself may be faulty. Breakers that are old, damaged, or were installed incorrectly may trip frequently for no apparent reason. Alternatively, faulty breakers may not trip when they are supposed to, leaving the home at risk of electrical fire. Some signs of a bad circuit breaker include:

• The circuit breaker getting hot and tripping frequently;
• The circuit breaker won’t reset;
• It has been over 10 years since the breaker was last serviced; and
• The breaker has scorch marks.

An important electrical safety tip to keep in mind is that resetting a breaker over and over again can cause what is called an arc flash, which is a small electrical explosion that can be deadly. If resetting the breaker once does not remedy the issue, it’s a good idea for the homeowner to hire an electrician near them who knows how to replace a circuit breaker safely. Mock warns, “Don’t take any chances with circuit breakers. Instead, call a licensed electrician who knows the safe ways to replace breaker boxes, upgrade circuits, and diagnose potential electrical problems in your home.” Wiring a breaker box is a job to leave to an experienced electrician.

A professional electrician can help determine the specific cause of a frequently tripping circuit breaker.

Most circuit breaker problems—aside from those explained in the sections above—will need to be inspected and addressed by a licensed electrician. According to the Electrical Safety Foundation International (ESFI) , each year “thousands of people in the United States are critically injured and electrocuted as a result of electrical fires, accidents, [or] electrocution in their own homes.” While homeowners may be tempted to save on electrician costs by attempting circuit breaker replacement or repair themselves, electrical work is not suitable for casual DIYers. “Yes, you have to pay, but you can save many hours of head-scratching by hiring an electrician. Electricians will also have all the right tools for diagnosing and repairing the circuit,” Haas adds. “Lastly, they will come with a warranty/guarantee should something arise, and they will typically return at no additional cost.”

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Master electrician Heath Eastman shows host Kevin O’Connor everything he needs to know about why and how breakers trip.

Heath Eastman talks about circuit breakers. Heath shows Kevin O’Connor that while resetting these breakers is simple, these are complex devices that monitor and protect circuits. First, the two talk about the different sizes of breakers before moving on to the different types. Finally, Heath shows Kevin how to test certain breakers to ensure they’re working properly.

Circuit breakers exist to protect people, appliances, and homes from dangerous electrical current. However, few people understand why the trip and how they operate. Master electrician Heath Eastman shows host Kevin O’Connor why this happens, and even explains a few different types of breakers.

All About Electrical Systems

Breakers protect circuits.

When electricity comes into the house, it flows through the electrical service panel. From there, the electricity flows out through different branches in the house, each controlled by a circuit breaker. Should a branch begin to overload and overheat, the breaker will trip to prevent damage.

Breaker Sizes

There are two main sizes of breakers in a house: 15 amp and 20 amp. The amp rating explains how much current the breaker can handle before it will trip, and each requires a certain size of wire. Fifteen-amp breakers require a 14-gauge wire, while 20-amp breakers require a 12-gauge wire.

How They Work

A 15-amp breaker won’t necessarily trip the moment it experiences a spike above 15 amps. Many devices draw more amps upon start-up, and these breakers allow those temporary spikes. However, should the breaker sense elevated amperage for longer than is typical, it will trip to prevent the circuit from overheating.

GFCIs and AFCIs

Beyond circuit overload protection, there are other types of breakers that offer additional coverage. These include GFCI breakers and relatively-new AFCI breakers .

GFCI (ground fault circuit interrupter) breakers need to experience the same amount of current going out as coming back through the circuit. If the breaker experiences a drop in returning current, it assumes that the circuit is leaking, whether it be through a water source or a person. When this imbalance occurs, the GFCI trips immediately.

AFCI (arc fault circuit interrupter) breakers sense when the circuit, a device, or an appliance is arcing (the current is jumping from the circuit and onto something else or someone). When the breaker recognizes the arc signature, it trips immediately. These breakers are relatively new and look similar to GFCI breakers, but they’re becoming a code requirement in most locations.

How to Test Breakers

Homeowners, electricians, and inspectors can test their breakers. There are devices that users can plug into an outlet and replicate an error. These devices, known as AFCI/GFCI testers, can trip the breaker altogether or replicate a ground or arc fault, triggering the breaker. This is one of the best ways to ensure that a breaker is working properly.

When to Call a Professional

If a circuit is continuously tripping, or you know that it should be tripping and isn’t, be sure to call in a professional. An electrician will be able to determine the cause of the issue and make sure your circuit breakers and electrical system are safe.

Heath explains what a circuit breaker is, why they trip and how it protects a home. A circuit breaker is a device, installed in the electrical panel, that controls whether power can be sent from the panel through a circuit. Heath explains this ability is controlled by a switch that can be operated either manually—like when a person wants to interrupt power for service—or automatically, like a breaker trip.

He says power overloads, current “leaks”, and arcs are the three reasons that would cause a breaker to trip. A Power overload happens when a device is calling for more power than a receptacle , or a circuit is designed to provide. Current “leaks” are caused when current strays from the circuit for whatever reason, though it happens most commonly when moisture is present. Arcs can happen when the wire breaks down over time (due to overloads but also due to other factors, like animals chewing the wire and other decay) but what Heath sees the most is human error.

If a specific receptacle is consistently tripping the breaker, Heath advises to have a licensed electrician identify the problem to ensure the work is done safely.

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Circuit Breaker Tripping: Troubleshooting Guide

Hubert Miles | Licensed Home Inspector, CMI, CPI

Updated on January 5, 2024

A circuit breaker tripping results from short circuits, overloaded circuits, and ground faults. In each case, an unintended excessive flow of current triggers the trip. You must reset the circuit breaker by flipping it back on to restore power.

Circuit breakers trip because they cannot handle the amount of current running through them. Tripping the circuit breaker interrupts the flow of electricity and protects your devices or appliances from damage.

Get FREE estimates from licensed electricians in your area today. Whether you need to replace an outlet, hang a ceiling fan, a new electrical panel, or repair wiring, We Can Help!

Without electrical circuit breakers , the possibility of electrical fires would be much higher.  

This guide looks at what causes circuit breakers to trip, what you can do, and how to identify a bad breaker.

What Would Cause a Circuit Breaker to Trip

There are three leading causes of circuit breaker trips:

• circuit overload
• electrical faults (i.e., ground faults and arc faults)
• short circuits

Below are factors that can cause circuit breaker trips. 

Circuit Overload

A circuit overload happens when the flow of electric current running through the circuit exceeds the amperage of the devices it serves. 

For example, if your microwave is a 12.5 amp appliance, you can run it on a 15 amp circuit. That means your microwave is safe as long as the amperage running through the circuit is 15 amps .

However, if the circuit receives an excessive electrical load over 15 amps , it will automatically trip to protect your device from damage. If the circuit doesn’t trip, the excess current will fry the circuit in your microwave. 

Also, if you operate too many appliances and devices on one circuit, its internal mechanism heats up, causing the breaker to trip. 

Circuit overload is the most common reason for breakers tripping. 

Ground Faults

A ground fault occurs when the active wire comes into contact with a ground wire made of bare copper. Sometimes, this fault may happen when the hot wire touches the metal box connected to the ground wire.

Excessive current flows once the active wire touches the ground wire, flowing into the earth. If you step on the affected area, ground faults can cause shock and even electrocution. The uncontrolled flow of electricity will cause the circuit breaker to trip.

Arc Faults 

An arc fault happens when exposed faulty wiring touches, causing the electric current to arc at the meeting point. As a result, sparks occur, which can ignite an electric fire.

A corroded or loose connection is the main culprit for arc faults. Circuit overloads, ground faults, or short circuits trip an AFCI (Arc Fault Circuit Interrupter) circuit breaker.

Arc faults result from damaged, loose, or corroded terminals and wires. The arc fault builds up over time as the heat due to the cable damage and terminals build up to the point of ignition.

Short Circuits

A short circuit occurs when an active wire touches a neutral wire, and the electrical current takes an unintended path of least resistance. 

The common cause of short circuits is frayed wires coming into contact when the wires touch. The electrical current flow increases significantly, causing the circuit breaker to trip to stop the electricity from damaging appliances.

It is a short circuit because the current bypasses the proper circuit wiring channels and flows through a shorter, unplanned pathway.

Short circuits occur 

• When insulation melts and wires are exposed
• Within appliances with damaged internal wiring
• Due to damaged and frayed extension or appliance cords

How do You Fix a Breaker that Keeps Tripping?

A dedicated circuit breaker tripping indicates too much current flowing through the wiring or connection to the outlet.

Here is a step-by-step guide to follow when you notice the first trip:

• Begin by turning off all the appliances and unplug electrical devices from the outlet. Also, switch off light fixtures and unplug those that you can. This prevents any appliances from damaged when the breaker is reset and a sudden surge of power comes through.
• Open the circuit panel or box and locate the on and off buttons of the circuit breaker. You may notice an orange or red color on the breaker when it is off. 
• Flip the switch from off to on to reset the circuit breaker. Once the breaker is reset, you can switch and test the appliances to see if the electrical power is flowing.
• Keep safe as you reset the breaker by working from the side of the electrical box instead of the front. That way, you will avoid any sparks (should there be any) when you switch the breaker back on.
• Some people prefer to switch the main electrical switch when working on the circuit breaker for added safety. 

Types of Circuit Breakers

Standard circuit breaker.

Standard circuit breakers monitor the modulation of the electric current coming into your devices and appliances. 

This circuit breaker stops the current from flowing when it detects the excessive flow of electricity. 

Standard circuit breakers come in two forms:

• Single-pole circuit breakers
• Double pole circuit breakers

Single-Pole Circuit Breakers

Single-pole circuit breakers are the most common breakers in homes and buildings. They monitor the electric current’s flow in one wire and trip if that wire experiences a very high influx of electricity.

These breakers deliver only 120 volts and work well for 15 to 30 amp circuits. Single-pole circuit breakers come with one switch in the back. 

Double-Pole Circuit Breakers

The double-pole circuit breakers monitor the current in two wires simultaneously. You will notice two switches on the back of these breakers.

The double-pole circuit breakers will trip even if only one of the wires receives too much current. They can accommodate between 15 to 200 amps while delivering 240 volts. 

Single-pole breakers are a good fit for lighting fixtures and other standard home outlets. On the other hand, double-pole breakers work for larger appliances like dryers and washing machines. 

Ground Fault Circuit Interrupter (GFCI)

The GFCI circuit breaker interrupts the line due to ground faults. They trip when the current starts to follow an uncharted path into the ground. These ground fault surges occur when a foreign conductor, like water, comes in contact with a receptacle .

At the same time, they offer protection against circuit overloads and short circuits. 

GFCI circuit breakers come built into specialized outlets required for wet areas in the home, including :

• Outdoor areas like the balcony, patio, porches, and decks
• Laundry rooms
• Swimming pools
• Six feet from a sink
• Six feet from the bathroom

These breakers help prevent shock or electrocution should the electrical outlet contact water. 

Arc Fault Circuit Interrupter (AFCI) 

The AFCI circuit breaker detects normal and abnormal arc faults, so it will trip when it detects a dangerous arc fault that can cause a fire. 

The AFCI circuit breaker doesn’t work to protect devices and appliances plugged into an outlet. It works to prevent electrical fires due to faulty connections and wiring. The internal sensing mechanism in the circuit breaker senses the conditions of an electric arc, and the circuit trips to avoid an electric fire.

AFCI protection can also be built into an outlet. The National Electrical Code (NEC) requires these types of breakers to feature in :

• Common rooms
• Laundry areas

AFCI and GFCI circuit breakers can co-exist and complement each other for the best protection.

Combination All Fault Circuit Interrupter (CAFCI)

The CAFCI breaker senses and reacts to any electrical fault, including ground and arc faults.

CAFCI is a relatively new technology that meets new NEC requirements for circuits requiring arc and ground fault protection.

Do Circuit Breakers Get Weak?

A circuit breaker can wear out and become weak. If a breaker trips frequently, the thermal or magnetic element can lose calibration, causing it to trip at lower amp loads than intended. A breaker constantly under thermal stress caused by overloading the circuit will eventually trip more frequently.

Let’s not forget breakers are not impervious to damage. As the internal mechanical parts wear out, they become very sensitive and may not hold under normal load amperage and temperatures.

Electricians refer to this as a bad breaker .

Will a Bad Breaker Keep Tripping

By definition, bad breaker malfunctions, so it will keep tripping until it is either replaced or rectified .

A licensed electrician performs this simple test to see if a breaker will keep tripping and determine if it can be repaired or replaced in the following steps.

• The electrician will switch off all the fixtures and appliances in the house. Also, unplug everything.
• Find the malfunctioning circuit breaker . The electrician will go to the electrical box and locate the breaker lighting orange or red or the one with the switch off.
• They will ascertain that it is the correct circuit breaker. After that, the electrician puts the breaker off.
• With the switch on, the breaker is back on as well. The electrician will plug the appliances into the outlet with the problem circuit breaker. Now, they will turn the devices and appliances on. 

If the breaker trips, the electrician will investigate the circuit’s current amount. The breaker is bad if the current is according to the appliance’s rating.

How You know if a Circuit Breaker is Bad

Breakers do wear out after a while. It has a problem if the breaker doesn’t stay on after resetting it.

Since the circuit breaker controls the electric flow in the house, it is essential to monitor it and catch signs that it has gone bad early.

Here are key signs that denote a bad circuit breaker :

It Frequently Trips

Frequent tripping could be because of a bad breaker. After tripping and resetting, your circuit breaker should stay on unless it detects high current flow. 

To ensure that the issue is not the electricity but the circuit breaker, call an electrician to examine your electricity’s flow and determine whether it is the cause of the constant tripping.

If it is not, then the circuit breaker is the problem. 

The Breaker Overheats

Electrical systems will heat up when active. Typically a breaker can heat to about 60°C (140°F) before problems arise.

Terminations for standard rated breakers: UL 489 Paragraph 7.1.4.2.2 says the temperature rise on a wiring terminal at a point to which the insulation of a wire is brought up as in actual service shall not exceed 50°C (122°F). Terminations for 100% rated breakers: UL489 Paragraph 7.1.4.3.3 says the temperature rise on the termination shall not exceed 60°C (140°F). Handles, knobs, and other user surfaces: UL489 Paragraph 7.1.4.1.6 says the maximum temperature on handles, knobs, and other surfaces subject to user contact during normal operation shall not exceed 60°C (140°F) on metallic and 85°C (185°F) on nonmetallic surfaces. Source: https://www.clipsal.com/faq/fa173839

Call an electrician immediately if the breaker becomes too hot.

There are Scorch Marks

Scorch marks around receptacles, appliances, and the electrical box should tell you your circuit breaker has gone bad.

The burn marks indicate that wiring insulation has melted off and the circuit wires are now sparking and emanating heat or fire. That means that the circuit breaker did not interrupt the excess current and reached the wires and burned them. 

You may see melted wire sheathing on the wire where it connects to the breaker.

Professional electricians can use a  thermal imaging infrared camera to locate the heat source. The infrared camera allows them to pinpoint the problem area through the walls and other construction material.

A Burning Smell

Sometimes you may smell the insulation burning, but no scorch marks are present to denote which outlet is the problem. 

With the help of the infrared camera, an electrician can help locate electrical issues. 

If you encounter a burning odor, shut off the main power and call for emergency service from an electrician. 

The electrical wires burn because power surges through the circuit, melting the wire insulation.

What is Nuisance Tripping

Nuisance tripping is when a breaker trips without a fault to warrant the interruption to the electric current flow.

Nuisance tripping occurs due to several reasons:

Stringent Protection on Circuits 

Sometimes the circuit is protected by stringent conditions that detect any variance as a fault and cause a trip. 

Such stringent conditions can be tuned to accommodate the home’s or building’s electric needs.

A Highly Sensitive Circuit Breaker

In some cases, the circuit breaker has been set to susceptible settings so that they can detect even the slightest fault, even a minor average variance.

For example, the manufacturer can set an AFCI circuit breaker to sensitive standards to detect another circuit’s arc. This common issue may occur in a daisy chain where the circuit breakers connect in a linear series. There may be a faulty electrical outlet you are unaware of on the circuit. It is common for multiple rooms to share a breaker in older houses.

The Breaker Encounters Power Under Different Conditions

The variation in the current is normal, but the breaker responds to it by tripping because the flow is outside the breaker’s regular operation.

Your circuit breaker is tripping because the voltage it is encountering is not within the standard operation. You will need to adjust the circuit breaker or the voltage to eliminate nuisance tripping.

The Breaker Trips with Nothing Plugged in

A breaker tripping with nothing plugged in occurs when a hot, neutral wire is touching somewhere in the circuit. The common causes include frayed or damaged electrical wires, loose connections, faulty electrical receptacles, light switches , or dimmers.

Electrical wire damage happens when:

• wiring is chewed by animals such as rats, squirrels , raccoons, etc
• wire sheathing and insulation ages and become frayed
• wires rub against sharp edges such as punch-outs with missing grommets or wire clamps

Loose connections often occur when electrical wire nuts come loose or electrical tape wears out causing wires to touch.

Defective wiring can be anywhere along the circuit, so it’s best to contact a licensed electrician to troubleshoot why the breaker is tripping.

Replacing a Bad Circuit Breaker

• Check the electrical panel to see the compatible approved circuit breaker brands. Also, make a note of the brand of the electric panel . This is to help you determine if there are upgrades they could recommend for the hardware.
• Order online or go to the hardware store and purchase the breaker of the same voltage as the one you are replacing.
• Go and open the electrical box and switch off the bad breaker. Loosen the terminals and remove the wires using a pair of needle-nosed pliers. Ensure the pliers have rubber insulated handles to avoid shock or electrocution since you will use the pliers to grab the live wires from the terminal. That is a safety measure.
• Remove the bad breaker. Replace it with the new breaker and slip its clips into place. Remember to switch off the replacement breaker.
• Next, using the pliers, hold the wiring and tighten the screws on the terminal. It is crucial to ensure that the wires and screws in the terminals are in the right place.
• Turn the breaker on and replace the electrical panel cover.

Can a Breaker Fail Without Tipping

If you have a newer electrical panel , it’s not likely for a breaker to fail and not trip. However, in older breaker boxes like Federal Pacific , the breaker failing to trip is common.

The main reason Federal Pacific was investigated by the Consumer Products Safety Commission (CPSC) was widespread structure fires involving breakers failing to trip when an electrical overload was present. They found that the circuit breaker contacts would fuse to the bus bar.

Modern breakers will trip when a failure occurs as an added layer of safety. Most older breakers did not have these safeguards.

With AFCI breakers, if the Internal sensing mechanism fails, the breaker reverts to a standard breaker. The AFCI sensor mechanism will no longer work, but the breaker would still trip from overcurrent protection. Therefore, you should test the AFCI breaker regularly.

Conclusion 

Listen to your circuit breaker . It’s alerting you of a problem when it trips. That communication could be a problem with the breaker itself, the circuit, or the amount of electric current coming into your home.

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.

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Characteristics of Circuit Breaker Trip Curves and Coordination

Time-current curves are graphical representations of the trip characteristics of a circuit breaker. These curves show the relationship between the magnitude of the current flowing through the circuit breaker and the time it takes for the circuit breaker to trip.

This information is critical for ensuring that the circuit breaker provides adequate protection for the equipment it is connected to. Time-current curves are typically provided by the manufacturer of the circuit breaker and are based on industry standards such as the Institute of Electrical and Electronics Engineers (IEEE) and the National Electrical Manufacturers Association (NEMA).

How to read a Time-current Curve

Time-current curves are typically shown on a log-log plot. Figures along the horizontal axis of the curve represent the continuous current rating (In) for the circuit breaker, figures along the vertical axis represent time in seconds.

To determine how long a breaker will take to trip: find the current multiple of (In) at the bottom of the graph. Next, draw a vertical line to the point where it intersects the curve and then draw a horizontal line to the left side of the graph to find the trip time.

The total clearing time of a circuit breaker is the sum of the breaker’s sensing time, unlatching time, mechanical operating time and arcing time.

Curves are developed using predefined specifications such as operation at an ambient temperature of 40°C, so keep in mind that the actual operating conditions of the circuit breaker can cause variations in its performance.

Most curves have an information box that will define which circuit breaker the curve applies to. This information box may also contain important notes from the manufacturer such as the allowable deviation from trip times.

Overload Protection

The upper portion of the time-current curve shows the circuit breaker’s thermal response, the curved line indicates the nominal performance of the circuit breaker.

In thermal magnetic breakers, a thermal overload occurs when a bi-metal conductor inside the circuit breaker deflects after becoming heated by the load current, de-latching the operating mechanism and opening the contacts.

The larger the overload, the faster the bi-metallic strip will heat up and deflect to clear the overload. This is what is known as an “inverse time-curve.”

Long-Time Function

In electronic circuit breakers, the long-time function (L) simulates the effect of a thermal bi-metal element. The nominal pickup point where an electronic trip unit senses an overload is roughly around 10% of the selected ampere rating. Once picked up, the circuit breaker will trip after the time specified by the long-time delay adjustment has been achieved.

Short Circuit Protection

The lower portion of the time-current curve displays the short circuit response of the circuit breaker. In thermal magnetic breakers, tripping place when overcurrent’s of significant magnitude operate a magnetic armature inside of the circuit breaker which de-latches the mechanism.

Instantaneous Function

In electronic circuit breakers, the Instantaneous (I) function simulates the magnetic characteristic of a thermal-magnetic circuit breaker. This is achieved through the microprocessor which takes samples from the AC current waveform many times a second to calculate the true RMS value of the load current. Instantaneous tripping occurs with no intentional time delay.

Short-Time Function

Some electronic circuit breakers may be equipped with a Short-time function (S) which gives the circuit breaker a delay before tripping on a significant overcurrent. This allows for selective coordination between protective devices to ensure that only the device nearest to the fault open, leaving other circuits unaffected (see circuit breaker coordination below).

The I 2 t characteristic of the short time function determines the delay type. I 2 t IN will result in an inverse-time delay that resembles the time/current characteristics of fuses. This is similar to the long time function except with a much faster delay. I 2 t OUT provides a constant delay, usually 0.5 seconds or less as noted on the time-current curve.

Zone Interlock Function

Circuit breakers equipped with zone interlocking on short delay with no restraining signal from a downstream device will have the minimum time band applied regardless of setting, this is sometimes referred to as the maximum unrestrained delay.

When the instantaneous function is disabled, a short-time delay override is used to instantaneously trip circuit breakers in the event of a significant short circuit. This is called the short-time withstand rating and is represented on the trip curve as an absolute ampere value.

Related: Zone Selective Interlocking (ZSI) Basic Principles

Ground Fault Protection

Like the long-time function, the ground fault (G) element consists of a pickup and delay setting. When a phase-to-ground fault occurs, the sum of the phase currents are no longer be equal because the ground fault current returns through the ground bus. In a 4-wire system a fourth CT is installed on the neutral bus to detect this imbalance.

When a current imbalance occurs, the circuit breaker will pick up if the magnitude exceeds the ground fault pickup setting. If the breaker remains picked up for the time specified by the ground fault delay, the circuit breaker will trip. Ground fault protection is sometimes supplied with an I 2 t function which operates under the same principle as short-time delay.

Ground fault protection requires the least energy to trip the circuit breaker, often times with trip values set well below the long time pickup setting. When testing the overload or short circuit function of a circuit breaker, the ground fault protection will need to be disabled or “moved out of the way” for other functions to operate.

Use of the manufacturer’s test kit or rewiring the neutral CT input is the preferred method of primary-injection testing on a low voltage circuit breaker with ground fault protection, otherwise two poles can be connected in series to provide balanced secondary currents to the trip unit.

Related: Ground Fault Protection Systems: Performance Testing Basics

Circuit Breaker Coordination

Time-current curves are essential for the proper coordination of circuit breakers. In the event of a fault, only the circuit breaker closest to the fault should operate, leaving other circuits unaffected.

In the example below, three circuit breakers have been coordinated so that the tripping time of each breaker is greater than the tripping time for the downstream breaker(s) regardless of the fault magnitude.

Circuit breaker CB-3 is set to trip if an overload of 2000A or greater occurs for 0.080 seconds. Circuit breaker CB-2 will trip if the overload remains for 0.200 seconds, and circuit breaker CB-1 if the fault remains for 20 seconds.

If the fault occurs downstream of breaker CB-3 it will trip first and clear the fault. Circuit breakers CB-2 and CB-1 will continue to provide power to the circuit.

Each function of the trip unit should also be coordinated to prevent nuisance trips. If a circuit breaker is feeding a piece of equipment with large inrush currents for example, the instantaneous pickup value should be set higher than the short time pickup value to prevent tripping when the equipment is energized.

Related: Electrical Power System Coordination Studies Explained

References:

• Trip Curves and Coordination, Square D Data Bulletin 0600DB0105
• Basics of Circuit Breakers: Siemens STEP Series

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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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Keep losing power, and aren't sure why here are five of the most common reasons why a circuit breaker keeps tripping, and what you can do to fix the problem..

1. Ground Fault

2. short circuit, 3. circuit overload, 4. arc fault, 5. damaged breakers, circuit breaker keeps tripping, buell electric's blog.

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