MCB trip curve types explained: How to choose between B, C, D, K, and Z
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.
What determines a trip curve type
A trip curve’s type is set by its magnetic tripping threshold — the multiple of rated current at which the breaker’s short-circuit element fires. Each letter (B, C, D, K, Z) marks a different threshold range, called the tolerance band, within which a manufacturer can set the exact trip point for that curve type. A breaker’s tolerance band is what determines whether it will ride through a motor’s inrush current or trip on it — pick too tight a tolerance and you get nuisance tripping; too loose and you lose protection speed. For a full breakdown of how trip curves work and the standards behind them, see “Circuit breaker trip curves: A guide for panel builders“.
Trip curve types compared
Z curve
Trips at 2 to 3 times rated current — the tightest tolerance band of the group. Z curve breakers are built for loads with essentially no inrush tolerance, most often semiconductor equipment and other highly sensitive electronics where even a brief current spike needs to trip the circuit.
B curve
Trips at 3 to 5 times rated current. This is the standard curve for resistive and lighting circuits with minimal inrush — anywhere you want fast protection and don’t need to ride through a startup surge.
C curve
Trips at 5 to 10 times rated current. The most common general-purpose curve for mixed loads and medium inrush current, such as small motors and typical panel distribution circuits.
K curve
Trips at 10 to 14 times rated current. Suitable for loads with high inrush currents, mostly for use with motors and transformers.
D curve
Trips at 10 to 20 times rated current. D curve breakers are built for the highest inrush loads: large transformers, CNC/servo drives, and motors with heavy starting currents.
Comparison of tripping characteristics
Referring 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 curve for your panel
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.
Frequently asked questions
What’s the difference between B curve and C curve breakers?
A B curve breaker trips at 3 to 5 times rated current (AC), while a C curve trips at 5 to 10 times. C curves tolerate roughly double the inrush current before tripping, making them better suited to circuits with small motors or mixed loads, while B curves are typically used for resistive and lighting circuits with little to no inrush.
What’s the difference between C curve and D curve breakers?
A C curve trips at 5 to 10 times rated current, while a D curve trips at 10 to 20 times — roughly double the tolerance. C curves suit small motors and general distribution with moderate inrush, while D curves are reserved for high-inrush loads like large transformers and servo drives that would trip a C curve immediately.
What’s the difference between K curve and D curve breakers?
Both are designed for high-inrush loads, but D curve has a wider tolerance band — up to 20 times rated current on AC versus K curve’s 14 times. K curve is common for standard motor and transformer circuits, while D curve is reserved for the highest-inrush applications, like large transformers or CNC and servo drives.
Do I need a Z curve breaker for semiconductor equipment?
Z curve is the standard recommendation for semiconductor and other highly sensitive electronic loads, since it has the tightest tolerance band (2 to 3 times rated current on AC) of any standard trip curve — it trips on the smallest deviation from rated current, protecting equipment that can’t tolerate any meaningful overcurrent.
Author
Thomas Weinmann
Senior Product Marketing Manager, ABB Electrification Business. Thomas brings over 33 years of experience at ABB, specializing in electrical protection and control products, circuit protection devices, and low-voltage electrical systems for industrial and OEM applications. He holds a Bachelor’s Degree in Mechatronics, Robotics, and Automation Engineering from the University of Applied Sciences, Esslingen, Germany.
