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

Z Curve

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

B Curve

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

C Curve

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:

1. 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)
2. 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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