MCB Trip Curves Explained with Selection Examples

MCB Trip Curves indicate how quickly a miniature circuit breaker (MCB) trips under fault conditions. Different trip curves (B, C, D, K, Z) are designed for different load types and applications. Understanding them ensures correct selection for protection, coordination, and safety.

What is an MCB Trip Curve?

An MCB trip curve shows the relationship between fault current and tripping time. It has two parts:

• Thermal tripping (overload): Protects against small overloads by heating a bimetal strip, which bends and trips the breaker.
• Magnetic tripping (short-circuit): Trips almost instantly when high fault current flows through a coil.

Types of MCB Trip Curves

1. Type B Curve

• Trips at 3–5 × In (rated current).
• Used for resistive and light inductive loads (lighting, heating).
• Example: 10A MCB trips at 30–50A.

2. Type C Curve

• Trips at 5–10 × In.
• Standard curve for general use (fans, sockets, small motors).
• Example: 16A MCB trips at 80–160A.

3. Type D Curve

• Trips at 10–20 × In.
• For high inrush loads (large motors, transformers, X-ray machines).
• Example: 32A MCB trips at 320–640A.

4. Type K Curve

• Trips at 8–12 × In.
• Suitable for inductive loads with moderate inrush (motor groups, compressors).

5. Type Z Curve

• Trips at 2–3 × In.
• Highly sensitive — used for semiconductor protection and electronics.

Comparison Table of MCB Trip Curves

MCB Type	Magnetic Trip Range	Typical Applications
Type B	3–5 × In	Lighting, resistive loads
Type C	5–10 × In	General circuits, mixed loads
Type D	10–20 × In	Large motors, transformers
Type K	8–12 × In	Motor groups, compressors
Type Z	2–3 × In	Sensitive electronics

MCB Selection Examples

• Home Lighting Circuit: Type B curve, 10A, 6 kA MCB.
• Socket Outlets with Mixed Loads: Type C curve, 16A–20A.
• 3-Phase Induction Motor (10 HP): Type D curve, 32A.
• Electronics Lab: Type Z curve, 6A, high sensitivity.

Advantages of Choosing Correct Trip Curve

• Prevents nuisance tripping.
• Improves coordination between protective devices.
• Protects equipment from short-circuits and overloads effectively.
• Enhances safety and reliability of installations.

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FAQs

Q1. What is the most common MCB curve for homes?
Type C curve is most common for residential and commercial general circuits.

Q2. Why not use D curve MCBs everywhere?
D curve requires high fault current to trip, which may not be available in domestic wiring — making it unsafe for normal home circuits.

Q3. How to read an MCB trip curve graph?
The X-axis shows multiples of rated current, and the Y-axis shows tripping time (instantaneous vs thermal delay).

Conclusion

Correct selection of MCB trip curves ensures safety, prevents nuisance tripping, and protects equipment. Use B for lighting, C for mixed loads, D for high inrush motors, K for inductive loads, and Z for sensitive electronics.