Two breakers with the same 32 A rating can behave very differently when a motor starts or a cable faults, because their trip curve sets the magnetic trip point. This guide shows how each curve works, how to pick one and how to prove it will trip.
The letter printed before the ampere rating on an MCB tells you how fast it trips on a short circuit. We cover the IEC bands, thermal tests, selection by load and a fault current check with a calculator.
What Are MCB Trip Curves?
MCB trip curves are time versus current graphs that show how quickly a miniature circuit breaker opens for any overcurrent, from a mild overload to a dead short. Each curve has a slow thermal region for overloads and a fast magnetic band, which IEC 60898 1 fixes at 3 to 5 × In for type B, 5 to 10 × In for type C and 10 to 20 × In for type D.
- Type B: magnetic trip at 3 to 5 × In, for lighting, heaters and long cable runs
- Type C: magnetic trip at 5 to 10 × In, the default for mixed and small motor loads
- Type D: magnetic trip at 10 to 20 × In, for transformers, welders and big motors
- K and Z curves come from IEC 60947 2: K for inrush loads, Z at 2 to 3 × In for electronics
- All curves must not trip at 1.13 × In and must trip at 1.45 × In within one hour
The letter on the front of the breaker, such as C32, is the curve and the number is the rated current In. Reading MCB trip curves correctly is the first step before you calculate the MCB rating for a circuit, because the rating protects the cable while the curve decides how the breaker handles inrush and faults.
On the chart the horizontal axis is current as a multiple of In and the vertical axis is trip time, both on log scales. Every curve is really a band between a minimum and a maximum line, because the standard allows manufacturers a tolerance window.
How MCB Trip Curves Work: Thermal and Magnetic Regions
ABB explains that every MCB has only two protection elements with fixed settings, a bimetal strip for overload and a coil for short circuit. The bimetal heats and bends slowly, while the coil pulls the latch open within milliseconds once current crosses the magnetic threshold.
The thermal part of all MCB trip curves looks almost the same, as ABB notes, and only the magnetic band shifts left or right. That is why a B, C or D breaker of the same rating carries the same steady load, but reacts very differently to a motor start, as covered in motor starting methods compared.
| Test current | Requirement in IEC 60898 1 | What it proves |
|---|---|---|
| 1.13 × In | Must not trip within 1 hour from cold | Breaker carries rated load with margin |
| 1.45 × In | Must trip within 1 hour (2 hours above 63 A) | Cable is protected against sustained overload |
| 2.55 × In | Must trip between 1 and 60 s (120 s above 32 A) | Bimetal is working correctly |
| Lower band limit | Must not trip within 0.1 s | Inrush below this value is tolerated |
| Upper band limit | Must trip within 0.1 s | Short circuits are cleared instantly |
Electrical Engineering Portal notes that instantaneous trip is conventionally taken as opening within 0.1 s. On MCB trip curves for a 32 A breaker, that means about 160 A for type B, 320 A for type C and 640 A for type D at the upper band limit.
MCB Trip Curve Types B, C, D, K and Z
Magnetic band 3 to 5 × In, the most sensitive of the common curves.
Magnetic band 5 to 10 × In, rides through moderate inrush.
Magnetic band 10 to 20 × In for very high inrush.
ABB lists 10 to 14 × In, with a tighter thermal curve for motors.
Trips at 2 to 3 × In for very low let through.
Among MCB trip curves, CHINT points out that type B protects people and long cables in TN and IT systems, while type D suits LV to LV transformers and discharge lamps. The link between curve choice and earthing system is explained in TN, TT and IT earthing systems.
When a C curve breaker trips every time a pump starts, do not jump to a higher ampere rating. CHINT warns that a bigger rating can let the cable overheat before the breaker trips, so change the curve after checking the fault current instead.
Which MCB Curve to Use for Which Load?
With MCB trip curves the rule is simple: pick the most sensitive curve whose lower magnetic limit stays above the inrush peak of the load. Then confirm that the fault current at the far end of the cable is higher than the upper limit of that curve.
| Load | Typical inrush | Suggested curve | Reason |
|---|---|---|---|
| Incandescent lamps, heaters | Low, near 1 × In | B | Resistive load, maximum sensitivity |
| LED drivers and SMPS | High but very short | C | Capacitor charging spike |
| Small induction motors, fans | 5 to 8 × FLC | C | Standard choice for DOL starting |
| Control transformers | 10 to 15 × In | D or K | Magnetising inrush peak |
| Welders and large motors | Above 10 × In | D | Avoids nuisance trips |
| PLC cards, semiconductors | Must be limited | Z | Fast trip before damage |
Motor inrush depends on the full load current, so first find it with the help of motor full load current tables. Transformer inrush can be far higher than the rated current for a few cycles, a topic explained well in transformer inrush current.
- Clears faults faster and limits damage
- Works on long cables where fault current is low
- Better protection for people in TN systems
- Lower let through energy for cables
- Rides through motor and transformer inrush
- Needs a much higher fault current to trip instantly
- May fail the loop impedance check on long runs
- Can delay disconnection if cables are undersized
Solved Example: Will the MCB Trip Instantly?
A 230 V socket circuit is protected by a B32 breaker and the measured earth fault loop impedance at the far socket is 1.0 Ω. We need to know if the fault current exceeds the upper magnetic limit, so the breaker clears the fault inside 0.1 s.
If = 230 ÷ 1.0 = 230 A
Trip current Ia = 5 × 32 = 160 A
Ratio = 230 ÷ 160 = 1.44
Max Zs = 230 ÷ 160 = 1.44 Ω
Comparing MCB trip curves, the B32 trips magnetically, while a C32 would need 320 A and fail this check. The method follows the same logic as earth fault loop impedance Zs testing, and the prospective current can also be found from short circuit fault current calculation.
CHINT data for its eB series shows a reference calibration temperature of 30 °C, with the rated current multiplied by 0.96 at 40 °C. In a hot Indian panel, a breaker can therefore trip on overload at a lower current than its label suggests.
Coordination and Selectivity Between MCBs
Selectivity means only the breaker nearest the fault opens, while the upstream device stays closed. With MCB trip curves this is hard to achieve in the magnetic zone, because both trip within milliseconds once the fault is high enough.
A common rule is to keep the upstream rating at least twice the downstream rating and to use a higher curve upstream. Full time current studies are explained in circuit breaker coordination and selectivity, and fuse backup is covered in our fuse selection guide.
In motor circuits the MCB handles short circuits only, while a thermal overload relay gives accurate overload protection. The difference between MCBs, MCCBs and residual current devices is set out in MCB vs MCCB vs ELCB vs RCCB.
For a broader view of how breakers interrupt current, see this note on the working principle of a circuit breaker. If you are sizing the feeder at the same time, this primer on cable sizing basics is a useful companion.
Troubleshooting MCB Trips in the Field
| Symptom | Likely cause | Action |
|---|---|---|
| Trips at motor start | Inrush above lower magnetic limit | Check FLC and change C to D curve |
| Trips after hours of running | Overload or hot panel | Measure current and apply ambient factor |
| Trips when LED lights switch on | Many drivers charging together | Use C curve or stagger the loads |
| Does not trip on test fault | Loop impedance too high | Shorten cable, increase size or use RCD |
| Breaker feels hot | Loose terminal | Torque terminals and check load |
- Curve letter matches the load type
- Fault current exceeds the upper band limit at the far end
- Cable rating is at least the MCB rating after derating
- Ambient correction factor applied for hot panels
- Upstream device is rated for selectivity
- Breaking capacity is above prospective fault current
Where the loop impedance is too high for any curve, an RCD gives earth fault protection, and the options are listed in RCD types AC, A, F and B. Run through the electrical safety audit checklist after any change to protective devices.
When you order a spare, copy the full code such as C16 6 kA from the old unit. The breaking capacity matters as much as the curve, since the breaker must interrupt the full prospective fault current safely.
MCB Trip Curves FAQ
The difference is the current at which the magnetic element trips instantly. Type B trips at 3 to 5 times In, type C at 5 to 10 times and type D at 10 to 20 times.
All three have a similar thermal region for overloads. The choice depends on load inrush, because a B curve will nuisance trip on a motor start that a C or D curve rides through easily.
Most homes use type B or type C breakers on final circuits. Type B suits lighting and long socket runs, because it trips at a lower fault current and so works with higher loop impedance.
Type C is often chosen where air conditioners, pumps or many LED drivers cause inrush. The fault current at the farthest point must still exceed the upper magnetic limit of the chosen curve.
You can, but only after checking the earth fault loop impedance at the end of the circuit. A C curve needs roughly twice the fault current of a B curve to trip instantly, so a long cable may no longer pass.
If the check passes and nuisance trips came from inrush, the change is usually sensible. Never solve nuisance tripping by raising the ampere rating above the cable rating.
These are the conventional test currents of IEC 60898 1 for the thermal part of the breaker. At 1.13 times In the breaker must not trip within one hour from cold.
At 1.45 times In it must trip within one hour for ratings up to 63 amps. Together these two points prove the breaker carries rated load yet protects the cable against sustained overload.
K and Z curves are defined for industrial breakers under IEC 60947 2 rather than the domestic standard. ABB lists K at 10 to 14 times In for motors and transformers with high inrush.
Z curve breakers trip at 2 to 3 times In and are used for semiconductors and sensitive electronic circuits. They limit let through energy so that delicate parts survive a short circuit.
The starting current of the motor is reaching the lower magnetic limit of the breaker curve. A direct online motor can draw 5 to 8 times its full load current for a few seconds at start.
Changing from a B to a C curve, or from C to D, usually solves it. Check the loop impedance first, and consider a soft starter where the supply is weak.
Related Articles
- How to Calculate MCB Rating for a Circuit
- MCB vs MCCB vs ELCB vs RCCB
- Circuit Breaker Coordination and Selectivity
- Earth Fault Loop Impedance Zs Testing
- IDMT Relay Characteristics, TMS and PSM
External References
- ABB: What You Need to Know About MCB Trip Curves (PDF)
- CHINT Global: Guide to MCB Trip Curves
- Wikipedia: Circuit Breaker
What We Learn Today
- MCB trip curves combine a slow thermal region for overloads with a fast magnetic band that sets the instant trip point for short circuits.
- Type B trips at 3 to 5 times In, type C at 5 to 10 times and type D at 10 to 20 times under IEC 60898 1.
- Always confirm that the fault current at the far end of the cable exceeds the upper magnetic limit, using U0 divided by the loop impedance.