MCB tripping curves show how quickly a miniature circuit breaker responds to overcurrent. B, C and D identify different magnetic trip ranges: typically 3–5, 5–10 and 10–20 times rated current for the IEC/EN 60898-1 AC characteristics compared here. The complete curve also includes the slower thermal response to overloads. 1
For a panel designer or equipment specifier, the task is to accommodate normal starting current while retaining effective fault protection. Curve selection is one step in the MCB selection guide, alongside current rating, cable protection and breaking capacity.
B, C and D curves at a glance
The table gives a starting point for comparison. An application name alone does not determine the correct curve.
| Curve | Magnetic trip range | Loads to investigate | Main selection consideration |
|---|---|---|---|
| B | 3–5 × In | Predominantly resistive loads, such as heaters | Relatively low starting current |
| C | 5–10 × In | Loads with moderate starting current, such as some motors | Starting-current magnitude and duration |
| D | 10–20 × In | High-inrush equipment, such as transformers | Inrush tolerance and sufficient fault current |
In means rated current. The ranges above follow the AC characteristics documented for ABB S300P under IEC/EN 60898-1; the load examples are general starting points, not equipment-specific recommendations. 1, 2
On a small screen, swipe across the diagram to inspect the labels.
Some product families specify narrower bands. Schneider Electric, for example, lists 10–14 × In for its AC C60 Multi-9 D curve. That figure and a 10–20 × In reference range should not be combined without identifying the product scope. 3
GYM10 B, C and D tripping curves
On a small screen, swipe across the diagram to inspect the labels.
How to read an MCB tripping curve
A time-current chart plots current on the horizontal axis and operating time on the vertical axis. Many charts use logarithmic scales, so equal distances do not represent equal numerical increments. Check whether the current axis shows amperes or the ratio I/In before reading it. 8
The curve has two functional regions:
- Thermal region: sustained overload heats a bimetallic element. A larger overload generally produces a shorter operating time.
- Magnetic region: sufficiently high current operates the electromagnetic release rapidly. The B, C or D designation distinguishes this pickup range. 2
On a small screen, swipe across the diagram to inspect the labels.
Published characteristics often show a tolerance band rather than one exact line. At a chosen current, the band can give a range of possible operating times under the stated test conditions. Read both boundaries, together with the notes on temperature and whether the breaker starts cold. 8
A tripping-time calculation you can follow
Start by calculating the current multiple:
Current multiple = actual current ÷ rated current = I/In
For the ETI B10 example in ETI's published article:
- Rated current is 10 A and the example current is 20 A.
- I/In = 20 ÷ 10 = 2.
- At 2 × In, ETI's Figure 3 and Table 1 give an operating-time range of 32–175 seconds. 2
The arithmetic locates the point on the horizontal axis; the manufacturer's curve supplies the time interval. These are ETI example values, not a specification for a GEYA B10 breaker.
Why a D curve is not simply “slower”
A D curve has a higher magnetic pickup range, not one universally longer trip time. Compare complete characteristics at the same current multiple to assess the response. 2
Likewise, reaching the lower edge of a magnetic band is not the same as reaching its guaranteed operating boundary. Being inside the 5–10 × In band does not, by itself, determine a C breaker's exact operating time. 8
Choosing a curve: check starting current and fault current
Start with the equipment's normal current and starting-current profile. A short inrush pulse and a sustained overload can have similar peak currents but require different interpretations on a time-current chart. The manufacturer's starting data gives a more useful basis than labels such as “commercial” or “industrial.” 4
Then consider the other end of the problem: the minimum fault current the circuit can deliver. Long conductors and higher fault-loop impedance can limit that current. Moving to a higher magnetic threshold may avoid startup trips while making rapid fault disconnection harder to achieve. 5
Where the design relies on magnetic operation, compare the calculated minimum fault current with the relevant operating boundary, including its tolerance. The required disconnection time depends on the installation and protection arrangement; it cannot be settled by the curve letter alone. 5
Keep these checks distinct:
- Rated current and cable protection: use the load and installation conditions when choosing an MCB current rating.
- Curve: evaluate normal starting behaviour against the required fault response.
- Breaking capacity: check the available short-circuit level at the installation point. The MCB breaking-capacity guide explains this separate rating. 4, 5
A qualified electrical designer should assess those conditions before a curve change is used to resolve repeated tripping.
Temperature, adjacent devices and supply frequency
The installation can differ from the conditions used to publish a curve:
- Temperature: thermal behaviour depends on ambient temperature. A 30°C reference appears in some MCB documentation, but the relevant temperature and correction values belong to the selected series.
- Adjacent loaded devices: heat from neighbouring breakers can affect the temperature within a distribution board. Apply the manufacturer's grouping and enclosure guidance.
- Supply frequency: magnetic behaviour can require correction outside the specified frequency range. Check the product's frequency rating and published correction data. 6
These effects call for the correct product data, not a universal derating percentage. An AC chart also should not be treated as evidence of DC suitability; use the specified DC rating and characteristic where applicable. 8
When MCB and MCCB curves differ
Many MCBs have fixed trip characteristics. An MCCB may instead have adjustable thermal-magnetic or electronic protection, depending on its trip unit. Electronic units can offer long-time, short-time and instantaneous functions; those features are not universal to every MCCB. 4
If the design needs adjustable pickup or delay for coordination, compare the actual trip units and their settings. Selecting an MCCB solely because a standard MCB trips during startup does not establish that it will meet the protection requirements.
Other questions about trip curves
Are K and Z curves the same across manufacturers?
Will different curve letters ensure only the downstream breaker trips?
No. Selectivity concerns the operation of a particular upstream and downstream combination across the relevant fault-current range. Use the manufacturer's selectivity tables and applicable conditions; choosing different letters alone does not establish selective operation. 7
Continue the MCB selection process
Once the curve has been checked, return to the MCB selection guide to review the remaining selection criteria. When the circuit requirements are defined, compare the available GEYA miniature circuit breakers and request the characteristic for the exact model under consideration.
Technical sources
- ABB: MCBs technical details (PDF), particularly printed pages 1/9–1/10 for S300P characteristics and the associated test-condition notes.↩↩↩
- ETI: Most Common Tripping Characteristics in Miniature Circuit Breakers Explained, particularly Figure 3 and Table 1 for the B10 example.↩↩↩↩
- Schneider Electric: B, C, D, K and Z curves for C60 Multi-9 circuit breakers.↩↩
- Schneider Electric Electrical Installation Guide: Fundamental characteristics of a circuit-breaker.↩↩↩
- Schneider Electric Electrical Installation Guide: Calculation of minimum levels of short-circuit current.↩↩↩
- Hager: MCB technical information (PDF), application notes on temperature, grouping and frequency.↩
- Schneider Electric Electrical Installation Guide: Coordination between circuit-breakers.↩
- ABB: Circuit protection characteristics of UL 489 miniature circuit breakers (PDF), general explanation of chart regions and tolerance bands. Its UL-specific test values are not applied to the IEC comparison table above.↩↩↩↩
