Key takeaways
- B, C and D mainly identify the instantaneous magnetic trip band: 3–5, 5–10 and 10–20 times rated current under IEC/EN 60898-1.
- A higher curve letter can tolerate more inrush, but it also requires more fault current to operate magnetically and may affect disconnection-time compliance.
- Breaking capacity is checked against the maximum prospective short-circuit current at the installation point; it is separate from the trip curve.
- Selectivity and back-up protection describe different outcomes and must be verified for the exact upstream/downstream device pair, voltage and fault-current level.
- ABB coordination tables or software—not a generic curve sketch—are the final authority in the instantaneous short-circuit region.
For ABB System pro M compact S200 miniature circuit breakers, the B, C or D marking primarily tells you where the electromagnetic release is expected to operate during a high overcurrent. It does not tell you the breaker’s interrupting rating, guarantee discrimination with an upstream device, or replace a short-circuit and disconnection-time calculation.
What an MCB trip curve actually shows
An MCB normally combines two release mechanisms. The thermal release responds to sustained overloads and operates on an inverse-time basis: more current generally means a shorter trip time. The electromagnetic release responds much faster to high overcurrent, such as a short circuit. The B, C and D designation changes the magnetic operating band; it is not a different continuous-current rating.
B, C and D instantaneous trip bands
IEC/EN 60898-1 magnetic release comparison
| Curve | Magnetic operating band | What the band means | Selection implication |
|---|---|---|---|
| B | Above 3 × In to 5 × In | Lower instantaneous threshold of the three | More sensitive to short-duration inrush; lower fault current is needed to enter the magnetic region |
| C | Above 5 × In to 10 × In | Intermediate instantaneous threshold | Allows more inrush than B, but needs a higher fault current for magnetic operation |
| D | Above 10 × In to 20 × In | Highest instantaneous threshold of the three | Can ride through larger inrush, but the protection calculation must support the higher threshold |
The band is a tolerance zone, not one exact pickup value. Below the lower boundary, the magnetic release must not operate under the stated test condition; at the upper boundary, it must operate within the standard’s specified time. Between those boundaries, either behavior can be compliant. That is why a single line at 5, 10 or 20 times rated current is not an adequate model of a real breaker.
Worked example: a 16 A breaker
Multiplying the rated current In by the curve limits gives the magnetic band. For a 16 A device, B spans 48–80 A, C spans 80–160 A and D spans 160–320 A. These figures do not mean the breaker will always trip at the first number or wait until the second. They define the region in which the magnetic release transitions from required non-operation to required operation.
Magnetic bands for a 16 A MCB
| Curve | Lower boundary | Upper boundary |
|---|---|---|
| B16 | 3 × 16 A = 48 A | 5 × 16 A = 80 A |
| C16 | 5 × 16 A = 80 A | 10 × 16 A = 160 A |
| D16 | 10 × 16 A = 160 A | 20 × 16 A = 320 A |
How to choose the curve without creating a protection problem
The practical objective is to let expected starting or energization current pass without nuisance tripping while still clearing the lowest credible fault current within the required time. Moving from B to C or D may solve one side of that equation and make the other side worse.
Check inrush as both current and time
A motor start, transformer energization, capacitor charging pulse and LED-driver inrush can reach similar peak currents but last for very different periods. Plot the manufacturer’s worst-case inrush envelope against the breaker’s upper trip boundary at the relevant ambient and installation condition. A peak multiplier with no duration is not enough to judge compatibility.
Check the minimum fault current and disconnection time
The far end of a long or high-impedance circuit may produce much less fault current than the supply end. If that minimum value does not reach the magnetic operating region, clearing depends on the slower thermal part of the curve. The design must still meet the applicable shock-protection and conductor-protection disconnection requirements. Increasing the curve from C to D without repeating this calculation can turn nuisance-trip relief into delayed fault clearance.
Apply the actual installation conditions
Thermal behavior is influenced by ambient temperature, adjacent loaded devices, enclosure conditions and conductor termination. The reference curve is a laboratory characteristic, not a guarantee that every installation behaves identically. Apply ABB’s correction or application guidance where the installation differs from the reference conditions.
Trip curve is not breaking capacity
The trip curve describes when the release commands an opening operation. Breaking capacity describes the short-circuit level the device is rated to interrupt under a stated standard, voltage and test sequence. A C-curve device is not inherently able to interrupt more fault current than a B-curve device.
Keep these short-circuit terms separate
| Term | What it describes | How it is used |
|---|---|---|
| Prospective short-circuit current | The fault current that could flow at a point in the installation before a protective device limits it | Calculate or measure it at the installation point; compare the maximum value with the applicable device or combination rating |
| Icn | Rated short-circuit capacity used for household and similar MCB applications under IEC/EN 60898-1 | Use the value marked or documented for the relevant voltage and pole arrangement |
| Icu | Rated ultimate short-circuit breaking capacity under IEC/EN 60947-2 | Shows the maximum tested breaking duty under that standard; it is not interchangeable with Icn |
| Ics | Rated service short-circuit breaking capacity under IEC/EN 60947-2 | Indicates the defined service-performance level after the specified short-circuit test sequence |
Use the rating that belongs to the exact device, approval, voltage, frequency and pole configuration. If the maximum prospective short-circuit current exceeds the breaker’s applicable standalone capacity, the answer is not to choose a different trip curve. The design needs a device with sufficient capacity or an ABB-approved back-up combination.
What selectivity means between two breakers
Two protective devices in series are selective when an overcurrent on the load side causes the downstream device to open while the upstream device remains closed. The aim is continuity: only the affected branch is disconnected.
Total and partial selectivity
| Result | Meaning | Evidence required |
|---|---|---|
| Total selectivity | The downstream device alone clears overcurrents up to the stated limit of the combination | Manufacturer data for the exact pair and stated electrical conditions |
| Partial selectivity | Selectivity is assured only up to a stated limit current Is | Confirm that the maximum prospective short-circuit current at the downstream location does not exceed Is |
| Not demonstrated | Both devices may operate for some faults, even if their rated currents differ | Do not infer selectivity from brand, frame size or curve letter |
Time-current comparison works best in the overload region
For overload coordination, compare the downstream device’s maximum operating-time curve with the upstream device’s minimum non-operating or operating boundary, including tolerances. The curves need adequate separation across the current range being assessed. Comparing two nominal center lines can overstate the available margin.
The short-circuit region needs manufacturer coordination data
At high fault currents, current limitation, contact motion, arc voltage and let-through energy affect both devices before a simple time-current plot can describe the interaction. ABB states that selectivity limits in the instantaneous region are specific to the device combination and are obtained from test data. Use ABB’s selectivity tables, SOC or the current ABB coordination tool for the exact pair.
Back-up protection is useful—but it is not selectivity
Back-up protection, also called cascading, is a tested coordination in which an upstream current-limiting device reduces the short-circuit stress seen by a downstream device. This can allow the combination to be used at a prospective fault level above the downstream breaker’s standalone breaking capacity, but only up to the value stated by the manufacturer.
The desired outcome is different from selectivity. A selective combination aims to keep the upstream device closed. A back-up combination aims to interrupt the fault safely; operation of the upstream device may be part of that result. One ABB table may show a selectivity limit and a separate back-up value for the same pair. Neither value should be substituted for the other.
A defensible ABB S200 selection workflow
Document the decision in this order
- 01
Identify every device exactly
Record the full ABB order code, number of poles, rated current, curve, applicable standard, rated voltage and breaking-capacity marking. Do the same for the upstream protective device.
- 02
Define normal load and inrush
Use measured data or the equipment manufacturer’s maximum starting or energization current and duration. Include operating cycles and simultaneous starts where relevant.
- 03
Calculate both fault-current extremes
Determine the minimum fault current used for disconnection-time verification and the maximum prospective short-circuit current used for interrupting-capacity and coordination checks.
- 04
Check the complete time-current curve
Verify that expected inrush remains below the breaker’s operating boundary while the minimum fault current produces acceptable clearing. Include tolerances and installation corrections.
- 05
Verify short-circuit capacity
Confirm that the applicable Icn, Icu or approved back-up rating is sufficient at the stated voltage. Do not mix values from different standards or product variants.
- 06
Verify the exact upstream/downstream pair
Use current ABB selectivity and back-up tables or software. Record whether selectivity is total or partial, the limit current Is, the back-up value and all conditions attached to the table.
- 07
Retain the calculation and source revision
Keep the device codes, fault-current assumptions, curve or table reference and document revision with the panel schedule. This makes later substitutions reviewable.
Common mistakes to avoid
Choosing D curve only to stop nuisance trips. This may mask an undersized circuit, an abnormal start, an incorrect breaker rating or a fault-current problem. Diagnose the event and repeat the protection checks before changing the curve.
Assuming different current ratings guarantee selectivity. Two devices can have separated overload curves and still operate together during a high-current fault. Check the exact combination table.
Comparing Icn directly with Icu or Ics. They belong to different standards and test duties. Use the value applicable to the installation and the marking on the selected product.
Treating back-up protection as permission to use any lower-rated downstream breaker. Cascading applies only to the tested combination, conditions and maximum value published by the manufacturer.
Replacing an S200-family device by visible dimensions alone. Similar front profiles can hide different standards, curves, capacities, terminal arrangements and accessories. Preserve the complete order code and coordination evidence.
Bottom line
Start with the B, C or D curve only after the load and its inrush are known. Finish the design only after minimum fault current, disconnection time, maximum prospective short-circuit current, breaking capacity and the exact ABB coordination data have all been checked. The curve letter is one input to that process—not the conclusion.
