7.3 Circuit-Breaker Curves & Breaking Capacity
Key Takeaways
- MCB Type B, C and D characteristics differ mainly in the instantaneous (magnetic) trip current multiples of In
- Type B typically trips instantaneously around 3–5 In, Type C around 5–10 In, and Type D around 10–20 In (IEC 60898 teaching ranges — confirm device datasheet)
- Breaking capacity (Icu ultimate, Ics service) must be adequate for the prospective fault current at the point of installation
- Series or backup protection may allow a downstream device to be used where fault level exceeds its standalone rating — only when manufacturer/cascade data supports it
- Curve selection manages start/inrush versus fault sensitivity; it does not replace Ib ≤ In ≤ Iz continuous coordination
Circuit-Breaker Curves & Breaking Capacity
Quick Answer: MCB curve type (B, C, D) sets the instantaneous magnetic trip band as a multiple of In. Breaking capacity (Icu / Ics) is the fault current the breaker can interrupt without catastrophic failure. Select curve for load behaviour; select breaking capacity for prospective fault current at the board — neither replaces Ib ≤ In ≤ Iz.
Time–Current Curves in Plain Language
A thermal-magnetic circuit-breaker has two regions on its time–current curve:
- Thermal / long-time region — inverse time: modest overloads take longer to trip; heavier overloads trip sooner. This region delivers overload protection tied to In.
- Magnetic / instantaneous region — above a threshold multiple of In, the breaker opens very quickly (cycles), clearing short-circuit current.
Curve letters B, C, D (IEC 60898 family used widely on Australian final-circuit MCBs) primarily move that instantaneous threshold. They are not different continuous ratings: a 20 A Type B and a 20 A Type C both have In = 20 A for Ib ≤ In ≤ Iz. They disagree on how many times In is needed before the magnetic trip fires.
Type B, C and D — Instantaneous Multiples
Teaching ranges commonly quoted for instantaneous trip (confirm the live product standard/datasheet):
| Curve | Typical instantaneous trip band | Typical application themes |
|---|---|---|
| Type B | About 3–5 × In | Circuits with low inrush — many resistive loads, some general final circuits where fault sensitivity is desired |
| Type C | About 5–10 × In | General-purpose Australian commercial/domestic MCBs — fluorescent/LED drivers, modest inductive loads, many socket circuits |
| Type D | About 10–20 × In | High inrush — large motors, transformers, some welders — where Type C would nuisance-trip on start |
Why the Band Matters
Suppose In = 16 A, Type C. Instantaneous trip might begin near 5 × 16 = 80 A and be guaranteed by about 10 × 16 = 160 A (band teaching — exact limits per datasheet). A motor start drawing 90 A for a fraction of a second may sit inside or outside the magnetic region depending on type and actual curve. Type D raises the magnetic threshold so start current is less likely to instantaneous-trip, while thermal overload protection for sustained excess current remains anchored to In.
| Scenario | Better curve theme | Risk of wrong choice |
|---|---|---|
| Lighting / low inrush final circuit | Often B or C per design practice | Type D may be unnecessarily “deaf” to some faults |
| General GPOs / mixed loads | Commonly C in AU practice | Type B may nuisance-trip on some electronic loads |
| Large DOL motor | Often D (or dedicated motor protection) | Type B/C nuisance-trips on start |
Capstone items love asking whether to change In or curve when motors trip on start. If continuous load is fine and Ib ≤ In ≤ Iz holds, the answer is often curve / motor protection, not blindly upsizing In (which may then violate In ≤ Iz).
Breaking Capacity — Can It Interrupt the Fault?
Prospective fault current at a switchboard can be several kiloamperes. The breaker must interrupt that current. Two markings appear on many devices:
| Symbol | Meaning (teaching) |
|---|---|
| Icu | Rated ultimate short-circuit breaking capacity — the maximum fault current the breaker can interrupt under specified conditions; after clearing at Icu, the device may not be fit for further service without inspection/replacement per manufacturer rules |
| Ics | Rated service short-circuit breaking capacity — the fault current the breaker can interrupt and remain suitable for continued service (often expressed as a percentage of Icu on MCCBs) |
If prospective fault current at the point of installation exceeds the breaker’s breaking capacity (and no valid backup arrangement applies), the device may fail violently — contacts weld, case ruptures, fault continues. That is a safety defect, not a paperwork nicety.
Prospective Fault Current
Prospective fault current depends on supply transformer size/impedance, service mains, MEN arrangement and the impedance of conductors to the fault location. Near the main switchboard on a robust supply, fault level is highest; at the end of a long final subcircuit it is lower. Designers and assessors care most about the highest fault level the device might see — typically at the board where it is installed.
For the capstone: you may be given a prospective fault current figure and asked whether a 6 kA MCB is adequate on a board with 10 kA prospective fault current. Without cascade data, 6 kA < 10 kA → not adequate standalone.
Series / Backup (Cascade) Protection Themes
Sometimes a downstream breaker with a modest Icu is installed where the raw prospective fault current is higher, because an upstream fuse or MCCB has been proven (by manufacturer cascade tables) to limit let-through energy so the combination is safe. This is backup or cascading protection.
Teaching cautions:
- Backup is not a verbal excuse — it requires documented combination data for the specific upstream and downstream devices.
- Backup can conflict with discrimination: the upstream device may also open on a heavy fault, blacking out more of the installation (Section 7.4).
- “The main fuse will save us” without datasheet support is not competent design.
| Arrangement | Idea | Watch-out |
|---|---|---|
| Standalone | Downstream Icu/Ics ≥ prospective fault current | Simplest to defend in assessment |
| Backup / cascade | Upstream device limits energy so downstream combination is rated | Need manufacturer tables; may sacrifice selectivity |
| Series-rated systems | Specific tested combinations | Do not invent pairings |
Curve Versus Breaking Capacity Versus Coordination
Keep three questions separate:
- Continuous? → Ib ≤ In ≤ Iz.
- Start / inrush without nuisance magnetic trip? → curve type (B/C/D) or dedicated motor protection.
- Can it interrupt the fault safely? → Icu/Ics vs prospective fault current (or validated backup).
A Type D 32 A MCB with Icu = 6 kA can still be wrong on a 15 kA board. A Type B 20 A MCB with Icu = 10 kA can still be wrong if Iz is only 18 A. Passing one check never proves the others.
MCCB Adjustable Trips (Brief)
Larger MCCBs may offer adjustable long-time (Ir), short-time and instantaneous settings. Conceptually they still implement overload and fault regions; the “curve” becomes a set of dials rather than a fixed B/C/D letter. Settings must respect cable Iz (continuous), discrimination studies, and manufacturer limits. Do not set instantaneous so high that fault clearing becomes dangerously slow without a documented reason.
Common Exam Traps
- Upsizing In to stop motor-start trips instead of selecting Type D / motor protection — then failing In ≤ Iz.
- Reading Icu on the label but comparing it to load current instead of prospective fault current.
- Assuming every MCB is 6 kA and every Queensland board is below 6 kA without checking.
- Confusing Ics with residual current (mA) — completely different quantity.
Bridge Forward
Even a correctly rated, correctly curved breaker is only half the story if an upstream device trips for every downstream fault. Section 7.4 covers discrimination and selectivity.
What is the main practical difference between Type B, C and D MCBs of the same In?
A DOL motor on a correctly sized cable nuisance-trips a Type C MCB on every start, while running current is well below In. What is the most appropriate first design response taught for this symptom?
If prospective fault current at a distribution board is 10 kA and a proposed MCB is marked Icu = 6 kA with no validated backup combination, what is the breaking-capacity verdict?
What do Icu and Ics describe on a circuit-breaker?