How long until the breaker trips? Try it
Educational model. The band is drawn through the IEC/EN 60898-1 test points at 30 °C; the line between them is an interpolation. The “sample breaker” is one notional device: its thermal time constant is estimated from test reports, not taken from the standard. Look up the trip time of your own breaker on the manufacturer’s curve. In a warm enclosure a breaker trips sooner: by about 5 % in current for every 10 °C above 30 °C. Cable heating is shown as an order of magnitude: one time constant estimated from tests on cables in thermal insulation, installation method A1, 30 °C. It is not a fire prediction. The short-circuit current is calculated for a 20 m line with a notional supply impedance of 0.35 Ω. The real current depends on your supply; an electrician determines it by measuring Zs.
What happens inside the breaker
Bimetal strip: overload
Current heats a strip made of two metals that expand differently, so it bends. The larger the overload, the sooner it reaches the latch. A small overload can take minutes or hours; a strip that is already warm trips sooner.
Coil and plunger: short circuit
A large current magnetises the coil, which pulls in the plunger and knocks the latch open within milliseconds. The curve letter sets where this begins: B at 3–5 × In, C at 5–10 × In, D at 10–20 × In.
Cable: what is being protected
A cable heats up gradually and cools down the same way. PVC insulation is rated for 70 °C in continuous use. The breaker has to switch off before the cable gets there, which is why its rating must suit the cross-section.
How to read the time-current chart
Current runs from left to right in amperes and time from bottom to top; both scales are logarithmic. The shaded band is where the standard allows a breaker of this curve and rating to trip: for any current, the lower edge is the earliest permitted trip time and the upper edge the latest. The solid line is one sample breaker inside the band. The vertical line marks the current of the chosen scenario; the dot rises with time and stops when it meets the line. Rings mark the IEC test points.
IEC/EN 60898-1 test points (a–e)
The standard does not prescribe a single curve. It fixes a few test points that every breaker must pass, and the curves of real devices run anywhere between them. The table shows each requirement and how the sample breaker meets it.
| Point | Current | Start | Requirement | Sample breaker B16 | Sample breaker C16 | Sample breaker D16 |
|---|---|---|---|---|---|---|
| a | 1.13 × In | cold | must not trip · ≥ 1 h | does not trip | does not trip | does not trip |
| b | 1.45 × In | warm, right after a | must trip · < 1 h | 42 s | 42 s | 42 s |
| c | 2.55 × In | cold | must trip · 1 s – 1 min | 18 s | 18 s | 18 s |
| d | B 3 · C 5 · D 10 × In | cold | must not trip · ≥ 0.1 sEN 60898-1 — must trip within: B 45 s · C 15 s · D 4 s | 12 s | 4.2 s | 1 s |
| e | B 5 · C 10 · D 20 × In | cold | must trip · < 0.1 s | 10 ms | 10 ms | 10 ms |
Point b starts with the breaker already warmed by the current of point a, as in the test itself. Point c allows 1–60 s for In ≤ 32 A and 1–120 s above. Points d and e depend on the curve letter; the upper time limits of point d come from EN 60898-1 and are not part of IEC 60898-1. Times are for an ambient temperature of 30 °C.
Worked scenarios
The five scenarios of the simulator with the numbers from the model. Protection is B16 on 2.5 mm² copper cable unless the row says otherwise; the short circuit is at the end of a 20 m line.
| Scenario | Protection | Load | Current | Breaker | Cable |
|---|---|---|---|---|---|
| 1. Kettle | B16 · 2.5 mm² | Kettle | 8.7 A · 0.54 × In | stays onStandard — must hold for: ≥ 1 h | Cable settles at: 38 °C |
| 2. + heater | B16 · 2.5 mm² | Kettle + Heater | 17.4 A · 1.09 × In | stays onStandard — must hold for: ≥ 1 h | Cable settles at: 62 °C |
| 3. + oven | B16 · 2.5 mm² | Kettle + Heater + Oven | 30.4 A · 1.9 × In | Tripped · bimetal · 38 sStandard — must trip within: 2 s – 2.2 min | Cable at trip: 35 °C · Without the breaker: > 120 °C |
| 4. Short circuit 20 m away | B16 · 2.5 mm² | Short circuit at the end of the line | 310 A · 19.4 × In | Tripped · electromagnet · 10 msStandard — must trip within: 5 ms – 0.1 s | Cable heating: +0.8 °C · Cable would withstand (from 70 °C): ≈ 0.86 s |
| 5. B16 on 1.5 mm² | B16 · 1.5 mm² | Kettle + Heater | 17.4 A · 1.09 × In | stays onStandard — must hold for: ≥ 1 h | Cable settles at: 88 °C |
| Fit B10 and run | B10 · 1.5 mm² | Kettle + Heater | 17.4 A · 1.74 × In | Tripped · bimetal · 49 sStandard — must trip within: 2.5 s – 3 min | Cable at trip: 36 °C · Without the breaker: > 80 °C |
The numbers come from the educational model described above, not from a particular product; the real trip time of your breaker is on the manufacturer’s curve. Cable ratings follow IEC 60364-5-52, Table B.52.2 (method A1, 30 °C); national tables can differ, for example 15.5 A for 1.5 mm² in Germany. The withstand time uses k = 115 from IEC 60364-4-43, Table 43A, starting from the 70 °C operating temperature.
Common questions
Why does a breaker not trip when the load is above its rating?
A breaker is allowed to carry a small overload. Up to 1.13 times its rating it must not trip within an hour; between 1.13 and 1.45 times it may or may not trip, and at 1.45 times it must trip in under an hour. That is why the breaker has to suit the cable: the cable must survive the same overload.
What is the difference between the bimetal strip and the electromagnet?
The bimetal strip reacts to heat, so it clears an overload with a delay that shrinks as the current grows. The electromagnet reacts to the current itself and trips almost instantly once the current passes the threshold of the curve letter. An ordinary breaker has both.
Does C16 trip later than B16?
On overload both meet the same thermal requirements, so the delay is similar. The difference is the instant trip: C needs 5–10 × In instead of 3–5 × In, so a moderate short circuit that B16 clears instantly may be cleared by C16 only through its slower thermal release.
Does the simulator show the exact trip time of my breaker?
No. It is an educational model: the band follows the IEC test points and the sample breaker is one plausible device inside it. Real devices differ between manufacturers and depend on temperature and previous load. For a design, use the manufacturer’s curve and have an electrician check the circuit.
Sources and further reading
- Siemens — Miniature Circuit Breakers, §9.1–9.2 (2025)
- Schneider Electric — Electrical Installation Guide
- ABB — Comparison of tripping characteristics for MCBs (2CDC400002D0201)
- Schneider Electric — FAQ FA369327: conventional currents 1.13 and 1.45 × In
- Eaton — Tripping characteristics for MCBs (CA08103002Z-EN)
- ABB SACE — Electrical installation handbook, vol. 2 (1SDC010001D0202)
- Schneider Electric — Electrical Installation Guide: withstand of cables under short-circuit conditions
- IEC 60364-5-52 ed. 3.0 — preview with the titles of tables B.52 (VDE Verlag)
- IET Wiring Matters / ERA Technology — current ratings for cables in thermal insulation
- IET Wiring Matters — maximum Zs values (Cmin = 0.95, CLC/TR 50480)
- MCB ratings and B, C, D curves explained
- Cable cross-sections explained