A miniature circuit breaker protects a circuit in two ways. A thermal bimetal trips it on sustained overload after a delay that shortens as the overload grows. A magnetic coil trips it almost instantly on short-circuit current. Manufacturers and test laboratories verify both against the time-current requirements of IEC 60898-1 for household and similar breakers, and IEC 60947-2 for industrial breakers.
Time-Delay (Thermal) Tests
IEC 60898-1 defines a conventional non-tripping current and a conventional tripping current, tested in sequence at a reference ambient temperature of 30°C:
| Test current | Requirement (breakers rated up to 63 A) |
|---|---|
| 1.13 × In | Must not trip within 1 hour (conventional time), starting from cold |
| 1.45 × In | Applied immediately after the 1.13 × In test, raised within 5 s; must trip within 1 hour |
| 2.55 × In | From cold, must trip between 1 s and 60 s (In ≤ 32 A) or 1 s and 120 s (In > 32 A) |
For breakers rated above 63 A the conventional time becomes 2 hours. Thermal tripping depends on the starting temperature of the bimetal, so "from cold" and a controlled ambient temperature are essential for repeatable results.
Instantaneous (Magnetic) Tests and Curve Types
The B, C and D tripping curves are defined by the instantaneous tripping range:
| Type | Must not trip within 0.1 s at | Must trip within 0.1 s at |
|---|---|---|
| B | 3 × In | 5 × In |
| C | 5 × In | 10 × In |
| D | 10 × In | 20 × In |
Type B suits resistive loads and long cable runs, C suits general commercial and motor loads, and D suits loads with high inrush such as transformers.
What a Test System Needs
- A high-current, low-voltage AC source with low waveform distortion and closed-loop current regulation.
- Accurate timing from milliseconds to hours.
- A temperature-controlled enclosure, so thermal tests start from a known ambient.
- Automatic sequencing — particularly the step from 1.13 × In to 1.45 × In within 5 seconds.
The LISUN LS-B60 combines time-delay and instantaneous stations under PLC control for 1- to 4-pole breakers rated 3–125 A. The supplier specifies an AC output of 10–350 A for time-delay tests at 1.13, 1.45 and 2.55 × In, 100–1500 A for instantaneous tests at 3–10 × In, current accuracy class 0.5, distortion ≤5%, and a heated chamber from room temperature to 50°C with ±1°C stability. It applies 1.13 × In and automatically raises the current to 1.45 × In after 5 seconds. It also performs temperature-rise tests.
Frequently Asked Questions
Why does a breaker that passed at the factory trip early in a hot switchboard? Thermal trips are calibrated at a reference temperature. At higher ambient temperatures, less current is needed to trip, which is why derating applies in hot enclosures.
Do RCBOs need the same tests? The overcurrent part of an RCBO is tested in the same way. The residual-current function is tested separately for operating current and time.