The difference between overload and short circuit, and how the tripping characteristic (B/C/D) of circuit breakers is chosen. Overcurrent covers two phenomena, both of which must be interrupted before the conductor is damaged.
Introduction
Overcurrent covers two different phenomena: overload (an excessive, but not violent, current through an otherwise fault-free circuit) and short circuit (a very large current due to a fault connection with low impedance). Both must be interrupted before the conductor is damaged.
Overload protection
The rated current of the protective device must be coordinated with the conductor's current-carrying capacity so that the conductor does not overheat. The relationship between design current, rated current and current-carrying capacity is laid down in DS/HD 60364-4-43 — the values are looked up there.
Short-circuit protection
In the event of a short circuit, the equipment must trip before the conductor reaches a critical temperature. This sets requirements both for the breaking capacity (the equipment must be able to interrupt the expected short-circuit current) and for the energy (I²t) the conductor is exposed to until disconnection.
Tripping characteristics B, C and D
Miniature circuit breakers have a thermal element (against overload) and a magnetic element (against short circuit). The characteristic describes how quickly the magnetic tripping occurs relative to the rated current:
- 01B — trips at relatively low overcurrent; typically for ordinary domestic and lighting circuits.
- 02C — tolerates a higher short-term inrush current; often used for circuits with motors or many electronic supplies.
- 03D — for loads with heavy inrush surges.
| Characteristic | Magnetic release | Typical use |
|---|---|---|
| B | 3–5 · In | Housing and lighting groups |
| C | 5–10 · In | Motors, many electronic supplies |
| D | 10–20 · In | Transformers, powerful switching surges |
Eksempel: En 16 A C-sikring udløser magnetisk et sted mellem 5 · 16 = 80 A og 10 · 16 = 160 A. Til at eftervise garanteret udkobling ved fejl regnes med den øvre grænse (160 A). (Kilde: DS/EN 60898-1.)
Coordination with the leader
Overbelastningsbeskyttelsen skal koordineres med lederen efter de to uligheder IB ≤ In ≤ Iz og I2 ≤ 1,45 · Iz (se artiklen om kabeltværsnit). Sikringens mærkestrøm må ikke overstige lederens belastningsevne. (Kilde: DS/HD 60364-4-43.)
Breaking capacity — Icn, Icu and Ics
The protection must be able to interrupt the expected short-circuit current at the location without being damaged itself. This is expressed as a breaking capacity:
- 01Icn — mærkebrydeevne for automatsikringer til husholdning (DS/EN 60898-1), angives i ampere på sikringen (fx 6000 = 6 kA).
- 02Icu — ultimate breaking capacity for circuit breakers (DS/EN 60947-2): the breaker may be damaged afterwards but must have broken safely.
- 03Ics — operating breaking capacity (DS/EN 60947-2): the switch is still functional after the break. Often expressed as a percentage of Icu (e.g. 50%, 75%, 100%).
Fuse links vs. automatic fuses
| Type | Characteristic | Application |
|---|---|---|
| gG/gL | Full range — protects against both overload and short circuit | General cable and installation protection |
| aM | Partial area — short-circuit protection ONLY | Motor circuits (overload is covered by motor protection) |
| Automatic fuse (MCB) | Reconnectable, thermal + magnetic release | Housing and light industry |
The usage classes of fuses are defined in DS/EN 60269. An aM fuse alone does NOT protect against overload — it must be paired with a thermal relay/motor protection.
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