Trip current by rating
| Rating In, A |
Holds 1.13·In, A |
Trips instantly, A | ||
|---|---|---|---|---|
| Type B 3–5·In |
Type C 5–10·In |
Type D 10–20·In |
||
| 6 | 6.8 | 18–30 | 30–60 | 60–120 |
| 10 | 11.3 | 30–50 | 50–100 | 100–200 |
| 16 | 18.1 | 48–80 | 80–160 | 160–320 |
| 20 | 22.6 | 60–100 | 100–200 | 200–400 |
| 25 | 28.2 | 75–125 | 125–250 | 250–500 |
| 32 | 36.2 | 96–160 | 160–320 | 320–640 |
| 40 | 45.2 | 120–200 | 200–400 | 400–800 |
| 50 | 56.5 | 150–250 | 250–500 | 500–1000 |
| 63 | 71.2 | 189–315 | 315–630 | 630–1260 |
Trip curve types
| Type | Instantaneous trip |
Application |
|---|---|---|
| A | 2–3·In | Semiconductors and measuring circuits, very long runs. Rare. |
| B | 3–5·In | Lighting, socket circuits, lines without inrush currents; long runs and networks with low short-circuit current. |
| C | 5–10·In | The universal residential choice. Mixed loads with moderate inrush - sockets, household appliances. |
| D | 10–20·In | Motors, transformers, welders and other loads with high inrush currents. |
| K | 8–12·In | Inductive loads and electric motors, industrial use. |
| Z | 2–3·In | Electronics and semiconductor devices, sensitive equipment. |
Two releases
A breaker has two independent trip mechanisms. The thermal one responds to overload with a delay - from seconds to an hour. The magnetic one responds to a short circuit and trips instantly (hundredths of a second). In the table above, the "Holds" column (1.13·In) is the current the breaker carries indefinitely without tripping; it is the same for all types, because they respond to overload identically. The "Trips instantly" columns give the short-circuit current range in which the magnetic release acts: below the lower bound the breaker will not yet trip instantly, above the upper bound it is guaranteed to. The type (B, C, D) is exactly what sets this range.
Overload (thermal release), same for B, C and D
1.13·In - the conventional non-tripping current: the breaker carries
the load indefinitely. 1.45·In - the conventional tripping current:
it disconnects in less than 1 hour. 2.55·In - disconnection in 1–60 s
(ratings up to 32 A) or 1–120 s (above 32 A). In other words, the breaker tolerates
a small overload (up to ~13%), and the higher the excess current, the faster it
trips. These thresholds (1.13 and 1.45·In) apply to residential B, C and D breakers
per IEC 60898. Industrial curves A, K and Z (IEC 60947-2) use different overload
thresholds - 1.05 and 1.30·In.
Breaking capacity
The highest short-circuit current the breaker
can interrupt without being destroyed. It is printed in a rectangle on the case:
4500, 6000 or 10000 (A). For apartment
panels 4.5–6 kA is usually enough; closer to the service entrance and substation
10 kA units are used. What the other markings on the case mean - see the
breaker markings reference.
How this relates to the cable
For the breaker to trip on a short circuit at the end of a line, the short-circuit current there must exceed its instantaneous trip threshold from the table - hence the limit on line length (maximum cable length). Type B trips at a lower current than C, so it allows twice the length. Reference material, values per IEC 60898; the actual design should be done by a qualified electrician.