What Is a Fuse? Fuse Types: MCB, NH Blade, Cartridge and Glass Fuses
What a fuse does, how a miniature circuit breaker differs from a fuse link, NH blade, cylindrical cartridge, D-type and glass fuses, gG vs aM classes and selection rules.
A fuse is a protective device that opens a circuit by itself when current exceeds the permitted value; it protects cables from overheating and installations from short-circuit damage. There are two main groups: fuse links (IEC 60269), whose element melts and must be replaced, and miniature circuit breakers (IEC 60898-1), which can be reset after tripping.
> Related Category: Browse all types and ratings in Fuses, Miniature Circuit Breakers and Blade Fuses.
1. Fuse types compared
| Type | Current range | Breaking capacity | After operation | Typical use |
|---|---|---|---|---|
| Miniature circuit breaker (MCB) | 1–125 A | 3 – 4.5 – 6 – 10 kA | Reset the toggle | Home and commercial boards, lighting and socket circuits |
| RCBO | 6–63 A | 4.5–10 kA | Reset the toggle | Overcurrent plus residual current protection in one device |
| NH blade fuse | 2–1,250 A | 100–120 kA | Replace the link | Main board incomers, transformer outputs, industrial distribution |
| Cylindrical cartridge fuse | 0.5–125 A | 50–120 kA | Replace the cartridge | Control circuits, motors, semiconductors, solar strings |
| D-type (bottle) fuse | 2–63 A | 50 kA | Replace the link | Older building installations, meter boards |
| Glass fuse | 0.05–20 A | 35 A – 1.5 kA | Replace | Electronics, inside appliances, instruments |
2. Miniature circuit breakers
An MCB has two releases: a bimetal (thermal) strip that bends on overload and a coil (magnetic) that acts instantly on short circuit. The curve letter gives the magnetic trip multiple: B (3–5 times), C (5–10 times), D (10–20 times). See MCB B, C, D Tripping Curve Difference.
- Poles: 1P, 1P+N, 2P, 3P, 3P+N and 4P.
- Choose at least 4.5 kA for dwellings and 6–10 kA for commercial and industrial boards.
- Above 125 A use a moulded case circuit breaker.
3. NH blade fuses
Ceramic body, quartz sand filling and blade contacts; fitted in a fuse base or fuse switch-disconnector. They limit and clear very high fault currents within milliseconds.
| NH size | Current range |
|---|---|
| 000 / 00 | 2–160 A |
| 1 | 16–250 A |
| 2 | 35–400 A |
| 3 | 315–630 A |
| 4 | 500–1,250 A |
A link fits only a blade fuse base of the same size and is pulled off-load with an insulated fuse handle.
4. Cylindrical cartridge and glass fuses
Cylindrical cartridge fuses are named by diameter × length and sit in DIN-rail fuse holders:
| Size (mm) | Maximum current |
|---|---|
| 8.5 × 31.5 | 20 A |
| 10 × 38 | 32 A |
| 14 × 51 | 50 A |
| 22 × 58 | 125 A |
Solar strings use 10 × 38 gPV cartridges rated 1,000 V DC; an AC cartridge cannot quench a DC arc. Glass fuses (5 × 20 mm and 6.3 × 32 mm) serve electronics and carry a speed letter: F fast, T time-lag.
5. What do gG and aM mean?
| Class | Range | Use |
|---|---|---|
| gG | Overload + short circuit (full range) | Cable and general line protection |
| aM | Short circuit only (partial range) | Motor circuits; rides through starting current, needs an overload relay |
| gR / aR | Very fast | Semiconductors in drives, UPS units and rectifiers |
| gPV | DC, full range | Photovoltaic strings |
6. Selecting and connecting a fuse
Basic rule: load current ≤ fuse rating ≤ cable current-carrying capacity. The fuse protects the cable, so it is sized for the cable, not the appliance.
| Copper cable size | Typical fuse | Circuit |
|---|---|---|
| 1.5 mm² | 10 A | Lighting |
| 2.5 mm² | 16 A | Sockets |
| 4 mm² | 20–25 A | Oven, air conditioner, water heater |
| 6 mm² | 32 A | Hob, sub-main |
- Breaking capacity must exceed the prospective short-circuit current at the point of installation.
- On an MCB the supply usually enters the top terminal and the load leaves the bottom; the line conductor passes through the breaker, the neutral goes to the neutral bar.
- Never replace a blown fuse with a higher rating or bridge it with wire; find the cause first.
- For amp sizing see the Miniature Circuit Breaker Sizing and Selection Guide.
Frequently asked questions
What does a fuse do?
It opens the circuit when current exceeds a set value, preventing cables from overheating and a short circuit from turning into a fire.
What is the difference between an MCB and an NH blade fuse?
An MCB is reset by its toggle after tripping and is made up to 125 A. An NH fuse link melts and must be replaced, but covers currents up to 1,250 A and faults up to 120 kA.
Why does a fuse blow or a breaker trip?
Three causes: overload (too many loads on one circuit), short circuit (line to neutral or line to earth), and heating from ageing or a loose terminal. If the RCD trips, the cause is earth leakage.
Can an aM fuse replace a gG fuse?
No. An aM fuse gives no overload protection; it clears short circuits only and is used with an overload relay in motor circuits. Cable protection needs gG.
How many watts can a 16 A breaker carry?
On a 230 V single-phase circuit, 16 × 230 ≈ 3,680 W. Continuous loads should stay near 80% of that (2,900 W).