Installation technology · Topic 3 of 5
Protective devices
Protective devices switch a circuit off automatically when something goes wrong, before cables overheat or someone gets a shock.
- I – Current
What can go wrong
- Overload: too much current in a healthy circuit, for example too many heaters on one socket circuit.
- Short circuit: a fault between line and neutral, causing a very large current.
- Earth fault: a fault between line and earth, such as a line conductor touching a metal case.
The devices
| Device | Standard | Protects against |
|---|---|---|
| Rewireable fuse | BS 3036 | Overload and short circuit (older installations) |
| Cartridge fuse for homes | BS 88-3 (was BS 1361) | Overload and short circuit |
| Industrial cartridge fuse | BS 88-2 | Overload and short circuit |
| Plug fuse | BS 1362 | The flexible cord of an appliance |
| Circuit-breaker (MCB) | BS EN 60898 | Overload and short circuit |
| RCD (RCCB) | BS EN 61008 | Earth leakage only |
| RCBO | BS EN 61009 | Overload, short circuit and earth leakage |
MCB types
The type letter tells you how much current trips the breaker instantly, as a multiple of its rating (In):
| Type | Instant trip | Typical use |
|---|---|---|
| B | 3 to 5 × In | Homes: sockets, lighting, heaters |
| C | 5 to 10 × In | Commercial: small motors, lots of lighting |
| D | 10 to 20 × In | Industrial: large motors, transformers |
RCDs
An RCD compares the current going out on the line with the current coming back on the neutral. If some leaks away to earth, for example through a person, and the difference reaches its rating, it trips. A 30 mA RCD gives additional protection against electric shock. BS 7671 requires it for socket-outlets rated up to 32 A and for mobile equipment up to 32 A used outdoors.
An RCD on its own (an RCCB) gives no protection against overload or short circuit, so it is always used with fuses or MCBs. An RCBO combines both in one device.
Exam tip: a Type B 32 A MCB trips instantly somewhere between 96 A (3 × 32) and 160 A (5 × 32). Maximum Zs calculations use the top of the range, 5 × In.
Try it: Maximum Zs calculator
Mini test
Pick an answer to see if you're right and why.