Overcurrent protection: breakers, short circuits and cable section
How AREI treats overcurrent, short circuits, overload and the relationship between protection and wiring.
Uitleg
This page summarizes AREI Book 1 rules about protection against overcurrent. The main references are chapter 4.4, section 5.2.4, the diagram requirements in 3.1.2.2 and the commissioning checks in 6.4.5.
Looking for the meaning of voltage, current, power, breakers or cable section first? Start with Electricity basics, Protection devices or Wiring.
In short
- Overcurrent is any current greater than the conductor's admissible current.
- A breaker or fuse must protect against overload and short circuit.
- Protection must match cable section, installation method, conductor arrangement and environment.
- Short-circuit protection needs sufficient breaking capacity.
- The diagram must show the type and characteristics of overcurrent protection and wiring.
Overload or short circuit?
Overload
The circuit carries more current for too long than the wiring can safely handle.
- Typically caused by too many or too-heavy consumers.
- The cable heats up.
- Protection must trip before damage occurs.
Short circuit
A fault creates a very high current.
- Protection must disconnect quickly enough.
- Breaking capacity must match the expected short-circuit current.
- Fault energy must not damage the conductors.
The core rule: protection and cable belong together
AREI does not start with “which breaker is convenient?” It starts with the electrical conductor that must be protected. Its admissible current depends on, among other things:
- conductor section;
- insulation and cable type;
- installation method;
- ambient temperature;
- grouping with other conductors.
The overcurrent device must protect the conductor. In practice, its rated current and tripping characteristic must not exceed the safe limit of the wiring.
| Information | Why it matters |
|---|---|
| Cable section | Helps determine admissible current and voltage drop |
| Cable type | Determines thermal and mechanical properties |
| Installation method | A tube, wall, air or bundle changes the result |
| Breaker rating | Must match the conductor and load |
| Breaking capacity | Must match the expected short-circuit current |
The breaker protects the cable
A larger breaker is not automatically extra margin. If its rating is higher than the safe current of the conductor, the cable can heat up before the protection trips.
What belongs on the one-line diagram?
For household installations, AREI requires the one-line diagram to show at least the characteristics of the electrical wiring: type, section and conductor count. The installation method, RCD protection and overcurrent protection must also be visible.
In Condui
Enter cable section and breaker rating together. Validation is only useful when both values match what is actually installed.
Typical diagram errors
| Error | Why it is a problem |
|---|---|
| Breaker rating is missing | The overcurrent protection cannot be assessed |
| Cable section is missing | It is impossible to determine whether the breaker protects the conductor |
| Different sections in one circuit are unclear | The smallest section often sets the safe limit |
| A sub-board has no clear supply | Short-circuit and overcurrent logic becomes unreadable |
| PV, battery or EV is added without protection data | New sources or heavy consumers change the design |
Existing installations
Part 8 contains specific provisions for existing or old installations. Some older protective devices or wiring may remain under conditions, but that does not mean new modifications automatically receive the same exception.
When redrawing an existing installation, clearly record what is existing, what was changed and which parts are new.