RCBO, RCD, MCB Explained: Complete Guide to Electrical Protection Devices
Electrical Protection Devices Explained
No single device protects against everything. A well-designed installation layers several, each doing a different job.
Quick answer
Think of the devices as covering six different faults: overload and short circuit (MCB), earth leakage (RCD), both together (RCBO), arcing (AFDD), surges (SPD) and loss of the supply’s PEN conductor (PEN fault detection). RCBOs, SPDs and PEN units are often combined in one enclosure.
What protects against what
| Device | Protects against | Read more |
|---|---|---|
| MCB | Overload and short circuit | RCBO vs MCB vs RCD |
| RCD | Earth leakage, shock and fire risk | RCD trip sensitivity |
| RCBO | Overcurrent and earth leakage on one circuit | RCBO types explained |
| AFDD | Arcing faults that MCBs and RCDs cannot see | Arc fault detection |
| SPD | Transient overvoltages from lightning and switching | Do I need surge protection? |
| PEN fault detection | Loss of the PEN conductor on a PME supply | What is PME? |
Choosing the specification
- RCD type (A, B, F) follows the equipment: see What is a Type B RCD?
- Pole count follows the supply: see 4-pole vs 2-pole RCDs
- Upstream devices may need a time delay: see time delay RCDs
- Enclosure rating follows the location: see IP40 vs IP65
Pre-assembled units
Many installs combine several of these in one pre-wired unit. See fully assembled consumer units and the WCED range comparison.
Frequently asked questions
Can one device replace the others?
No. Each protects against a different fault. Combined units package several functions in one enclosure, but the functions remain separate.
Is an RCD enough on its own?
No. An RCD has no overcurrent protection, so it is paired with an MCB or used as an RCBO.
Where do I start?
Start with the guide for the job in front of you, or use the Knowledge Base home page.
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