ProcirelMCB, MCCB, RCCB and ELCB: Which Breaker Does What
Breakers protect different faults. MCBs and MCCBs interrupt overloads and short circuits, RCCBs detect current leaking to earth and save lives. A compliant installation layers them each device with its own job.
> At a glance: 7 minute guide · part 7 of 10 in the electrical engineering complete guide track · includes a worked example and a quick-reference table.
## Overcurrent family
Here is the working theory in one pass. MCBs (to ~100 A) protect final circuits; curves B, C, D describe magnetic trip tolerance to inrush. MCCBs cover heavier feeders to ~1600 A with adjustable trips. Both protect equipment and wiring from heat they do not protect people from earth faults.
| D | e | v | i | c | e | | | | | | | | | | |
| — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — |
| P | r | o | t | e | c | t | s | | a | g | a | i | n | s | t |
| T | y | p | i | c | a | l | | r | a | t | i | n | g | | |
| T | r | i | p | | s | p | e | e | d | | | | | | |
| MCB | Overload + short circuit | 2–63 A | Curve B/C/D instant + thermal | | | | | | | | | | | | |
| MCCB | Feeder overcurrent | 16–1600 A | Adjustable, fast magnetic | | | | | | | | | | | | |
| RCCB/RCD | Earth leakage (people) | 30/100/300 mA | ~30 ms at rated fault | | | | | | | | | | | | |
| RCBO | Both MCB + RCD jobs | varies | Combined | | | | | | | | | | | | |
| ELCB (old) | Earth voltage | | Obsolete replace | | | | | | | | | | | | |
## Earth-fault family
RCCBs compare live and neutral currents (residual current). A 30 mA unit trips when the difference leakage through a person or insulation exceeds threshold. Older voltage-operated ELCBs are obsolete. RCDs trip on residual current; RCBOs combine RCD + MCB in one slot.
## Coordination and selectivity
Upstream devices should let downstream ones clear local faults. Stagger ratings and curves so the breaker nearest the fault operates first. Regular RCD test-button checks ( quarterly) verify the mechanism still trips at speed.
## How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1.
2.
3.
4.
### Worked example
A workshop circuit tripping every motor start had a B-curve MCB seeing inrush as a fault. Swapping to C-curve kept protection while tolerating the starting surge. Run the numbers yourself with the Electrical Power Calculator and the result should agree to within rounding.
> Practical note from the bench. Our installation diagrams always label each device’s job overcurrent, earth fault, or both so the layering reads at a glance.
## Common mistakes to avoid
– Using a 100 mA RCD where 30 mA personnel protection is required
– MCB curve too sensitive for motor inrush nuisance trips
– No selectivity: one fault blacking the whole board
## Key takeaways
– Overcurrent family the foundation of this guide; revisit it if any measurement here surprises you.
– Earth-fault family the foundation of this guide; revisit it if any measurement here surprises you.
– Coordination and selectivity the foundation of this guide; revisit it if any measurement here surprises you.
## Who this guide is for
Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the electrical engineering complete guide path. Intermediate readers use it as a reference the table. The worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.
## What you need before starting
Nothing exotic: the parts or tools named in the guide, a multimeter. The Electrical Power Calculator open in a tab. Identify the fault each layer must catch before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.
### Quick reference card
| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Overcurrent family |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Common mistakes to avoid |
## How this fits the electrical engineering complete guide track
This guide is one stop in the structured learning path. Start from the [electrical engineering complete guide](/tutorial/electrical-engineering-complete-guide) pillar page for the full map, or continue with [earthing systems](/tutorial/earthing-systems-explained) and [motor starter protection](/tutorial/dol-starter-circuit). For the arithmetic, open the [Electrical Power Calculator](/tools/electrical-power).
## Frequently asked questions
Does an RCD replace an MCB?
No an RCD ignores overload; you still need overcurrent protection (or an RCBO combining both).
Why test RCDs if they just sit there?
The trip mechanism can seize over years; the test button exercises it in milliseconds.
Is there a calculator for this?
Yes the [Electrical Power Calculator](/tools/electrical-power) tool runs the formulas from this guide instantly, client-side, with no signup.
## Your next step in this track
– The complete electrical engineering guide: [Electrical Engineering complete guide](/tutorial/electrical-engineering-complete-guide)
– Read next: [pid controller explained: proportional-integral-derivative in practice](/tutorial/pid-controller-explained)
– Also in this track: [single-phase vs three-phase power: the real differences](/tutorial/single-phase-vs-three-phase)
– Continue with: [power factor explained: why real power is not apparent power](/tutorial/power-factor-explained)
– Calculate as you go: [power calculator](/tools/electrical-power) · [transformer turns ratio tool](/tools/transformer-turns-ratio) · [cable sizing helper](/tools/wire-gauge-awg)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.
## Practical working notes
Component substitution is a legitimate experiment as long as it is deliberate. Swap one part, predict the effect, measure, and record. That single habit converts a parts bin into a teaching lab and makes every future guide in this track faster to absorb.
The fastest way to internalise this topic is to change one variable deliberately and predict the result before measuring. Wrong predictions are the curriculum, they show exactly which mental model needs revisiting, and the bench grades honestly.
## Experience notes
Inrush from motors or supplies. Curve C breakers tolerate it, and sizing follows the datasheet.
Isolated low-voltage work, yes. Mains and panel work needs a qualified electrician, every time.
## One last piece of advice
A note on safety margins, the theme under every page in this track: ratings are limits, not suggestions, and the margin you skip is the one that fails first in the field.
Working through Overcurrent familyand Earth-fault family with that habit in mind takes minutes, and it is the difference between reading about this topic and owning it.