---
title: "MCB, MCCB, ACB and RCCB: what each protects, and where"
description: "The four devices compared on rating, breaking capacity and adjustability — plus trip curves, Icu versus Ics, and choosing an RCD type for VFD circuits."
date: "2026-01-29"
author: "Divakar B"
source: "https://energycalchq.com/blog/mcb-mccb-acb-rccb-difference"
license: "© 2026 Divakar B. Quote with attribution to https://energycalchq.com/blog/mcb-mccb-acb-rccb-difference"
---

Four devices turn up on every distribution drawing, and the differences between
them are usually explained as a matter of size — MCB small, MCCB medium, ACB
large. That is true and it is not the useful distinction. They differ in what
they can *see*, how far they can be *adjusted*, and how much fault current they
can *break*, and those three properties decide where each one belongs.

The RCCB is not on the same list at all. It protects against something the other
three cannot detect.

## The short version

| | MCB | MCCB | ACB | RCCB |
|---|---|---|---|---|
| Typical range | 0.5–125 A | 16–1600 A | 630–6300 A | 25–125 A |
| Breaking capacity | 6–10 kA | 25–70 kA | 50–150 kA | none of its own |
| Detects overload | Yes, fixed | Yes, adjustable | Yes, adjustable | No |
| Detects short circuit | Yes, fixed | Yes, adjustable | Yes, adjustable | No |
| Detects earth leakage | No | Optional add-on | Yes, in the trip unit | **Yes, its only job** |
| Adjustable settings | None | Thermal, often magnetic | Everything, plus delays | Sensitivity is fixed |
| Maintainable | Replace | Replace | Serviceable, drawout | Replace |
| Where it belongs | Final circuits | Submains | Incomers, main bus | Wherever people are |

![Distribution hierarchy with an ACB incomer, MCCB submains, and MCBs with an RCCB on final circuits](/blog/protection-device-hierarchy.svg "Breaking capacity falls going down the hierarchy, because the impedance of the cable between levels reduces the fault current available.")

## The one that is different

An MCB, MCCB and ACB all measure current flowing **through** them, and they trip
when there is too much of it. That protects the cable and the equipment. It does
essentially nothing for a person touching a live part.

Current flowing through someone to earth is a small current — 30 mA can be
lethal, and a 32 A MCB will not notice 30 mA any more than it notices a light
bulb switching on. An RCCB compares the current going out with the current coming
back. If they differ, current is leaving the circuit somewhere it should not be,
and it trips.

| Sensitivity | Purpose |
|---|---|
| 10 mA | Very high risk — medical, some laboratory |
| **30 mA** | **Protection of people.** The figure for socket circuits and anywhere with human contact |
| 100 mA | Fire protection, some equipment protection |
| 300 mA | Fire protection on submains, or upstream of 30 mA devices for discrimination |

An RCCB has no overcurrent protection of its own. It must always sit behind an
MCB or MCCB, sized so the RCCB's own current rating is never exceeded. An
**RCBO** combines the two functions in one module, which costs more per way and
means a leakage fault trips only the affected circuit — usually the better
arrangement on anything the user cares about.

## RCD types: the mistake that VFDs cause

This one is missed constantly, and it turns a safety device into an ornament.

| Type | Detects | Use for |
|---|---|---|
| AC | Sinusoidal AC leakage only | Simple resistive and inductive loads |
| A | AC plus pulsating DC | Anything with electronics — the sensible default today |
| F | Type A plus mixed frequencies | Single-phase VFD loads, washing machines |
| B | Type A/F plus smooth DC | Three-phase VFDs, EV chargers, solar inverters |

The failure mode is specific and dangerous. A fault downstream of a rectifier
can produce leakage with a **DC component**, and DC leakage can saturate the
core of a Type AC device so that it no longer detects the AC leakage it was
fitted for. The device does not fail obviously — it stops working while looking
perfectly healthy.

If the circuit has a VFD, an EV charger or a solar inverter on it, the RCD must
be Type B (or Type F for single-phase drives). Type AC on such a circuit is a
false sense of protection.

## Trip curves

The trip curve says how fast the magnetic element operates as a multiple of the
rated current:

| Curve | Trips magnetically at | For |
|---|---|---|
| B | 3–5 × In | Resistive loads, long cable runs, domestic |
| C | 5–10 × In | The general purpose choice — mixed loads, small motors |
| D | 10–20 × In | Transformers, motors, anything with high inrush |

A B-curve device on a motor circuit trips on every start. A D-curve device on a
lighting circuit at the end of a long run may not trip on a genuine fault at all,
because the fault current at that distance never reaches ten times the rating.
That second failure is the more dangerous one, and it is why long final circuits
sometimes need a B curve or an earth fault loop impedance check rather than an
assumption.

For motors specifically, the right answer is usually not an MCB at all but an
**MPCB** — a motor protection circuit breaker with an adjustable thermal element
set to the motor's full load current and a fixed magnetic element around 13 × In
that lets the starting inrush pass. The [motor starter
calculator](/tools/motor-starter) picks one from the motor rating.

## Icu, Ics and Icn — three breaking capacities

Datasheets quote more than one, and the difference matters:

- **Icu — ultimate breaking capacity.** The device will interrupt this fault
  once, safely. It may not be usable afterwards.
- **Ics — service breaking capacity.** It will interrupt this fault and remain
  fit for service. Usually quoted as a percentage of Icu: 50 %, 75 % or 100 %.
- **Icn — rated breaking capacity** for MCBs to IS/IEC 60898, the domestic and
  similar standard. Typically 6 kA or 10 kA.

Specifying on Icu alone means accepting that after a serious fault the device is
scrap and possibly no longer protecting anything. On main incomers and anywhere
downtime is expensive, specify **Ics = 100 % of Icu**.

Get the fault level from the transformer with the [transformer sizing
calculator](/tools/transformer-sizing), and remember it falls as you move down
the system — which is why 10 kA MCBs are acceptable at a final distribution
board fed by a long submain, and would not be acceptable on the main bus.

## Discrimination, and the cheat that goes with it

Discrimination means a fault takes out only the device closest to it, leaving the
rest of the installation running. It is built with the adjustable settings on
MCCBs and ACBs: current thresholds that step up going upstream, and short time
delays on the upstream devices so the downstream one gets the first chance to
clear.

Those delays are not free. Every one of them extends the time the fault current
flows, and the [cable short-circuit
withstand](/blog/cable-short-circuit-withstand) calculation scales with √t. A
0.6 s delay upstream can push a cable up two sizes.

**Cascading** (or back-up protection) is the related trick: an upstream
current-limiting device is allowed to help a downstream one, so the downstream
device can be specified below the prospective fault level at its position. It is
cheaper, and it is only valid for **tested combinations** published by the
manufacturer — you cannot derive it yourself, and you cannot mix brands.

## Choosing, in order

The [MCB and MCCB sizing calculator](/tools/breaker-sizing) runs the sequence
below for a given circuit — rating, curve, breaking capacity and the
`Ib ≤ In ≤ Iz` check against your derated cable, with the panel-ambient
correction applied.

1. Fault level at this point in the system — that sets the minimum breaking
   capacity, and Ics rather than Icu where continuity matters.
2. Load current and the cable it protects — the device rating must sit between
   the design current and the derated cable capacity, `Ib ≤ In ≤ Iz`, which is
   step four of [sizing a cable correctly](/blog/how-to-size-a-cable-correctly).
3. Inrush behaviour of the load — which sets the curve, or sends you to an MPCB.
4. Whether anyone can touch it — which decides the RCD, and its sensitivity.
5. What electronics are downstream — which decides the RCD **type**.
6. Whether a fault here should take out anything else — which decides how much
   adjustability you need to pay for.