MCB, RCCB, RCBO and MCCB — what each one actually does
A plain guide to the four breakers in every Indian distribution board, the B/C/D curves, and how to pick the right one for lights, sockets, ACs and motors.
Open any modern distribution board and you’ll find a row of small switches that look alike but do quite different jobs. Picking the wrong one is how you get nuisance tripping — or worse, no trip when it matters.
MCB — protects the wiring
A Miniature Circuit Breaker trips on overload (too much current for too long) and on short circuit. Its job is to stop the cable from overheating. Ratings run from 0.5 A up to 63 A (some ranges go to 125 A).
The B, C and D curves
The letter tells you how much short surge the MCB tolerates before it trips instantly:
| Curve | Trips instantly at | Use for |
|---|---|---|
| B | 3–5 × rated current | Lighting, heaters, long cable runs |
| C | 5–10 × rated current | Sockets, ACs, small motors, general circuits |
| D | 10–20 × rated current | Motors, transformers, welding sets — high inrush |
RCCB — protects people
A Residual Current Circuit Breaker watches the difference between current going out and coming back. If some current leaks to earth — through a faulty appliance, damp wiring, or a person — it trips. It does not protect against overload, so it always works alongside MCBs.
- 30 mA — for protection against electric shock (homes, bathrooms, outdoor sockets).
- 100 mA / 300 mA — for fire protection on larger installations.
RCBO — both in one
An RCBO is an MCB and an RCCB in a single device, for one circuit. A leak on one circuit then trips only that circuit instead of the whole house — handy for geysers, outdoor and kitchen circuits.
MCCB — for bigger loads
A Moulded Case Circuit Breaker does the same job as an MCB for larger currents — from about 16 A up to 1600 A — with a higher breaking capacity (25 kA, 36 kA, 50 kA and more) and, often, adjustable trip settings. You’ll find them as incomers in buildings, commercial panels and industrial feeders.
A simple home layout
- Incomer: a DP or FP isolator or MCB sized for your load.
- RCCB 30 mA after the incomer, covering the socket and wet-area circuits.
- MCBs per circuit: B or C curve for lights and fans, C curve for sockets and ACs.
Size every MCB to the cable it protects, not to the appliance. A 32 A breaker on a 1.5 sq mm wire will let that wire cook before it trips.
Bring us your load list and we’ll help you put the board together — L&T (Lauritz Knudsen), Havells, Legrand, Schneider and Siemens ranges are in stock.
Quick answers
What is the difference between an MCB and an RCCB?
An MCB trips on overload and short circuit — it protects the wiring. An RCCB trips on earth leakage — it protects people from shock. They do different jobs, so most boards have both.
Which MCB curve should I use for a home?
B curve suits lighting and resistive loads. C curve is the common choice for sockets, ACs and general circuits where there is some inrush. D curve is for motors and transformers with high starting current.
What rating of RCCB is used for personal protection?
30 mA RCCBs are used for protection against electric shock. 100 mA and 300 mA units are used for fire protection on larger installations.