What each device does
| Device | Protects against | Does not protect against | Where it goes |
|---|---|---|---|
| MCB (miniature circuit breaker) | Overload and short circuit | Earth leakage and shock | One per circuit |
| RCCB (residual current circuit breaker) | Earth leakage, shock, leakage fires | Overload and short circuit | Ahead of a group of MCBs |
| RCBO | Overload, short circuit and earth leakage, in one device | Nothing missing for a single circuit | One per circuit, in place of MCB |
| ELCB | Old name. Voltage-operated ELCBs sensed voltage on the earth wire; today people use "ELCB" to mean an RCCB | Voltage type misses faults when earth is poor | Replace old voltage-type units |
An RCCB compares the current going out on phase with the current coming back on neutral. If some of it is leaking to earth, through a damp wall, a faulty geyser element or a person, the difference trips it within a fraction of a second. An MCB cannot see that: 30 mA through a human body is far too small to trip a 16 A breaker, but it is enough to kill.
What the regulations require
Regulation 44 of the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 says that the use of electricity in an electrical installation shall be controlled by a residual current device, and that in domestic installations the residual operating current shall not exceed 30 milliampere. These are the binding safety regulations under the Electricity Act, and the regulation text sets no minimum load below which a house is exempt. The licensed contractor's test report and the Inspectorate's self-certification forms both include a question on whether an RCD of appropriate capacity is provided.
The CEA text says the RCD rating and tripping time should be as per the relevant standards. For RCCBs that is IS 12640 (Part 1); for MCBs, IS/IEC 60898-1. Buy ISI-marked devices only.
Choosing MCB ratings
The MCB protects the wire, so its rating follows the wire size and the load. Common practice in homes with copper wiring:
| Circuit | Typical wire | Typical MCB |
|---|---|---|
| Lights and fans | 1.5 sq mm | 6 A or 10 A |
| 6 A / 16 A socket circuit | 2.5 sq mm | 16 A |
| Split AC (each) | 2.5–4 sq mm | 16 A or 20 A, per AC rating |
| Geyser, oven, induction | 4 sq mm | 20 A or 25 A |
| Main incomer, single-phase house | 6–10 sq mm | 32–63 A isolator or MCB, per sanctioned load |
Curves: a B curve MCB trips instantly at 3 to 5 times its rating and suits lighting and sockets. A C curve trips at 5 to 10 times and suits motors, pumps, ACs and loads with inrush current. Never "solve" tripping by fitting a bigger MCB on the same wire; that just moves the failure point into the wall.
Breaking capacity is the other number on an MCB, printed in a small box as 6000 or 10000 (amperes). It is the fault current the MCB can safely interrupt. For houses, 6 kA is common; close to a large transformer or in commercial buildings the designer may ask for 10 kA. Poles matter too: SP (single-pole) for single-phase circuits, DP for an incomer that must break both phase and neutral, TP or TPN and four-pole for three-phase. A cheap unbranded MCB may carry the right numbers on the front and still fail to trip; buy from a dealer who can show the ISI licence for that model.
RCCB sensitivity and type
- 30 mA is for protection of people. Use it for all socket, bathroom, kitchen, outdoor and pump circuits. In a domestic installation the regulation caps the RCD at 30 mA.
- 100 mA and 300 mA devices protect against fire from leakage, not against shock. They are used upstream in larger buildings and commercial installations, with 30 mA devices below them.
- Current rating of an RCCB (25, 40, 63 A) is how much load it can carry, not when it trips. It must be at least the rating of the MCB or isolator ahead of it.
- Type AC detects only smooth AC leakage. Type A also detects pulsating DC leakage, which inverter ACs, washing machines, induction cooktops and EV chargers can produce. For new homes, Type A is the better choice; EV chargers have their own requirement, see our EV charger guide.
A single 30 mA RCCB for the whole house means one leaking appliance blacks out everything, and normal small leakages from many appliances add up and cause trips. Splitting circuits across two RCCBs, or using RCBOs on key circuits, gives fewer nuisance trips and easier fault-finding.
A sensible DB layout for a 2–3 bedroom house
A layout we commonly use for a single-phase house, to be adjusted for your load and the designer's calculation:
| Position | Device | Feeds |
|---|---|---|
| Incomer | Double-pole isolator or DP MCB, rated to the sanctioned load | Whole house, from the meter |
| Surge protection | Type 2 SPD with its own back-up MCB or fuse | Protects all circuits |
| Group 1 | 30 mA RCCB (Type A), 40 A or 63 A | Socket circuits, kitchen, bathrooms, outdoor, pump |
| Group 2 | Second 30 mA RCCB | Lighting and fans, AC circuits |
| Outgoing | SP MCBs, B curve for lights and sockets, C curve for AC and pump | One per circuit, labelled |
| Spare | Two to four empty ways | Future solar, EV, extra AC |
In a three-phase house, the incomer is a four-pole isolator, and you can use a four-pole RCCB or single-phase RCCBs on each phase group. Keep each circuit's neutral on the same RCCB as its phase. Mixed neutrals are the number one cause of "the RCCB trips when I switch on the light in another room".
Avoid the shortcuts we often find in older houses: an RCCB fitted but the socket circuits wired around it; the earth wire looped to the neutral to "stop tripping"; and a 32 A MCB on 1.5 sq mm wire because a heater kept tripping the 10 A one. Each of these turns off the protection while leaving the device in the board, which is worse than having no device at all, because the owner believes he is protected.
Upgrading an old fuse board to MCBs and RCCB
Many older houses still have rewireable fuses, or MCBs with no RCCB. Upgrading is worth doing, but an RCCB fitted to old wiring often trips straight away, and that is the RCCB doing its job. Plan the upgrade in this order:
- Test the wiring first. Ask for an insulation resistance test of each circuit. Very low readings mean the insulation has aged, and a rewire, not a new board, is the real fix. Our house wiring cost guide lists the signs that rewiring is due.
- Check the earth. Old houses often have no earth wire to the sockets, or an earth pit that has dried out. Without a good earth, a leaking appliance body stays live until someone touches it, and the RCCB then trips through that person. Get a new earth pit if the test value is poor.
- Separate the neutrals. Old wiring often shares one neutral between several circuits. Each circuit's neutral must be traced and taken to the same RCCB as its phase, or the RCCB will trip for no visible reason.
- Re-group the circuits. Put lights, sockets, kitchen, geyser and each AC on separate MCBs, rated for the wire actually in the wall rather than the load you would like to run.
- Add surge protection and spare ways. A new board is the cheapest time to fit an SPD and leave room for a future AC, solar or EV circuit. Our lightning and surge protection guide explains what an SPD does and does not cover.
The board change itself is a day's work, but tracing neutrals in old wiring can take longer. Use a licensed contractor who gives you a test report at the end; see electrical contractor licence in Karnataka. If you are moving to three-phase at the same time, read single-phase or three-phase first, so the new board is planned once.
Why it trips: finding the cause
- 1Switch off all MCBs below the RCCB and reset it
If it trips with everything off, the fault is in the RCCB or the wiring before the MCBs.
- 2Switch MCBs on one at a time
The circuit that trips the RCCB is the faulty one.
- ?Unplug every appliance on that circuitYes Plug back one by one to find the applianceNo Fault is in the wiring: call a licensed electrician
- 4Common culprits
Geyser element, water in a socket or outdoor light, pump cable, old fridge, damp junction box, neutral shared between circuits.
- ✓Repair the fault. Never bypass the RCCB
Other causes we see in the field: neutrals of two circuits mixed in the DB, so current goes out through one RCCB and returns through another; a surge during lightning; and moisture in conduits after the monsoon. An insulation resistance test with a megger finds wiring faults that a visual check misses. See the monsoon safety checklist.
Monthly test and replacement
The test button only checks the mechanism. It does not check your earthing, which needs a separate earth resistance test; see the earthing and testing checklist.
Rules, fees and amounts change from time to time. This article explains how things work in practice in Karnataka. Before you act, confirm the current rule with the jurisdictional office of the Department of Electrical Inspectorate, your ESCOM subdivision (BESCOM, MESCOM, CESC Mysuru, HESCOM or GESCOM), the Seva Sindhu portal or the helpline 1912.
Questions
What is the difference between MCB and RCCB?
An MCB trips on overload or short circuit and protects the wiring. An RCCB trips on earth leakage and protects people from shock. A house needs both, or RCBOs that combine them.
Is RCCB compulsory in Karnataka homes?
Regulation 44 of the CEA 2023 safety regulations requires electricity use to be controlled by a residual current device, not exceeding 30 mA in domestic installations. These regulations apply across India, including Karnataka.
Should I use 30 mA or 100 mA RCCB at home?
30 mA. It is the sensitivity that protects people from shock, and the regulation caps domestic RCDs at 30 mA. 100 mA and 300 mA are for fire protection upstream in larger installations.
What is the difference between ELCB and RCCB?
Old voltage-operated ELCBs sensed voltage on the earth wire and fail when earthing is poor. An RCCB senses current imbalance between phase and neutral. Today 'ELCB' is often used loosely to mean an RCCB.
Type AC or Type A RCCB?
Type A also detects pulsating DC leakage from inverter appliances, washing machines and EV chargers. It is the better choice for new homes.
Why does my RCCB trip when it rains?
Water is getting into an outdoor light, socket, junction box or pump cable and leaking current to earth. Switch circuits on one at a time to find it, and get the wiring dried and tested.
How often should I press the RCCB test button?
Once a month is a good habit. If it does not trip immediately, replace the RCCB.
Sources
Rules and figures in this article were checked against these sources on 30 Sep 2026. Schemes and tariffs change, so confirm with the official office before you act.
- CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 - Gazette
- IS 12640 (Part 1) RCCB product manual (BIS)
- IS/IEC 60898-1 circuit-breakers for household installations
- Which RCD type? (IET Wiring Matters)
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