Circuit breakers: MCB vs. RCD, and B, C and D trip curves

Open the consumer unit at home and you'll find a row of breakers. Some are "ordinary" and some have a test button (T). Understanding the difference isn't a luxury: it's understanding what protects your wiring from fire, and what protects your family from electric shock.

1) The miniature circuit breaker (MCB)

Its job is to protect the cable and circuit from:

• Overload: current above the rating for a period, tripped by the breaker's thermal element.
• Short circuit: a sudden huge current, tripped instantly by the magnetic element.

The breaker is marked with a code like C16: the letter is the trip curve, and the number the rated current in amps.

Trip curves B, C and D

IEC 60898 sets the instantaneous trip range for each curve as multiples of the rated current In1:

The curve and rating are chosen at design to match cable size, length and load. Fitting a bigger breaker because it "trips a lot" is a serious mistake; it can overload the cable and cause a fire.

2) The residual current device (RCD)

It compares the current going out on the phase with the current returning on the neutral. If they differ, some current has "leaked" to earth — perhaps through a person — and it trips immediately. So it is protection for people first and foremost.

The 30 mA requirement

In Saudi Arabia the binding reference is the Saudi Electrical Code SBC 401, based on the IEC 60364 series with amendments suited to local conditions2. What follows is from the international standard, to explain the requirement.

IEC 60364-4-41 requires additional protection by an RCD with a rated residual current not exceeding 30 mA for socket-outlet circuits rated up to 32 A likely to be used by ordinary persons for general use, for circuits of mobile equipment used outdoors up to 32 A, and for final lighting circuits in dwellings3. (The 2005 edition of the standard set 20 A for socket-outlets and did not cover lighting circuits4.)

The guide explains the reason for this additional protection: other protective measures are not infallible, because of poor maintenance, carelessness, insulation wear or contact with water, and a high-sensitivity RCD trips fast enough to prevent injury or death from shock to a healthy person3.

Schneider Electric's electrical installation guide recommends limiting the number of sockets protected by one RCD (e.g. 10 sockets per RCD)3. See why the RCD trips.

Test your RCD

The T button on the RCD simulates a leak to test the trip mechanism. Press it periodically per the manufacturer's instructions: if it doesn't trip, the RCD needs checking by a technician.

3) RCBO: two in one

A device combining MCB protection (overload and short circuit) with RCD protection (leakage) for a single circuit. Its advantage is that a leak on one circuit doesn't cut power to other circuits.

4) Other protection you may hear about

• SPD (surge protection): protects appliances from transient overvoltages. See electrical faults.
• Main switch: isolates the whole board for maintenance.

5) The electrical path at home and the documents to ask for

The binding reference is SBC 4012. This path helps you request documents and tests from a professional; it gives no wiring steps and does not replace an approved electrical design.

6) Tips for your consumer unit

1. Ask the electrician to label every breaker with what it feeds (kitchen, room 1, living room AC…).
2. Make sure there are RCD(s) for socket circuits, especially bathrooms, kitchens and outdoors.
3. Never replace a breaker with a higher rating without checking the cable size.
4. Heat or a burning smell at the board is a danger sign: switch off the power and call a professional immediately.