MCB, MCCB, RCD & RCBO Explained
A clear guide to the four most common low-voltage protective devices — what each one protects against, the difference between overcurrent and earth-leakage protection, and how they combine to keep a UAE installation safe.
Every low-voltage installation needs protective devices that disconnect a circuit automatically when something goes wrong. The four most common are the MCB, MCCB, RCD and RCBO. They are often confused, but each guards against a specific danger, and choosing the right combination is essential for both safety and compliance.
Broadly, there are two distinct threats: too much current flowing in a circuit (overload or short circuit), and current leaking to earth through a fault or through a person. Overcurrent devices protect the cable and equipment from overheating and fault energy; residual-current devices protect people from electric shock. Understanding which device does which is the key to reading any distribution board.
How it works
An MCB (Miniature Circuit Breaker) protects against overcurrent. It contains two mechanisms: a thermal (bimetallic) element that trips on a sustained overload, and a magnetic element that trips almost instantly on a high short-circuit current. MCBs are used on final circuits — lighting, sockets and small power — typically up to around 125 A, with fixed trip settings and a defined trip curve (Type B, C or D).
An MCCB (Moulded Case Circuit Breaker) works on the same overcurrent principles but is built for higher ratings and fault levels, commonly from around 100 A up to many hundreds or thousands of amperes. MCCBs are used for main incomers, sub-mains and large feeders, and larger units often have adjustable thermal and magnetic trip settings so protection can be tuned and coordinated across the network.
An RCD (Residual Current Device) does not protect against overload at all. Instead it continuously compares the current flowing out in the line conductor with the current returning in the neutral. In a healthy circuit these are equal. If some current is leaking to earth — for example through a damaged cable or a person — the two no longer balance, and the RCD trips quickly to limit the risk of fatal electric shock. RCDs are rated by their tripping sensitivity, with 30 mA used for personal (additional) protection.
An RCBO (Residual Current Breaker with Overcurrent protection) combines both functions in one device: it provides the overcurrent protection of an MCB and the earth-leakage protection of an RCD. Because each RCBO protects a single circuit, a fault on one circuit does not trip the others — unlike a single RCD covering a whole group of circuits, where one fault can disconnect everything downstream.
In a typical board these devices work together as a coordinated system. A main MCCB protects the incoming supply, MCBs or RCBOs protect each final circuit against overcurrent, and RCD or RCBO protection provides earth-leakage protection where required, such as on socket circuits and in wet areas. Correct selection of ratings, breaking capacity and discrimination ensures that only the device nearest a fault operates, keeping the rest of the installation live.
Main types
In the UAE
- In Abu Dhabi, protective devices and their coordination must follow the Department of Energy / distribution company wiring regulations, which are based on the principles of IEC 60364, covering overcurrent and earth-fault protection.
- Additional protection by 30 mA residual-current devices is the recognised means of reducing shock risk on socket circuits and in wet locations, in line with international wiring practice adopted across the UAE.
- Devices and materials must comply with recognised BS/IEC standards and, where applicable, carry conformity marking referencing ESMA / Emirates standards, and discrimination between upstream and downstream devices is required for a reliable installation.
How GPR applies this
GPR selects and coordinates MCBs, MCCBs, RCDs and RCBOs for residential, commercial and industrial distribution boards across Abu Dhabi, matching each device to its circuit, fault level and protection duty. We provide 30 mA earth-leakage protection where required for shock safety, confirm breaking capacity in kA, and arrange discrimination through the distribution hierarchy so installations pass Department of Energy inspection and stay both safe and selective.
Frequently asked questions
What is the difference between an MCB and an RCD?
An MCB protects the circuit against too much current (overload and short circuit). An RCD protects people against electric shock by detecting current leaking to earth. They guard against different dangers.
What does an RCBO do that an MCB does not?
An RCBO combines overcurrent protection with earth-leakage (residual-current) protection in one device, and it does so per circuit so a single fault does not trip neighbouring circuits.
When do I use an MCCB instead of an MCB?
Use an MCCB for higher current ratings and fault levels — main incomers, sub-mains and large feeders — especially where adjustable trip settings or higher breaking capacity are needed.
What does a 30 mA RCD protect against?
It provides additional protection against fatal electric shock, tripping quickly if around 30 mA leaks to earth, and is used on socket-outlets and circuits in wet areas.
Why does one RCD trip several circuits at once?
A single RCD covering a group of circuits will disconnect all of them on one earth fault. Using individual RCBOs per circuit avoids this by isolating only the faulty circuit.