RCD vs MCB: How They Differ in Electrical Safety

Every year, many people are injured in electrical accidents at home. Not everyone knows that many of these injuries could be prevented by having a residual current device (RCD) and miniature circuit breaker (MCB) installed in the house’s consumer unit. These devices can save lives by protecting our lives from fatal electric shocks, and can also provide some protection against fire. RCD and MCB are both protection devices but there are significant differences between them. Let’s check them one by one.

Differences Between RCD and MCB

The differences between RCD and MCB are:

1. Definition

A residual current device (RCD) is the generic term for a device that simultaneously performs the functions of detection of the residual current, comparison of this value with the rated residual operating value, and opening the protected circuit when the residual current exceeds this value.

A miniature circuit breaker is a device that protects and controls circuits against overloads and short circuits by isolating them. It can also be used as the main switch.

2. Protection

The major difference between RCD and MCB is the protection type. A residual current device (RCD) cannot protect against overload or short-circuit current. It only protects against residual currents. But MCB has overload and short circuit protection.

3. Working principle

RCD operates by measuring the current balance between two conductors using a differential current transformer. The device opens contacts when it detects any difference in current between the line conductor and the neutral conductor. The supply and return currents must sum to zero, otherwise, there is a leakage of current to somewhere else. RCD is intended to operate within 25-40 milliseconds before the electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.

There are two types of operation in the MCB. One is due to the thermal effect of current and the other is due to the electromagnetic effect of over-current. The thermal operation of the miniature circuit breaker is achieved with a bimetallic strip. But during short circuit conditions, the sudden rising of electric current causes electromechanical displacement of the plunger associated with the tripping coil or solenoid of MCB.

4. Number of poles

Most common RCDs have 2 poles and 4 poles. But MCBs have lots of pole variations such as 1, 2, 3, 4, 1+N, and 3+N poles.

5. Test button availability

Series of product standards require such RCD to be fitted with a test button (marked “T”) on the front panel. The user can check and be certain that the device is working correctly. The test button allows the user to check if the RCD can trip in presence of an earth leakage current. If the device doesn’t trip, it must be examined to determine if the device is out of service. MCB does not have any test button.

6. Location in the circuit

RCDs are located in the downstream circuits but MCBs are located in the upstream circuits. The current rating of the RCD is the maximum permanent current for which the product is designed. It is protected against overloads by the circuit breaker located upstream on its power supply line. As a consequence: The rating of the residual current circuit breaker must be equal to or greater than the rating of the circuit breaker located upstream.

7. Applications

MCBs have wide application areas such as household applications, motors, heaters, cable protection, semiconductors, sensors, etc. RCDs are generally used in household applications and some special applications such as drives.

8. Price and market size

The average price of an MCB is cheaper than RCD. But the market size of MCBs is wider than RCDs.

9. Types

The following devices are all RCDs, equipped with additional functions and features, offering cost-effective solutions in different applications.

• Residual Current Circuit breaker (RCCB)
• Residual Current Circuit Breaker with Overload protection (RCBO)
• Earth leakage relay

Residual current protective devices are distinguished from one another in respect of their suitability for detecting different forms of residual current. Below is a table giving RCD classification according to the waveform of the earth leakage currents:

• Type AC, sensitive to alternating current only.
• Type A, sensitive to alternating and/or pulsating current with DC components.
• Type B to protect in case of alternating residual sinusoidal currents up to 1.000 Hz, pulsating direct residual currents, and smooth direct residual currents
• Type F, with a rated frequency of 50 Hz or 60 Hz is intended for installations when frequency inverters are supplied between phase and neutral or phase and earthed middle conductor and can provide protection in case of alternating residual sinusoidal at the rated frequency, pulsating direct residual currents, and composite residual currents that may occur.

Overloads and short circuits have different characteristics due to the type of load. That’s why there are many MCB types to make efficient protection. B, C, D, K and Z-type MCBs can be used in different applications.

• Z: Designed to protect circuits that need a very low short circuit trip setting (Ex: Semiconductors)
• B: Designed for cable protection (Ex: Control circuits, lighting)
• C: Designed for medium magnetic startups (Ex: Lighting panels, control panels)
• D and K: Designed to allow for high inrush loads (Ex: Motor or transformation circuits)

10. Rated current

The rated current of an RCD is between 16A…125A. But the rated current of an MCB is between 0.5A…125A.

11. Circuit symbol

12. History

MCB has a longer history. A modern miniature circuit breaker was invented by Hugo Stotz in 1924.

It is not clear when and by whom the first RCD was developed. First actually functioning residual current devices for use in low-voltage installations were designed in the 1940s with a focus on protection in the event of insulation failure. Their sensitivity was around 100 mA.

In conclusion, installing a residual current device (RCD) and miniature circuit breaker (MCB) in the house’s consumer unit can prevent electrical accidents and save lives. Although both devices are protection devices, they have significant differences. The major difference between RCD and MCB is the protection type, with RCD protecting against residual currents and MCB protecting against overload and short-circuit currents. RCDs are generally used in household applications, while MCBs have wider application areas. Understanding the differences between these devices is essential to ensure proper electrical safety measures in a home or building.

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