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
We can list the differences between RCD and MCB as follows.
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.
The major difference between RCD and MCB is the protection type. 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.
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 over 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 plunger associated with tripping coil or solenoid of MCB.
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, 3+N poles.
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.
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.
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.
Price and market size
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 rated frequency 50 Hz or 60 Hz are 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, Z type MCB’s 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)
The rated current of an RCD is between 16A…125A. But the rated current of an MCB is between 0,5A…125A.
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.
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