Features of 3 circuit protection devices (MCB, RCCB, and RCBO) that you probably forgot

Features of 3 circuit protection devices (MCB, RCCB and RCBO) that you probably forgot

Circuit protection devices

Traditionally, fuses were the primary devices chosen for final circuit protection, until the 1970s circuit breaker technology began to become dominant. Today, the three main circuit protection devices are:

Features of 3 circuit protection devices (MCB, RCCB and RCBO) that you probably forgot

  1. Miniature circuit breakers (MCB),
  2. Residual current circuit breakers (RCCB) and
  3. Residual circuit breakers with overcurrent protection (RCBO)

Miniature Circuit Breakers (MCBs)

MCBs provide overcurrent and short circuit protection only and are unable to detect residual current (earth leakage current) if it is not large enough to be classified as overload or short circuit.

Therefore, in many applications, RCBOs are a better option, but MCBs remains the most widely used devices, especially in older installations.

Circuit protection devices
Circuit Protection Devices

In addition, there are cases where it may be better to use an MCB in conjunction with a separate RCCB rather than an RCBO, as the use of separate devices can expand the choice of protection features.

MCB types //

MCB 20A 10 / 15kA Trigger type B 1P

In cases where MCBs have been selected as the most suitable protective device, the next step is to decide which type of MCB to use . BS EN 60898 defines several types of MCBs based on their operating characteristics. The most common characteristics of MCB are Types B, C and D , which cover the majority of commonly encountered applications.

These were recently joined by Type K features which provide improved protection in certain applications.

In consumer applications it is rare to see anything other than Type B MCBs being used , with the occasional use of Type C devices on high inrush circuits such as some lighting loads.

There are other specialized types as well, but these are rarely encountered in normal installation work and are therefore not considered in this technical article. Let’s see the most common MCB features in more detail //

MCB type B

Type B MCBs respond quickly to overloads and are built to trip when the current passing through them is between 3 and 4.5 times normal current at full load. They are suitable for protecting incandescent lighting circuits and outlets in domestic and commercial environments, where there is a low risk of overcurrent of an intensity that could cause the MCB to trip.

MCB type C

Type C MCBs react more slowly , and are recommended for applications involving inductive loads with high inrush currents, such as fluorescent lighting installations. Type C MCBs are designed to trigger between 5 and 10 times the normal current at full load.

Type D MCB

D-type MCBs are even slower and are configured to trigger between 10 and 20 times the normal current at full load. They are recommended only for circuits with very high inrush currents , such as those feeding transformers and welding machines.

Note However, MCBs with type K characteristics may offer better protection in some such applications.

Type K MCB

Type K MCBs are designed to trip between 8 and 12 times the normal current at full load by placing them between the traditional

Type C and Type D circuit breakers. In most cases, they provide improved wiring protection should be provided in circuits including motors, capacitors and transformers, where it would previously have been necessary to use devices type D. This reinforced protection is obtained without increasing the risk of unwanted tripping.

When selecting MCBs, it is essential to remember that their primary function is to quickly disconnect a fault current to protect cables and equipment downstream of the device, as well as to protect personnel from electric shock. This means that the devices must be chosen with nominal characteristics suitable for the application and the size of the cables used . Detailed instructions on how to do this are given in the IET wiring regulations.

Also note that Type B and Type C MCBs can be normally selected to provide trip times that provide adequate cable protection, but this may be more difficult with Type D and in some cases with Type K devices. , where a lower loop impedance (Zs) may be required to achieve the trip time prescribed in the wiring regulations.

Another essential characteristic of MCBs which must be considered is the breaking capacity. This must always be greater than the Prospective Short Circuit Current (PSCC) at the point where the MCB is to be installed, otherwise the device may not be able to safely clear faults.

In practice, it is usually not difficult to meet this requirement, as part of the routine testing of electrical installations, required by the IET wiring regulation, is to measure the source impedance of the power supply, from which the PSCC can be calculated. In fact, many types of installation tests perform this calculation automatically.

MCBs with a breaking capacity of 6 kA is very widely used and are generally chosen for domestic applications. Devices with breaking capacities of 10 kA and even 25 kA are normally used in commercial and industrial applications due to the inevitably higher fault levels associated with such installations.

Residual current circuit breaker (RCCB)

Intended primarily to minimize the risk of injury from electric shock, RCDs only provide protection against residual currents (earth leakage) and are not sensitive to overloads or short circuits. For this reason, they should never be used as the sole protection device for a circuit.

Circuits with RCCB protection must always include separate overload and short circuit protection. Most often this is an MCB, but it could be, for example, a fuse.

Like MCBs, RCCBs are available in various different types which are denoted by letters. This is a potential source of confusion, which is why it is useful to remember that a type B MCB, for example, is not related to a type B RCCB.

The types of RCCB most likely to be encountered are

  • AC type,
  • Type A,
  • Type B,
  • Type B + and
  • Types.

Type RCCB AC are sensitive to alternating currents and are suitable for most household and commercial applications.

RCCB type A additional protection compared to AC type devices, as they are sensitive not only to pulsed direct current currents . Type A devices should be used in preference to type AC devices in applications with a lot of “electronic” loads , such as computer equipment or electronic ballast lighting systems.

RCCB type B are sensitive to alternating current, pulsating alternating current and constant direct current leakage currents. This type may be necessary for use in solar photovoltaic (PV) installations because photovoltaic panels produce a DC output and certain types of faults can cause DC currents to leak to earth. However, this will depend on the inverter used in the installation.

Type B + RCCB are similar to Type B, but respond to AC leakage currents over a wider frequency range, which is useful in some specialized applications.

It should be noted that Type B and Type B + devices can be used anywhere a Type AC or Type A device is specified. as they provide the same functionality as these types and more .

Type S RCCBs are selective devices intended for use in circuits that include more than one RCCB. For example, an installation may include an upstream RCCB to protect multiple downstream circuits, some of which also have their own RCCB or RCBO.

If a ground fault occurs on any of the downstream circuit protection devices with an RCCB or RCBO, that device should trip rather than the upstream RCCB. Using an S-type RCCB for the upstream device will provide the necessary selectivity.

RCCBs are offered with different sensitivities. In theory, a more sensitive device (i.e. with a lower tripping current) will provide more effective protection against the risk of electric shock, but in practice it will also be more sensitive to the risk of electric shock. “Unexpected tripping” – tripping in the absence of significant danger .

Advice on choosing devices with sensitivity is provided in the IET wiring regulations, which should always be consulted. However, for most household and small-scale commercial applications, 30mA devices are used.

Residual current circuit breaker with overcurrent protection (RCBO)

RCBOs are devices that combine the functions of an MCB and an RCCB in one device. They, therefore, provide overload, short-circuit protection devices against residual current (earth fault) and are normally used as the only protection device in the circuit. In principle, it would be possible to produce a wide range of RCBOs with different combinations of MCB and RCCB characteristics.

In practice, manufacturers limit themselves to the most common combinations. These typically include an MCB type B or C characteristic, combined with an RCCB type AC or type A characteristic.

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When other combinations are required, it means that an RCBO cannot be used and the necessary protection must be provided by using a separate MCB and RCCB. It should be noted, however, that field adaptable residual current modules are still available, which can be used to convert some older MCBs to RCBO systems, thus allowing a simple upgrade of existing installations.

A recent development to note is the introduction of the so-called compact RCBOs . In the past, RCBOs were invariably larger, typically much larger than regular MCBs. This meant they took up more space in consumer units and distribution boards, making wiring more difficult.

The new compact RCBOs are not much larger than a regular MCB and therefore make the installation process much easier. Compact RCBOs are available with the same feature combinations as standard RCBOs.

The nominal current, the breaking capacity, and the sensitivity to the residual current of the RCBOs are chosen in exactly the same way as for the separate MCBs and RCCBs.

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