RCDs, RCBOs, SPDs & AFDDs: Modern Protective Devices Explained

A modern consumer unit contains several types of protective device, each designed to detect and respond to a specific type of electrical fault. Understanding what each device does, how it works, and when to use it is essential knowledge for anyone studying for the 18th Edition (BS 7671) qualification or the 2391 Inspection and Testing qualification.

This guide covers all the main protective devices found in domestic and commercial installations: MCBs, RCDs (including Types AC, A, B, and F), RCBOs, SPDs, and AFDDs. For each device, we explain how it works, which standards govern its use, and when to specify it in an installation.

The 18th Edition with Amendment 2 introduced significant changes to the requirements for RCD types, surge protection, and arc fault detection. If you hold an older qualification, many of these requirements will be new. If you are studying for the first time, this guide covers everything you need to know about circuit protection for the exam and for real-world installation work.

Miniature Circuit Breakers provide overcurrent protection. They detect when the current flowing through a circuit exceeds a safe level — either due to an overload (too many appliances) or a short circuit (a direct fault between line and neutral). When this happens, the MCB trips and disconnects the circuit, protecting the cable from overheating and potentially causing a fire.

MCBs replaced older rewirable fuses and cartridge fuses. Unlike a fuse, which must be replaced after it blows, an MCB can simply be reset once the fault has been cleared. MCBs also provide more consistent and reliable tripping characteristics than fuse wire.

MCB Types and Tripping Characteristics

MCBs are classified by their instantaneous tripping characteristic — the multiple of rated current at which they trip magnetically (instantly) rather than thermally (with a time delay). The type determines what kind of load the MCB is suited to.

• ✓Type B: trips at 3 to 5 times rated current. Standard for domestic installations — lighting, socket outlets, immersion heaters, showers. Suitable for resistive and lightly inductive loads.
• ✓Type C: trips at 5 to 10 times rated current. Used for circuits with moderate inrush currents — commercial fluorescent lighting banks, small motors, air conditioning compressors. Avoids nuisance tripping from startup surges.
• ✓Type D: trips at 10 to 20 times rated current. Used for circuits with very high inrush currents — large motors, transformers, X-ray equipment, welding machines. Rarely seen in domestic work.

Common Domestic MCB Ratings

• ✓6A — lighting circuits
• ✓16A — immersion heater, single radial circuit
• ✓20A — radial power circuit (up to 20A)
• ✓32A — ring final circuit, cooker circuit
• ✓40A — shower circuit (up to 9.5kW)
• ✓50A — large shower or cooker (high-rated)

Residual Current Devices provide earth leakage protection. An RCD continuously monitors the balance between the current flowing out on the line conductor and the current returning on the neutral conductor. In a healthy circuit, these should be equal. If current is leaking to earth — for example, through a person touching a live part — the RCD detects the imbalance and disconnects the circuit.

A 30mA RCD rated for personal protection must disconnect within 40ms when the earth leakage current reaches 150mA (five times the rated residual operating current). At the rated current of 30mA, the RCD must trip within 300ms. This speed of disconnection is fast enough to prevent ventricular fibrillation in most circumstances, which is why 30mA RCDs are considered life-saving devices.

Type AC

Type AC RCDs detect sinusoidal AC earth leakage only. They are the simplest and cheapest type, and were once the standard for domestic installations. However, they cannot detect pulsating DC or smooth DC leakage currents, which are produced by electronic equipment containing rectifiers — including LED drivers, EV chargers, IT equipment, and variable speed drives.

Type AC RCDs are being phased out. BS 7671 Amendment 2 (Regulation 531.3.3) specifies that Type AC should not be used where loads producing DC components are present. Since almost every modern installation contains such loads, Type AC is no longer appropriate for new installations in most cases.

Type A

Type A RCDs detect sinusoidal AC and pulsating DC earth leakage currents. Pulsating DC is the waveform produced by single-phase rectifiers, which are found in the majority of modern electronic equipment: LED drivers, phone chargers, laptop power supplies, and many domestic appliances with electronic controls.

Type A is now the minimum requirement for most circuits in domestic and commercial installations. BS 7671 Regulation 531.3.3 states that where the characteristics of the load are not known, or where earth fault currents with DC components are expected, at least a Type A RCD shall be used. In practice, this means Type A for virtually all new installations.

Type B

Type B RCDs detect AC, pulsating DC, and smooth DC earth leakage. Smooth DC leakage can be produced by three-phase rectifiers, variable frequency drives (VFDs), and certain types of EV charger (particularly three-phase chargers without built-in DC fault detection).

Type B RCDs are significantly more expensive than Type A and are only required for specific applications. They are commonly specified for circuits supplying three-phase variable speed drives, some EV charging installations, and photovoltaic (solar PV) inverters where smooth DC fault currents are possible.

Type F

Type F RCDs detect AC, pulsating DC, and mixed-frequency earth leakage. They are designed for circuits containing single-phase variable frequency inverters — such as washing machines, dishwashers, and air conditioning units with inverter-driven motors. Type F provides enhanced sensitivity to the complex waveforms produced by frequency inverters.

While Type F is available and offers advantages for inverter-driven loads, Type A remains the minimum regulatory requirement for most circuits. Type F should be considered where the manufacturer of the connected equipment specifies it, or where enhanced detection of frequency-varied fault currents is needed.

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18th Edition (2382)

RCD types and selection criteria are key topics in the 18th Edition qualification. Understanding Regulation 531.3.3 and Amendment 2 changes is essential for the exam.

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An RCBO combines the functions of an MCB and an RCD in a single device. It provides both overcurrent protection (like an MCB) and earth leakage protection (like an RCD) for an individual circuit. This means each circuit has its own independent protection — if a fault occurs on one circuit, only that circuit is disconnected. All other circuits remain live and unaffected.

Advantages of RCBOs

• ✓Individual circuit protection — a fault on one circuit does not trip other circuits
• ✓Eliminates nuisance tripping affecting multiple circuits (a common complaint with shared RCDs)
• ✓Easier fault finding — the tripped device immediately identifies which circuit has the fault
• ✓Critical circuits (freezer, alarm system, medical equipment) remain live during faults on other circuits
• ✓Available in Type A, Type B, and Type F variants to match the load requirements

RCBOs vs Dual RCD Configuration

In a dual RCD (split-load) consumer unit, multiple ring and radial circuits share each RCD. Understanding different circuit types helps determine the best protection arrangement. If an earth fault occurs on any one circuit, the RCD trips and all circuits on that bank lose power. For example, if the kitchen socket circuit develops a fault, the lighting circuits on the same RCD bank will also go dark.

With an RCBO board, each circuit has its own device. Only the faulty circuit trips. This is particularly important for circuits where loss of supply could cause inconvenience, financial loss, or safety issues — such as freezers, security systems, or circuits supplying medical equipment.

Cost Consideration

RCBOs are more expensive per device than MCBs (typically 25 to 40 pounds each, compared to 5 to 10 pounds for an MCB). A full RCBO board for a typical domestic installation with 8 to 12 circuits will cost more than a dual RCD board. However, the improved discrimination, reduced nuisance tripping, and easier fault diagnosis often justify the additional cost. Full RCBO boards are increasingly considered best practice.

Surge Protection Devices protect electrical installations from transient overvoltages — brief but intense voltage spikes that can damage or destroy sensitive electronic equipment. These surges can be caused by lightning strikes (direct or nearby), switching of large loads on the supply network, or faults on the electricity distribution system.

BS 7671 Amendment 2 significantly strengthened the requirements for surge protection. Regulation 443 now requires a risk assessment to determine whether surge protection is needed. The assessment considers the consequences of an overvoltage event, and in practice, SPDs are required in the majority of new domestic installations.

SPD Types

• ✓Type 1 (T1): protects against direct lightning strikes. Installed at the origin of the installation (typically at or near the meter). Required where a building has a lightning protection system (LPS) or is supplied by an overhead line. Can handle very high energy surges (up to tens of thousands of amps).
• ✓Type 2 (T2): protects against indirect surges and switching transients. The most common type in domestic installations. Installed at the consumer unit. Handles surges from nearby lightning strikes, switching on the HV network, and load switching within the building.
• ✓Type 3 (T3): provides point-of-use protection for particularly sensitive equipment. Installed close to the equipment being protected (for example, at a socket outlet supplying a computer or medical device). Used as additional protection alongside a Type 2 SPD, not as a substitute for it.

When SPDs Are Required

The risk assessment under Regulation 443 considers whether the consequences of a surge could affect human life (medical equipment, life support), public services (emergency lighting, fire detection), commercial or industrial activity (IT systems, production equipment), or large numbers of individuals (hotels, schools, hospitals). Where the consequence of an overvoltage event is serious, SPDs must be installed unless a documented risk assessment demonstrates the risk is acceptable.

In practice, the majority of new domestic installations now require at least a Type 2 SPD. The only common exception is where the property has no sensitive electronic equipment and the risk assessment concludes the consequences of a surge are negligible — which is increasingly rare in modern homes.

Arc Fault Detection Devices detect dangerous arcing in electrical circuits. Arcing occurs when current flows across a gap in a conductor — for example, through a damaged cable, a loose connection, or a nail driven through a cable in a wall. Arcing generates intense heat at the point of the fault and can ignite surrounding materials, causing a fire.

The critical point about arcing faults is that they may not draw enough current to trip an MCB, and they may not involve earth leakage, so they will not trip an RCD. An arcing fault can burn for an extended period, generating enough heat to start a fire, while all the protective devices in the consumer unit remain closed. AFDDs address this gap in protection.

How AFDDs Work

AFDDs use electronic analysis of the current waveform to distinguish between dangerous arcing and normal arcing (which occurs in switches, motor brushes, and plug contacts). They detect two types of fault:

• ✓Series arcs: occur in a break in a single conductor (damaged cable, loose terminal). Current is limited by the load, so an MCB will not trip. The arc generates localised heat that can ignite insulation or nearby materials.
• ✓Parallel arcs: occur between two conductors (line-to-neutral or line-to-earth through carbonised insulation). May draw enough current to trip an MCB eventually, but can start a fire before the MCB operates, especially at lower fault currents.

BS 7671 Requirements

Regulation 421.1.7 recommends the use of AFDDs in specific locations where the risk of fire is elevated:

• ✓Premises with sleeping accommodation — HMOs (houses in multiple occupation), care homes, sheltered housing, hotels
• ✓Locations with a risk of fire — due to the nature of processed or stored materials (woodworking shops, agricultural buildings with hay or straw)
• ✓Locations with combustible construction materials — timber-framed buildings, thatched properties
• ✓Fire-propagating structures — buildings where fire could spread rapidly
• ✓Premises containing irreplaceable goods — museums, galleries, listed buildings, archives

AFDDs are not yet mandatory for all domestic installations. However, the direction of travel in the regulations is toward wider adoption. Several European countries already require AFDDs more broadly, and future editions of BS 7671 are expected to extend the recommendation or make it a requirement for more installation types.

Selecting the correct protective devices for an installation requires consideration of several factors: the type of load, the risk environment, the regulatory requirements, and the client's budget. Here is a systematic approach to specifying protection for each circuit.

Every Circuit Needs Overcurrent Protection

Every circuit must have an MCB or RCBO rated to protect the cable. Understanding earthing and bonding is essential alongside protective device selection. Select the MCB type based on the load characteristics: Type B for domestic resistive loads, Type C for moderate inrush loads, Type D for high inrush loads. Ensure the MCB rating does not exceed the current-carrying capacity of the smallest cable in the circuit.

Most Circuits Need 30mA RCD Protection

BS 7671 requires 30mA RCD protection for socket outlets rated up to 32A, circuits supplying mobile equipment outdoors, circuits in bathrooms and shower rooms, cables concealed in walls at a depth of less than 50mm, and circuits in caravans and marinas, among others. In practice, most circuits in a domestic installation require 30mA RCD protection. Provide this via a shared RCD or individual RCBOs.

RCD Type Selection

• ✓Type A minimum for most circuits — this is the default choice for domestic and commercial installations under BS 7671 Amendment 2
• ✓Type B where smooth DC fault currents are possible — three-phase VFDs, certain EV chargers, some PV inverters
• ✓Type F where frequency inverters are present — inverter-driven appliances if the manufacturer specifies it
• ✓Type AC should no longer be specified for new installations where electronic loads are present

SPD Assessment at the Origin

Carry out the risk assessment required by Regulation 443. In most new domestic installations, a Type 2 SPD at the consumer unit will be required. Where the property is supplied by an overhead line or has a lightning protection system, a Type 1 SPD is also needed. Consider Type 3 SPDs for circuits supplying particularly sensitive equipment.

AFDD for High-Risk Locations

If the installation is in a premises with sleeping accommodation, has a risk of fire due to stored materials or combustible construction, or contains irreplaceable goods, specify AFDDs on final circuits. Combined AFDD/RCBO devices provide overcurrent, earth leakage, and arc fault protection in a single unit.

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The 2391 qualification covers testing and verification of all protective devices — MCBs, RCDs, RCBOs, and SPDs. You will learn to test RCD trip times, verify MCB ratings against cable sizes, and assess whether the correct protective devices have been specified.

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