All about GFCI/RCD devices

In electrical engineering, standards exist to protect people and animals from electric shock.

This is achieved through a combination of:

• Basic protection as a precaution against direct contact with active parts by means of basic insulation in the fault-free state,
• Fault protection in the case of indirect contact with touchable parts, for example the metallic housing of an electrical appliance, which can become dangerous active parts in the event of a fault.

The use of residual current devices serves as additional protection for improved protection against electric shock, improved fault protection, and improved fire protection.

An RCD does not limit the current strength of the flowing fault current, but limits its duration.

Additional protection refers to a measure under certain conditions. This includes the residual current device (RCD) with its protective effect in the event of simultaneous failure of basic protection and fault protection. This means that in the electrical system or electrical appliance, a double or even multiple fault occurs. The RCD does not prevent electric shock and does not reduce the magnitude of the fault current through the human body. Depending on the magnitude, however, the duration of current flow through the body must be limited in such a way that the risk of ventricular fibrillation as an immediate life-threatening heart rhythm disorder is reduced to a minimum. The maximum permissible rated residual current IΔnfor personal protection is 30 mA. The use of the RCD as the sole protection against electric shock, i.e. without basic and fault protection, is not permitted.

An RCD with a rated residual current not greater than 30 mA must be provided for:

• Electrical outlets with a rated current not greater than 32 A, which are intended for use by laypersons and for general use
• Final circuits for portable equipment used outdoors with a rated current not greater than 32 A
• For lighting circuits (only in residential buildings)

An RCD can provide additional protection in the following cases:

• Improper handling or abusive use of electrical equipment
• Manipulation of electrical systems, e.g. by children (nail in socket)
• Damage to the electrical system due to external influences (e.g. moisture, interrupted protective grounding, broken insulation)
• Current flow through the body to ground due to non-compliance with the five safety rules by electrical professionals
• Handling of open electrical appliances for training purposes
• Faults in an electrical system or electrical appliance caused by non-professionals or improper work by professionals.

For fault protection, residual current devices (RCDs) must be used when the condition for automatic disconnection of the power supply by overcurrent protection devices in case of ground fault cannot be fulfilled. This is often the case when a TT system exists due to the type of ground connection. Due to the lack of electrical connection between the system grounding conductor and the operating grounding conductor, the fault current is mainly limited by the propagation resistance of the system grounding conductor RA.

The following condition applies to personal protection:

RA ≤ UT/IΔn with

• RA as the propagation resistance of the system ground including the protective conductor
• UT as contact voltage with a maximum of 50 V AC without time limit
• IΔn as the rated residual current of the residual current circuit breaker

Thus, a maximum propagation resistance would be possible of:

RA = 50V/30 mA ≈ 1,67 kΩ

In the following cases, RCDs with a rated residual operating current greater than 30 mA may be used for:

• Distribution circuits
• End circuits, if required for reasons other than protection against electric shock.

Under the condition of the 4.6-fold rated residual operating current, the following propagation resistances would be possible depending on the respective RCD used:

According to VdS regulations, in order to protect against electrically ignited fires, the fault current between the phase conductor and protective conductor or ground must not exceed 420 mA. Fault current circuit breakers with a rated fault current of up to 300 mA can be used for this purpose. Depending on the rated fault current, the following heat outputs can occur at a fault location:

These heat outputs are significantly lower than they would be with overcurrent protection devices alone. In addition, for fire protection, dedicated arc fault protection devices are also available, which must be installed in addition to the residual current circuit breaker and provide protection against cable fires that can occur in the event of cable breakage.

The residual current device (RCD) triggers at the latest when the rated residual current is reached and switches off the affected circuit on all poles from the upstream network, including the neutral conductor in four-pole switches. The internal test circuit is also disconnected because its current limiting resistance is not designed for continuous operation (misuse). The protective earth conductor is not part of the residual current device and is not disconnected.

Residual currents occur when a part of the current flows back to the power source through an unwanted current path. This current path can be the protective earth conductor, the enclosure of an electrical operating device, the earth including all metallic structures in electrical contact with the earth, as well as the body of a human or animal. The residual current device forms the arithmetic sum of all instantaneous values of the currents in the phase conductors and the neutral conductor. In an installation without an earth fault, the sum is always zero.

The summation is performed by a summation current transformer. Depending on the number of poles, two, three or four primary windings are passed through it. They are constructed in such a way that their induction effects cancel each other out in the fault-free state. No magnetic flux is generated in the transformer core and therefore no voltage is induced in the secondary winding. If a residual current flows back to the power source through such an unwanted current path, the sum of all currents through the summation current transformer is no longer zero. This results in a magnetic flux in the transformer core, which induces a voltage in the secondary winding. The secondary current triggers a switch via the holding magnet trigger and switches off the circuit on all poles.

The summation current transformer works like a transformer and is also frequency-dependent. Therefore, it can only detect alternating residual currents or pulsating direct residual currents. In the case of smooth direct residual currents, there is no transmission, and therefore no induction in the secondary winding - the residual current is not detected. In a mixed form (smooth direct residual current overlaid with alternating residual current), the alternating residual current can only be weakened or not transmitted at all, as the iron core is partially or completely saturated by the smooth direct residual current.

All-current-sensitive RCDs (e.g. type B) sometimes have a second transformer core for additional detection of smooth direct residual currents, which can be equipped with a Hall sensor to detect the magnetic field directly, and/or additional electronics to better detect (or suppress) frequency responses and their current dependencies and thus offer different types for the corresponding application purpose.

Components of a two-pole residual current circuit breaker:

1. Switching mechanism
2. Secondary winding
3. Summation current transformer toroid
4. Test button

The summation current transformer contains a toroidal core wound from crystalline or nanocrystalline soft magnetic tape. Ferrite cores are not suitable due to their low permeability and saturation induction. In order to achieve the necessary power to trigger the residual current circuit breaker, toroidal tape cores of a certain size and mass are required, typically weighing around 40 g. The cores are often encapsulated with insulation, and no force may be exerted on the cores through any possible shrinkage of the resins, as this would change the magnetic properties. Plastic housings, into which the cores are loosely inserted, are also common. Two to four working current windings made of thick copper wire are wound around the core, as well as the secondary winding and possibly a test winding, both made of thin wire.

The switch lock is the mechanism that connects the manual operation (lever or push button) and the trigger of the summation current transformer with the switch contacts. Inside the switch lock, there is the release spring, which is pre-tensioned during the switch-on (manually), and ensures the necessary force and speed for a safe disconnection. Additionally, the mechanism for tripping is housed here. The pre-tensioned switch lock can be triggered with minimal force and cannot be blocked from the outside.

The summation current transformer acts on the switch lock, for example, via a latching magnet trigger. This is connected to the secondary winding of the summation current transformer. A latching magnet trigger consists of a permanent magnet, two legs with magnetic shunt, a soft magnetic material armature, and an excitation winding. The magnetic flux of the permanent magnet passes through both legs and the armature. As a result, the armature is held against the force of the spring directed towards the switch lock trigger. If a current flows through the excitation winding, a second magnetic flux is generated. In one half-wave, the total flux is amplified, and in the other half-wave, it is weakened so that the spring pulls the armature away from the pole faces of the two legs. This leads to the triggering of the switch lock and the shutdown of the affected circuits.

The protective function of a residual current circuit breaker does not occur in the following cases:

• A person touches active parts of different potentials. These are two or more phase conductors with different phase angles or a phase conductor and the neutral conductor. The person is located on an electrically relatively well-insulated location against the earth and has no contact with grounded objects or the protective conductor.
• When a transformer (such as an isolation transformer) separates the circuit and a person simultaneously touches both poles on the secondary side.
• In the event of an overcurrent as overload or short-circuit, protection can only be ensured by automatic disconnection of the power supply through an overcurrent protection device.
• A conductor fault is not detected because no fault current flows to earth.
• Depending on the type of fault current, there is a risk that a residual current circuit breaker will not trip. It does not have the ability to detect all types of current (especially direct current).
• A residual current circuit breaker of type B+ for advanced fire prevention detects fault currents with frequencies up to 20 kHz only to earth. To detect such a fault current between two active conductors, an additional arc fault detection device would be required.

Residual current devices, are classified into types based on the type of fault current they can detect. In ascending order of sensitivity, the types are classified as Type AC, Type A, Type F, Type B, and Type B+.

Here is the classification of each type:

1. Type AC: This type is designed to detect purely sinusoidal alternating fault currents that can occur suddenly or rise slowly. It functions properly as long as a smooth direct current fault current does not exceed 6 mA. (No longer permitted in Germany.)
2. Type A: In addition to the functionality of Type AC, this type also detects pulsating direct current fault currents. Type A is the most commonly used RCD type for standard applications.
3. Type F: In addition to the functionality of Type A, this type can detect a mixture of fault currents with different frequencies up to 1 kHz. Such fault currents can occur, for example, in single-phase electrical devices with frequency converters. It functions properly as long as a smooth direct current fault current does not exceed 10 mA.
4. Type B: In addition to the functionality of Type F, this type can detect smooth direct current fault currents. It can detect different fault current waveforms regardless of phase angle, polarity, and whether they occur suddenly or rise slowly. Type B is also referred to as all-sensitive.
5. Type B+: In addition to the functionality of Type B, this type can detect sinusoidal alternating fault currents up to a frequency of 20 kHz. Type B+ is primarily used for advanced fire protection measures.

This classification based on the type of fault current allows for the selection of the appropriate RCD type for specific requirements and devices in an electrical installation. It is important to consider the regulations and standards of the specific country, as the permissibility of certain types may vary.

There are also combined RCDs with circuit breakers (LS) (e.g. 30 mA RCD and 13 A circuit breaker), which are called RCBOs (commonly referred to as "FI/LS"). An RCBO with a pole number of 1P + N typically has the same installation width (or the same number of modular units, abbreviation TE) as a two-pole circuit breaker or a two-pole RCD (two TE).

RCD sockets (SRCD) (commonly referred to as "GFCI sockets") monitor connected loads for earth fault currents (additional safety). They are used where, for example, there are no RCDs installed in existing installations with legal protection (grandfathering), but increased safety is still desired. They do not replace an RCD according to DIN EN 61008-1 (VDE 0664-10), where it is required.

If the individual units of an RCD, such as the differential current detection circuit, the evaluation of the differential current and the load switch, are located in physically separate housings, this unit is called a modular residual current protection device (MRCD).

The most important parameter is the rated fault current, IΔn, at which a residual current circuit breaker must trip at the latest. Values for IΔn are 10 mA, 30 mA, 100 mA, 300 mA, 500 mA, and 1 A. In practice, the tripping of a purely sinusoidal alternating fault current typically occurs between 0.6 · IΔn and 0.8 · IΔn.

The non-trip fault current, IΔn0, is equal to 0.5 · IΔn for a purely sinusoidal alternating fault current. A residual current circuit breaker should not trip below half of the rated fault current.

For different fault current waveforms, the following trip ranges are defined:

• 0.5 · IΔn to 1 · IΔn for purely sinusoidal alternating fault currents
• 0.35 · IΔn to 1.4 · IΔn for pulsating direct fault currents
• 0.25 · IΔn to 1.4 · IΔn for half-wave rectified currents with a phase control angle of 90°
• 0.11 · IΔn to 1.4 · IΔn for half-wave rectified currents with a phase control angle of 135°
• Up to 1.4 · IΔn for pulsating direct currents superimposed with a smooth direct fault current of maximum 6 mA
• 0.5 · IΔn to 1.4 · IΔn for mixed-frequency fault currents
• 0.5 · IΔn to 2 · IΔn for smooth direct fault currents

The rated current In is a predetermined value that a residual current circuit breaker can continuously carry on each phase conductor. Preferred values for In include 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 63 A, 80 A, 100 A, and 125 A.

According to DIN EN 61008-1 (VDE 0664-10):2013-08 (manufacturer specifications) for residual current circuit breakers without time delay, the maximum permissible trip times are 0.3 seconds at a current of IΔn, 0.15 seconds at 2 · IΔn, and 0.04 seconds at 5 · IΔn. This makes the occurrence of (lethal) ventricular fibrillation very unlikely but cannot be completely ruled out, among other reasons because the physiological effect of a current pulse depends on the phase of the heartbeat it falls into.

For selective residual current circuit breakers - those with time delay - the maximum permissible trip times are 0.5 seconds at a current of IΔn, 0.2 seconds at 2 · IΔn, and 0.15 seconds at 5 · IΔn.

Non-trip times are only defined for selective residual current circuit breakers. The shortest non-trip times are 0.13 seconds at a current of IΔn, 0.06 seconds at 2 · IΔn and 0.05 s at 5 · IΔn.

To achieve selectivity, residual current circuit breakers can be connected in series. In this configuration, only the residual current circuit breaker immediately associated with the faulty circuit should trip without any time delay. A residual current circuit breaker with a time delay is connected upstream as an additional protective device and is marked with an S-symbol for selectivity. Selectivity is achieved when:

• The shortest non-trip time of the upstream residual current circuit breaker with time delay is greater than the highest trip time of the downstream residual current circuit breaker without time delay.
• The rated fault current of the upstream residual current circuit breaker with time delay is at least three times the value of the downstream residual current circuit breaker without time delay (total selectivity).

Residual current circuit breakers with time delay are often referred to as selective or time-delayed residual current circuit breakers. Like with overcurrent protective devices, the goal is to achieve higher availability of the electrical installation through selectivity. Additionally, the following points should be noted:

• Residual current circuit breakers with time delay cannot be used for additional protection measures as their rated fault current is at least 100 mA. In this case, the current-time characteristic for the highest trip time is always in a range where there is an increased risk of ventricular fibrillation.
• The downstream residual current circuit breaker must not have a higher sensitivity (detection according to fault current waveform) compared to the upstream residual current circuit breaker. For example, a Type B residual current circuit breaker should not be installed downstream of a Type A residual current circuit breaker.

To prevent unwanted tripping, residual current circuit breakers with short-time delay are used. Causes of unwanted tripping can include:

• Voltage surges due to switching operations and atmospheric influences.
• Equalization processes following the connection or load change of capacitive or inductive devices.

The maximum allowable trip times are the same as those for residual current circuit breakers without time delay. Manufacturers use their own specific markings, such as:

• ABB indicating AP-R and using the term "short-time delayed."
• Siemens using a K-symbol and terms like "super-resistant" or "short-time delayed."
• Doepke using the symbols G or KV and the term "short-time delayed."

The use of residual current circuit breakers with time delay (selectivity) is also possible if the additional protection measure can be omitted.

In German standards, the following terms were previously used:

• Residual-current circuit breaker (FI) for devices independent of mains voltage (without auxiliary power source),
• Differential current circuit breaker (DI) for devices dependent on mains voltage (with auxiliary power source).

In trade, the following can also be found:

• Personnel protective device is a marketing name and not technically defined.
• Personnel protective switch is a designation used for residual-current circuit breakers in supply lines and extensions as well as in intermediate plugs, but is otherwise not precisely defined. BGI608 provides specifications for such portable protective devices when used as a power source for so-called small construction sites.

The following designations were used for residual-current circuit breakers combined with circuit breakers:

• FI/LS circuit breaker when they were independent of mains voltage,
• DI/LS circuit breaker when they were dependent on mains voltage.

The distinction between mains voltage-independent and mains voltage-dependent protective devices is not made in English-language standards and is also not used in IEC and EN standards. The following designations are used in international device standards:

In the installation regulations for electrical systems, residual-current circuit breakers are uniformly referred to by the overarching term RCD. Differentiation between FI, DI or special designs is no longer made in the installation regulations for electrical systems. The protection objective is decisive here. This must be realized with different designs depending on the place of use.

The use of residual-current devices is mandatory in many countries for new installations or modifications in residential and industrial settings, at least for sockets (up to 20 A or 32 A) (such as DIN VDE or ÖVE), in addition to the installed overcurrent protection devices. A residual-current device with a tripping current differential of 300 mA is often required by some power supply companies as a fire protection measure for the entire electrical system if the house supply is not via underground cables but through aerial power lines.

In Europe, except for Great Britain, residual-current devices (RCDs) that are not dependent on mains voltage are mandatory. The underlying safety philosophy questions the reliability of electronic amplifier circuits used in the simpler and smaller electronic differential-current switches (DI switches) used in the English-speaking world.

In Germany, residual-current devices (RCDs) have been required in new buildings since May 1984 for rooms with a bathtub or shower according to DIN VDE 0100-701 (the only exception being fixed water heaters).

Since June 2007, all socket circuits intended for use by laypersons and for general use in new buildings must also be equipped with a residual current device with a rated residual current not exceeding 30 mA. This applies to final circuits with a rated current up to 20 A indoors and up to 32 A outdoors (DIN VDE 0100-410:2007-06, section 411.3.3, transitional period until January 2009).

Since October 2018, these requirements also apply indoors to socket circuits up to 32 A, as well as to lighting circuits in residential buildings (DIN VDE 0100-410:2018-10, section 411.3.3, transitional period until July 2020).

Residual-current devices are also required for swimming pools, outdoor pools, and rooms and cabins with sauna heaters. The often-misunderstood term "Feuchtraum"(moisture-prone room) does not refer to bathrooms or toilets in living spaces. According to the definition in DIN 68800, a room is considered a moisture-prone room if the humidity is above 70% for a prolonged period. Kitchens in apartments and bathroom areas in apartments and hotels are explicitly classified as dry rooms with respect to installation in accordance with DIN VDE 0100-200:2008-06 section NC.3.3 (since these rooms only experience occasional moisture).

There is no obligation to retrofit old installations in Germany. This means that an installation may continue to be operated and repaired if it complied with the applicable standards and guidelines at the time of its construction and still complies with them today (grandfathering).

However, in Germany, the retrofitting of an RCD is unavoidable under the following circumstances:

• when changes in use are made
• in case of expansion of use, construction measures or renovations that interfere with the substance (not just repairs/restoration)
• if new legal regulations that require retrofitting come into force (observe TAB)
• after expired transitional periods
• in case of immediate dangers to persons and property

In agriculture, too, residual current circuit breakers must be used, especially in animal husbandry. The reduction of the permanently permissible touch voltage to 25 V AC voltage and 60 V DC voltage has been eliminated according to DIN VDE 0100-705:2007-10.

According to DIN VDE 0100-530:2018-06, RCDs for additional protection in AC systems must correspond to:

• DIN EN 61008-1 (VDE 0664-10) and DIN EN 61008-2-1 (VDE 0664-11) for residual current circuit breakers without built-in overcurrent protection (RCCBs); or
• DIN EN 61009-1 (VDE 0664-20) and DIN EN 61009-2-1 (VDE 0664-21) for residual current circuit breakers with built-in overcurrent protection (RCBOs); or
• DIN EN 62423 (VDE 0664-40) for residual current circuit breakers with and without built-in overcurrent protection (RCBOs and RCCBs).

In contrast, PRCD and SRCD (according to DIN VDE 0662) do not provide additional protection in the sense of DIN VDE 0100-410, but only serve to locally increase the level of safety.

In Austria, a residual current device has been legally required since 1980. According to ÖVE E8001-1/A1:2013-11-01, residual current devices with a rated residual current of maximum 30 mA are required for all circuits that contain sockets and whose rated current does not exceed 20 A.

The use of the AC type is not generally prohibited. In most cases (threat of damage in the event of a power failure), a type G residual current device that is short-time delayed and surge-proof must be used. A fuse with the rated current of the residual current device is only allowed if explicitly stated by the manufacturer; otherwise, for example, a 40 A residual current device must be protected with a maximum of 25 A. Due to these peculiarities, several manufacturers sell Austria-specific (and significantly more expensive) variants of their products, which are referred to, for example, as short-time delayed, type G, fuseable or fuse-protected.

On construction sites, an additional protection must be provided for all socket circuits with a rated current of up to 32 A and in agricultural and horticultural facilities (not in the adjacent residential buildings), in sauna areas, in swimming pools, in outdoor swimming facilities, in experimental setups in classrooms, in medically used rooms, in bathrooms, on campsites, on boat docks, and in hand-held wall lamps in changing rooms, regardless of their rated current.

In Switzerland, until 2009, according to the Low Voltage Installation Standard (NIN) 2005 4.7.2.3.1-8, a maximum of 30 mA was required for bathrooms, outdoor sockets, damp and wet rooms, corrosive environments, explosive atmospheres, construction sites, trade fairgrounds, marketplaces, and electrical test arrangements (all sockets ≤ 32 A).

For installations in corrosive environments, explosive and fire-prone rooms, and in agricultural businesses, 300 mA is required for the entire installation, with all sockets in agriculture being equipped with 30 mA residual current devices.

As of January 1, 2010, the new NIN 2010 came into force. From now on, every freely accessible socket ≤ 32 A must be protected by a maximum of 30 mA residual current protective device (RCD). Exceptions include sockets in IT systems where operational safety is more important and the room can only be accessed by a instructed group of people.

In residential construction, type A is generally used for all applications.

For testing the permissible disconnection time in the installation, 0.4 s applies to circuits ≤ 32 A. The testing with half and full differential current with a trigger time of <0.3 s is a pure device test and has no significance for the safety verification of electrical installations (SiNa).

The current (18th) edition of the IEE Electrical Wiring Regulations requires that all socket outlets in most installations have RCD protection, though there are exemptions. Non armoured cables buried in walls must also be RCD protected (again with some specific exemptions). Provision of RCD protection for circuits present in bathrooms and shower rooms reduces the requirement for supplementary bonding in those locations. Two RCDs may be used to cover the installation, with upstairs and downstairs lighting and power circuits spread across both RCDs. When one RCD trips, power is maintained to at least one lighting and power circuit. Other arrangements, such as the use of RCBOs, may be employed to meet the regulations. The new requirements for RCDs do not affect most existing installations unless they are rewired, the distribution board is changed, a new circuit is installed, or alterations are made such as additional socket outlets or new cables buried in walls.

RCDs used for shock protection must be of the 'immediate' operation type (not time-delayed) and must have a residual current sensitivity of no greater than 30 mA.

If spurious tripping would cause a greater problem than the risk of the electrical accident the RCD is supposed to prevent (examples might be a supply to a critical factory process, or to life support equipment), RCDs may be omitted, providing affected circuits are clearly labelled and the balance of risks considered; this may include the provision of alternative safety measures.

The previous edition of the regulations required use of RCDs for socket outlets that were liable to be used by outdoor appliances. Normal practice in domestic installations was to use a single RCD to cover all the circuits requiring RCD protection (typically sockets and showers) but to have some circuits (typically lighting) not RCD protected. This was to avoid a potentially dangerous loss of lighting should the RCD trip. Protection arrangements for other circuits varied. To implement this arrangement, it was common to install a consumer unit incorporating an RCD in what is known as a split load configuration, where one group of circuit breakers is supplied direct from the main switch (or time delay RCD in the case of a TT earth) and a second group of circuits is supplied via the RCD. This arrangement had the recognised problems that cumulative earth leakage currents from the normal operation of many items of equipment could cause spurious tripping of the RCD, and that tripping of the RCD would disconnect power from all the protected circuits.

GFCIs (Ground Fault Circuit Interrupters) are required in North America for socket-outlets located in areas with an easy path to ground, such as wet areas and rooms with uncovered concrete floors, to protect against electric shock.

In both Canada and the US, older two-wire, ungrounded NEMA 1 sockets may be replaced with NEMA 5 sockets protected by a GFCI (integral with the socket or with the corresponding circuit breaker) in lieu of rewiring the entire circuit with a grounding conductor. GFCI sockets have rectangular faces and accept Decora face plates, and can be mixed with regular outlets or switches in a multi-gang box with standard cover plates. In such cases, the sockets must be labeled "no equipment ground" and "GFCI protected."

The US National Electrical Code has required devices in certain locations to be protected by GFCIs since the 1960s. GFCIs are commonly available as an integral part of a socket or a circuit breaker installed in the distribution panelboard. Successive editions of the code have expanded the areas where GFCIs are required to include construction sites (1974), bathrooms and outdoor areas (1975), garages (1978), areas near hot tubs or spas (1981), hotel bathrooms (1984), kitchen counter sockets (1987), crawl spaces and unfinished basements (1990), near wet bar sinks (1993), near laundry sinks (2005), and in laundry rooms (2014).

GFCIs approved for protection against electric shock trip at 5 mA within 25 ms, while an Equipment Protective Device (EPD) is allowed to trip as high as 30 mA of current for equipment protection instead of people protection. The American Boat and Yacht Council requires both GFCIs for outlets and Equipment Leakage Circuit Interrupters (ELCI) for the entire boat, with ELCIs tripping on 30 mA after up to 100 ms to provide protection while minimizing nuisance trips. High-current RCDs with trip currents as high as 500 mA are sometimes deployed in environments (such as computing centers) where a lower threshold would carry an unacceptable risk of accidental trips, serving for equipment and fire protection instead of protection against the risks of electrical shocks.

According to Regulation 36 of the Electricity Regulations 1990

1. For a place of public entertainment, protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 10 mA.
2. For a place where the floor is likely to be wet or where the wall or enclosure is of low electrical resistance, protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 10 mA.
3. For an installation where hand-held equipment, apparatus or appliance is likely to be used, protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 30 mA.
4. For an installation other than the installation in (1), (2) and (3), protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 100 mA.

Residual current circuit breakers (RCCBs) can be used in all AC systems (TN, TT, and IT systems). In TN systems, they are primarily used as additional protection since fault protection is already provided by overcurrent protection devices. In TT systems, RCCBs often provide fault protection since triggering overcurrent protection devices is not guaranteed. In IT systems, their use should be the exception. A separate RCCB is required for each electrical device.

In new construction, there is nothing to prevent the entire power supply from being secured. At least two RCCBs should be installed in a sub-distribution board for apartments to ensure that the entire system is not shut down in case of a fault. However, this can be inconvenient, so it is recommended to limit the protected circuits using RCCBs. The selection should also take into account leakage currents of electronic loads (e.g., electronic ballasts) or their possible fault current type (e.g., built-in frequency converter in a washing machine).

RCCBs can also be triggered by external events, such as voltage surges caused by lightning strikes on overhead power lines. This can often lead to unpleasant side effects, such as heating or cooling systems being shut down even though there is no fault in the system. For this reason, circuit breakers have been developed that automatically switch the voltage back on two to three times shortly after being triggered. They remain permanently shut down only if the fault persists. These models are particularly useful for remotely controlled systems where there is no personnel on site to switch the circuit breaker back on.

The residual current circuit breaker was patented by Schuckert in 1903 under the name "sum current circuit" for detecting earth faults (DRP-No. 160,069). Kuhlmann described a method for measuring earth fault currents in the Berlin network at AEG. The technology, which today's residual current circuit breakers are also based on, was further developed by Nicholsen (1908, US-Pat-No. 959,787).

In the early 1950s, after numerous suggestions and technical studies on the basic applicability of the circuit as a protective device, a mature residual current circuit breaker for widespread use by electricity customers was presented for the first time. In 1951, a residual current circuit breaker with a trade name "Spiderweb" was developed by Schutzapparate-Gesellschaft & Co. mbH. KG, Schalksmühle/Westf. (Schupa), designed in two-, three-, and four-pole versions for a rated current of 25 A and voltages up to 380 V with a tripping fault current of 0.3 A. A lower tripping threshold was discussed but dismissed as economically unfeasible. The permissible leakage currents for heating devices at that time would have led to frequent false tripping with a lower tripping threshold.

In 1957, Gottfried Biegelmeier at Felten & Guilleaume in Austria developed a residual current circuit breaker. In Austria, these became legally required in private households in 1980, with the tripping current gradually reduced from originally 100 mA to 70, 65, and 30 mA. Since the beginning of 1985, this has also applied in Switzerland with the introduction of the regulation SEV 1000-1.1985.