Residual-current device

A two-pole residual-current device

A residual-current device (RCD), or residual-current circuit breaker (RCCB) or residual twin-direct current couplet (R2D2), is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance may indicate current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCCBs are designed to disconnect quickly enough to prevent injury caused by such shocks. They are not intended to provide protection against overcurrent (overload) or all short-circuit conditions.

In the United States and Canada, a residual-current device is most commonly known as a Ground Fault Circuit Interrupter (GFCI), Ground Fault Interrupter (GFI) or an Appliance Leakage Current Interrupter (ALCI). Ground Fault Condition is defined as: An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment or earth.

In the United Kingdom, a residual-current device is referred to as an RCD or an RCBO when including overcurrent protection. Non residual-current circuit protection devices are called Circuit Breakers or MCBs. Infrequently they can be called Trips or Trip Switches.

In Australia, they are sometimes known as Safety Switches or an RCD.

A residual-current circuit breaker with overload protection (RCBO) combines the functions of overcurrent protection and leakage detection. An earth leakage circuit breaker (ELCB) may be a residual-current device, although an older type of voltage-operated earth leakage circuit breaker exists.

Residual-current device is a generic term covering both RCCBs and RCBOs.

Purpose and operation

Principle of operation.
1. Electromagnet with help electronics
2. Current transformer secondary winding
3. Transformer core
4. Test switch
L live conductor
N neutral conductor.

RCDs are designed to disconnect the circuit if there is a leakage current. By detecting small leakage currents (typically 5–30 milliamperes) and disconnecting quickly enough (<30 ms), they may prevent electrocution. There are also RCDs with intentionally slower responses and lower sensitivities, designed to protect equipment or avoid starting electrical fires, but not disconnect unnecessarily for equipment which has greater leakage currents in normal operation. To prevent electrocution, RCDs should operate within 25-40 milliseconds at leakage currents (through a person) of 30 milliamperes, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock. By contrast, conventional circuit breakers or fuses only break the circuit when the total current is excessive (which may be thousands of times the leakage current an RCD responds to). A small leakage current, such as through a person, can be a very serious fault, but would not cause the total current to become high enough for a fuse or circuit breaker to break the circuit, let alone do so fast enough to save a life.

RCDs operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between the current flowing through the live conductor and that returning through the neutral conductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground, or to another circuit), and the device will open its contacts.

Residual current detection is complementary to over-current detection. Residual current detection cannot provide protection for overload or short-circuit currents, except for the special case of a short circuit from live to ground (not live to neutral).

Animated 3-phase RCD schematic.

For a RCD used with three-phase power, all live conductors and the neutral must pass through the current transformer.

Typical features

Opened 3-phase residual-current device

The diagram depicts the internal mechanism of a residual-current device (RCD). The device pictured is designed to be wired in-line in an appliance power cord. It is rated to carry a maximum current of 13 amperes and is designed to trip on a leakage current of 30 mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation.

The incoming supply and the neutral conductors are connected to the terminals at (1) and the outgoing load conductors are connected to the terminals at (2). The earth conductor (not shown) is connected through from supply to load uninterrupted.

When the reset button (3) is pressed the contacts ((4) and hidden behind (5)) close, allowing current to pass. The solenoid (5) keeps the contacts closed when the reset button is released.

The sense coil (6) is a differential current transformer which surrounds (but is not electrically connected to) the live and neutral conductors. In normal operation, all the current down the live conductor returns up the neutral conductor. The currents in the two conductors are therefore equal and opposite and cancel each other out.

Any fault to earth (for example caused by a person touching a live component in the attached appliance) causes some of the current to take a different return path which means there is an imbalance (difference) in the current in the two conductors (single phase case), or, more generally, a nonzero sum of currents from among various conductors (for example, three phase conductors and one neutral conductor).

This difference causes a current in the sense coil (6) which is picked up by the sense circuitry (7). The sense circuitry then removes power from the solenoid (5) and the contacts (4) are forced apart by a spring, cutting off the electricity supply to the appliance.

The device is designed so that the current is interrupted in milliseconds, greatly reducing the chances of a dangerous electric shock being received.

The test button (8) allows the correct operation of the device to be verified by passing a small current through the orange test wire (9). This simulates a fault by creating an imbalance in the sense coil. If the RCD does not trip when this button is pressed then the device must be replaced.

Form factors

A ground fault circuit interrupter circuit breaker (GFCI in USA and Canada) and residual-current breaker with overload (RCBO in Europe) are devices which combine the functions of a residual-current device with a circuit breaker. They detect both supply imbalance and overload current.

In Europe, RCDs can fit on the same DIN rail as the MCBs, however the busbar arrangements in consumer units and distribution boards can make it awkward to use them in this way. If it is desired to protect an individual circuit, an RCBO (Residual-current Circuit Breaker with Overcurrent protection) can be used. This incorporates an RCD and a miniature circuit breaker in one device.

Electrical plugs which incorporate an RCD are sometimes installed on appliances which might be considered to pose a particular safety hazard, for example long extension leads which might be used outdoors or garden equipment or hair dryers which may be used near a bath or sink. Occasionally an in-line RCD may be used to serve a similar function to one in a plug. By putting the RCD in the extension lead, protection is provided at whatever outlet is used even if the building has old wiring, such as knob and tube, or wiring that does not contain a grounding conductor.

GFI receptacles can be used in cases where there is no grounding conductor, but must be labeled as "Ungrounded". An ungrounded GFI receptacle will trip using the built in Test button, but will not trip using a GFI test plug, because the plug tests by shorting a small current from line to the non existent ground.

Electrical sockets with included RCDs are becoming common.

Combined with overcurrent devices

Residual-current and overcurrent protection may be combined in one device for installation into the service panel; this device is known as a GFCI breaker (Ground Fault Circuit Interrupter) in USA/Canada and as an RCBO (residual-current circuit breaker with overload protection) in Europe. In the US, RCBOs are more expensive than RCD outlets.

As well as requiring both line and neutral (or 3-phase) input and output, many GFCI/RCBO devices require a functional earth (FE) connection. For reasons of space some devices use flying leads rather than screw terminals, especially for the neutral input and FE connections.

More than one RCD feeding another is unnecessary, provided they have been wired properly. One exception is the case of a TT earthing system where the earth loop impedance may be high, meaning that a ground fault might not cause sufficient current to trip an ordinary circuit breaker or fuse. In this case a special 100 mA (or greater) trip current time-delayed RCD is installed covering the whole installation and then more sensitive RCDs should be installed downstream of it for sockets and other circuits which are considered high risk.

Testing

RCDs can be tested with the built-in test button to confirm functionality on a regular basis. RCDs if wired improperly may not operate correctly and are generally tested by the installer to verify correct operation. Use of a solenoid voltmeter from live to earth provides an external path and can test the wiring to the RCD. Such a test may be performed on installation of the device and at any "downstream" outlet.

Limitations

A residual-current circuit breaker cannot remove all risk of electric shock or fire. In particular, an RCD alone will not detect overload conditions, phase to neutral short circuits or phase-to-phase short circuits (see three phase electric power). Over-current protection (fuses or circuit breakers) must be provided. Circuit breakers that combine the functions of an RCD with overcurrent protection respond to both types of fault. These are known as RCBOs, and are available in 2, 3 and 4 pole configurations. RCBOs will typically have separate circuits for detecting current imbalance and for overload current but will have a common interrupting mechanism.

An RCD will help to protect against electric shock where current flows through a person from a phase (live / line / hot) to earth. It cannot protect against electric shock where current flows through a person from phase to neutral or phase to phase, for example where a finger touches both live and neutral contacts in a light fitting; a device can not differentiate between current flow through an intended load from flow through a person. Although the RCD may still trip if the person is in contact with the ground (earth) as some current will still pass through the persons finger and body to earth. Whole installations on a single RCD, common in the UK, are prone to 'nuisance' trips that can cause secondary safety problems with loss of lighting and defrosting of food. Invariably the trips are caused by deteriorating insulation on heater elements such as water heaters and cooker elements or rings. Although regarded as a nusiance the fault is with the deteriorated element and not the RCD: replacement of the offending element will resolve the problem; replacing the RCD will not.

In the case of RCDs that need a power supply, a dangerous condition can arise if the neutral wire is broken or switched off on the supply side of the RCD, while the corresponding live wire remains uninterrupted. The tripping circuit needs power to work and does not trip when the power supply fails. Connected equipment will not work without a neutral, but the RCD cannot protect people from contact with the energized wire. For this reason circuit breakers must be installed in a way that ensures that the neutral wire cannot be switched off unless the live wire is also switched off at the same time. Where there is a requirement for switching off the neutral wire, two-pole breakers (or four-pole for 3-phase) must be used. To provide some protection with an interrupted neutral, some RCDs and RCBOs are equipped with an auxiliary connection wire that must be connected to the earth busbar of the distribution board. This either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral.

Number of poles

RCDs may comprise two poles for use on single phase AC supplies (two current paths), three poles for use on three phase AC supplies (three current paths) or four poles for use on three phase & neutral supplies. RCDs can also be used with both AC alternating current and DC direct current power distribution systems.

Rated current

The rated current of an RCD is chosen according to the maximum sustained load current it will carry (if the RCD is connected in series with, and downstream of a circuit-breaker, the rated current of both items shall be the same).^{[citation needed]}

Sensitivity

RCD sensitivity is expressed as the rated residual operating current, noted I_Δn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their I_Δn value.

• High sensitivity (HS): 6 – 10 – 30 mA (for direct-contact / life injury protection)
• Medium sensitivity (MS): 100 – 300 – 500 – 1,000 mA (for fire protection)
• Low sensitivity (LS): 3 – 10 – 30 A (typically for protection of machine)

Note that the nominal value of residual current indicated above is not an absolute value. More information is presented below.

Type

Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the characteristics of the fault current.

• Type AC: RCD for which tripping is ensured
  • for residual sinusoidal alternating currents
• Type A: RCD for which tripping is ensured
  • as for type AC
  • for residual pulsating direct currents
  • for residual pulsating direct currents superimposed by a smooth direct current of 6 mA, with or without phase-angle control, independent of the polarity
• Type B: RCD for which tripping is ensured
  • as for type A
  • for residual sinusoidal currents up to 1 kHz
  • for residual sinusoidal currents superposed by a pure direct current
  • for pulsating direct currents superposed by a pure direct current
  • for residual currents which may result from rectifying circuits
    • three pulse star connection or six pulse bridge connection
    • two pulse bridge connection line-to-line with or without phase-angle monitoring, independently of the polarity

Break time

There are two groups of devices:

• G (general use) for instantaneous RCDs (i.e., without a time delay).
  • Minimum break time: immediate
  • Maximum break time: 200 ms for 1× I_Δn, 150 ms for 2× I_Δn, and 40 ms for 5× I_Δn
• S (selective) or T (time delayed) for RCDs with a short time delay (typically used in circuits containing surge suppressors)
  • Minimum break time: 130 ms for 1× I_Δn, 60 ms for 2× I_Δn, and 50 ms for 5× I_Δn
  • Maximum break time: 500 ms for 1× I_Δn, 200 ms for 2× I_Δn, and 150 ms for 5× I_Δn

Surge current resistance

The surge current refers to the peak current an RCD is designed to withstand using a test impulse of specified characteristics (an 8/20 µs impulse, named after the time constants of the rise and fall of current).

The IEC 61008 and IEC 61009 standards impose the use of a 0.5 µs/ 100 kHz damped oscillator wave (ring wave) to test the ability of residual-current protection devices to withstand operational discharges with a peak current equal to 200 A. With regard to atmospheric discharges, IEC 61008 and 61009 standards establish the 8/20 µs surge current test with 3 kA peak current but limit the requirement to RCDs classified as Selective.

History and nomenclature

The world’s first high-sensitivity earth leakage protection system (i.e. a system capable of protecting people from the hazards of direct contact between a live conductor and earth), was a second-harmonic magnetic amplifier core-balance system, known as the magamp, developed in South Africa by Henri Rubin. Electrical hazards were of great concern in South African gold mines, and Rubin, an engineer at the company F.W.J. Electrical Industries, initially developed a cold-cathode system in 1955 which operated at 525 V and had a tripping sensitivity of 250 mA. Prior to this, core balance earth leakage protection systems operated at sensitivities of about 10 A.

The cold cathode system was installed in a number of gold mines and worked reliably. However, Rubin began working on a completely novel system with greatly improved sensitivity, and by early 1956, he had produced a prototype second-harmonic magnetic amplifier-type core balance system (South African Patent No. 2268/56 and Australian Patent No. 218360). The prototype magamp was rated at 220 V, 60 A and had an internally adjustable tripping sensitivity of 12.5–17.5 mA. Very rapid tripping times were achieved through a novel design, and this combined with the high sensitivity was well within the safe current-time envelope for ventricular fibrillation determined by Charles Dalziel of the University of California, Berkeley, USA, who had estimated electrical shock hazards in humans. This system, with its associated circuit breaker, included overcurrent and short-circuit protection. In addition, the original prototype was able to trip at a lower sensitivity in the presence of an interrupted neutral, thus protecting against an important cause of electrical fire.

Following the accidental electrocution of a woman in a domestic accident at the Stilfontein gold mining village near Johannesburg, a few hundred F.W.J. 20 mA magamp earth leakage protection units were installed in the homes of the mining village during 1957 and 1958. F.W.J. Electrical Industries, which later changed its name to FW Electrical Industries, continued to manufacture 20 mA single phase and three phase magamp units.

At the time that he worked on the magamp, Rubin also considered using transistors in this application, but concluded that the early transistors then available were too unreliable. However, with the advent of improved transistors, the company that he worked for and other companies later produced transistorized versions of earth leakage protection.

In 1961, Dalziel, working with Rucker Manufacturing Co., developed a transistorized device for earth leakage protection which became known as a Ground Fault Circuit Interrupter (GFCI), sometimes colloquially shortened to Ground Fault Interrupter (GFI). This name for high-sensitivity earth leakage protection is still in common use in the U.S.A.^{[1][2][3][4][5]}

In the early 1970s most North American GFCI devices were of the circuit breaker type. GFCIs built into the outlet receptacle became commonplace beginning in the 1980s. The circuit breaker type, installed into a distribution panel, suffered from accidental trips mainly caused by poor or inconsistent insulation on the wiring. False trips were frequent when insulation problems were compounded by long circuit lengths. So much current leaked along the length of the conductors' insulation that the breaker might trip with the slightest increase of current imbalance. The migration to outlet receptacle based protection in North American installations reduced the accidental trips and provided obvious verification that wet areas were under electrical code-required protection. European installaions continue to use primarily RCDs installed at the distribution board, which provides protection in case of damage to fixed wiring; In Europe socket-based RCDs are primarily used for retro-fitting.

Regulation and adoption

Regulations differ widely from country to country. In most countries, not all circuits in a home are protected by RCDs. If a single RCD is installed for an entire electrical installation, any fault may cut all power to the premises.

Australia

In Australia, residual current devices have been mandatory on power circuits since 1991 and on light circuits since 2000.^[6]

Europe

Belgium

Belgian domestic installations are required to be equipped with a 300 mA residual current device that protects all circuits. Furthermore, at least one 30 mA residual current device is required that protects all circuits in "wet rooms" (e.g. bathroom, kitchen) as well as circuits that power certain "wet" appliances (washing machine, tumble dryer, dishwasher). Electrical underfloor heating is required to be protected by a 100 mA RCD. These RCDs must be of type A.

Italy

The Italian law (n. 46 March 1990) prescribes RCDs with no more than 30 mA residual current (informally called "salvavita" -- life saver) for all domestic installations to protect all the lines. The law was recently updated to mandate at least two separate RCDs for separate domestic circuits. Magnetic and thermal protection has been compulsory since 1968.

Germany

Germany requires the use of RCDs with no more than 30 mA on sockets up to 20 A which are for general use. This rule was introduced in June 2007 (DIN VDE 0100-410 Nr. 411.3.3).

Turkey

Turkey requires the use of RCDs with no more than 30 mA (hayati koruma rolesi) and 300 mA (yangın koruma rolesi) in all new homes since 2004. This rule was introduced in RG-16/06/2004-25494 .^[7]

Norway

In Norway, it has been required in all new homes since 2002, and on all new sockets since 2006.

United Kingdom

The previous 16th Edition of the IEE Electrical Wiring Regulations required use of RCDs for socket outlets that were liable to be used by outdoor appliances. Normal practice in domestic installations ^{[citation needed]} 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.

The current edition (17th) of the regulations requires that all socket outlets in most domestic installations have RCD protection, though there are exemptions. Cables buried in walls must also be RCD protected (again with some specific exemptions). {Refer to 17th Edition Amendment 1 effective from January 2012} 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 rating of no greater than 30 mA.

In certain commercial and industrial installations RCDs may be omitted.

North America

A Leviton GFCI "Decora^TM" receptacle in a North American kitchen. Local electrical code requires tamper-resistant receptacles in homes, and requires a GFCI for receptacles within 1 metre of a sink. The T-slot indicates this device is rated 20 amps and can take either a NEMA 5-15 or a NEMA 5-20 plug, though the latter type is rare on household appliances.

In North America, RCD (“GFCI”) receptacles invariably have rectangular faces and accept so-called decorator face plates. GFCI outlets can be mixed with regular outlets or with switches in a multigang box with a standard cover plate. Arc Fault Circuit Interrupters (AFCI) are required by current NEC code in locations such as sleeping rooms and incorporate GF protection. As a result, the slightly more expensive GFCI circuit breakers for load centers are less commonly used in residential applications.

In Canada and the United States, two-wire (ungrounded) (NEMA-1) outlets may be replaced with GFCIs to protect against electrocution, and a grounding wire does not need to be supplied to that GFCI. The outlet must be labeled as such. The GFCI manufacturers provide tags for the appropriate installation description. GFCI receptacles can be connected to also protect all the downstream receptacles on that circuit.

GFCI devices approved for protection against electric shock trip at 5 mA within 25 ms. A GFCI device which protects equipment (not people) is allowed to trip as high as 30 mA of current; this is known as an Equipment Protective Device (EPD). "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. These high-current RCDs serve for equipment and fire protection instead of protection against the risks of electrical shocks.

GFCI outlets are required by code in most places where an easy path to ground exists, such as wet areas, rooms with uncovered concrete floors and outdoor areas. In the U.S., successive editions of the National Electrical Code required GFCIs for additional areas: underwater swimming pool lights (1968); construction sites (1974); bathrooms and outdoor areas (1975); garages (1978); near hot tubs or spas (1981); hotel bathrooms (1984); kitchen counter receptacles (1987); crawl spaces and unfinished basements (1990); wet bar sinks (1993); and laundry sinks (2005).^[8]

See also

• Arc-fault circuit interrupter (AFCI)
• Earth leakage circuit breaker
• Insulation monitoring device
• Domestic AC power plugs and sockets

References

External links

Wikimedia Commons has media related to Residual current devices.

• GFCIs Fact Sheet (Consumer Product Safety Commission)
• Test of RCCB as per IEC 61008/61009 (Residual Current Device Testing)