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Residual-current device
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A residual current device (RCD), similar to that of a residual current circuit breaker (RCCB), 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 is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit.
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Encyclopedia
A residual current device (RCD), similar to that of a residual current circuit breaker (RCCB), 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 is sometimes caused by 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. RCDs are designed to disconnect quickly enough to mitigate the harm caused by such shocks although they not intended to provide protection against overload or short-circuit conditions.
In the United States and Canada, a residual current device is also known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). In Australia they are sometimes known as "safety switches" or simply "RCD"(or "RCDs" for the plural); in Italy they are also called "salvavita".
Purpose and operation
RCDs are designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5-30 milliamperes) than the currents needed to operate conventional circuit breakers or fuses (several amperes). RCDs are intended to operate within 25-40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.
In the United States, the National Electrical Code requires GFCI devices intended to protect people to interrupt the circuit if the leakage current exceeds a range of 4-6 mA of current (the trip setting is typically 5 mA) within 25 milliseconds. GFCI devices which protect equipment (not people) are allowed to trip as high as 30 mA of current. In Europe, the commonly used RCDs have trip currents of 10-300 mA.
RCDs operate by measuring the current balance between two conductors using a differential current transformer. The device will open its contacts when it detects a 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 (to earth/ground, or to another circuit, etc.).
Residual current detection is complementary to over-current detection. Residual current detection cannot provide protection for overload or short-circuit currents.
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 more as an additional fire-safety protection than as an effective protection against the risks of electrical shocks.
In some countries, two-wire (ungrounded) outlets may be replaced with three-wire GFCIs to protect against electrocution, and a grounding wire does not need to be supplied to that GFCI, but the outlet must be tagged as such. The GFCI manufacturers provide tags for the appropriate installation description.
Example
The photograph 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 doesn't latch mechanically and therefore trips out on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation.
The incoming supply live and the grounded 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 a fraction of a second, 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.
Rules and regulations
Rules and regulations differ widely from country to country. In Europe, the UK has only mandated the use of RCDs in new installations since July 2008. In contrast, Germany requires the use of RCDs on all sockets up to 20A which are for general use. This rule was introduced in June 2007 (DIN VDE 0100-410 Nr. 411.3.3). In Norway, it has been required in all new homes since 2002, and on all new sockets since 2006 ().
In the U.S., the National Electrical Code requires GFCIs in bathrooms, kitchens (near the sink, and specifically not including the refrigerator outlet, which is usually on a dedicated circuit), garages, outdoor areas, crawl spaces, unfinished basements, near wet bars, swimming pools, and spas in residential construction. In Australia they are required on all power circuits, and all new lighting circuits.
Use and placement
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 will cut all power to the premises. Normal practice in domestic installations in the UK is to use a single RCD for all RCD protected circuits but to have some circuits that are not protected at all (sockets usually are on the RCD; lamp holders usually aren't; other circuits vary by who installed the system). GFI receptacles in the USA have connections to protect downstream receptacles so that all outlets on a circuit may be protected by one GFI outlet.
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 in the US and as an RCBO in Europe. In the US, RCBOs are more expensive than RCD outlets.
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 mis-wired 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. Over-current protection (fuse or circuit breaker) 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 1, 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.
Whole installations on a single RCD, common in the UK, are prone to nuisance trips that can cause safety problems with loss of lighting and defrosting of food. RCDs also cause nuisance trips with appliances where earth leakage is common and not a cause of injury or mortality, such as water heaters.
History and nomenclature
In the early 1970s most GFCI devices were of the circuit breaker type. However the most commonly used GFCIs since the early 1980s are built into outlet receptacles. The problem with those of the circuit breaker type was that of many false trips due to the poor alternating current characteristics of 120 volt insulations, especially in circuits having longer cable lengths. So much current leaked along the length of the conductors' insulation that the breaker might trip with the slightest increase of current imbalance.
Types
A Residual Current Breaker with Overload (RCBO) is a combination of an RCD and a miniature circuit breaker (MCB).
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.
It is common to install an RCD in a consumer unit in what is known as a split load configuration where one group of circuits is just on the main switch (or time delay RCD in the case of a TT earth) and another group is on the RCD.
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 tub 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 you provide protection at whatever outlet is used even if the building has old wiring.
Electrical sockets with included RCDs are becoming common. In the U.S. these are required by law in wet areas (See National Electrical Code (US) for details.)
In North America, RCD ("GFCI") sockets are usually of the decora size (a size that harmonizes outlets and switches, so that there is no difference in size between an outlet cover and a switch cover). For example, using the decora size outlets, RCD outlets can be mixed with regular outlets or with switches in a multigang box with a standard cover plate.
Active/passive latching/non-latching
RCDs may be obtained that have different behaviours if the circuit they are protecting is de-energised.
- One type will trip on power failure and not re-make the circuit when the circuit is re-energised. This type is known as non-latching or active.
- Another type will re-make the circuit when the circuit is re-energised. This type is known as latching or passive.
The first type is used when the power-drawing equipment is regarded as a safety hazard if it is unexpectedly re-energised after a power failure e.g. lawn-mowers and hedge trimmers.
The second type may be used on equipment where unexpected re-energisation after a power failure is not a hazard. An example may be the use of an RCD on a circuit providing power to a food freezer, where having to reset an RCD after a power failure may be inconvenient.
Main characteristics
The following key parameters determine the RCD:
- Number of poles [2P or 3P or 4P]
- Rated current [in A]
- Sensitivity [in mA]
- Type [AC or A or B]
- Break time [in ms]
- Surge current resistance [in A]
Number of poles [2P, 3P or 4P]
RCDs may comprise two poles for use on single phase supplies (two current paths), three poles for use on three phase supplies (three current paths) or four poles for use on three phase & neutral supplies (four current paths).
Rated current [in A]
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).
Sensitivity [in mA]
RCD sensitivity is expressed as the rated residual operating current, noted I?n. Preferred values have been defined by the IEC,
thus making it possible to divide RCDs into three groups according to their I?n value.
- High sensitivity (HS): 6 – 10 – 30 mA (for direct-contact / life injury protection),
- Medium sensitivity (MS): 100 – 300 – 500 – 1000 mA (for fire protection),
- Low sensitivity (LS): 3 – 10 – 30 A (typically for protection of machines).
It should be borne in mind that nameplate rating and real trip current are not necessarily the same. For example UK 30mA RCDs must trip at an imbalance current lower than 30mA.
Type [AC or A or B]
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:
- for residual sinusoidal alternating currents,
- for residual pulsating direct currents,
- for residual pulsating direct currents superimposed by a smooth direct current of 0.006 A, 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 1000 Hz,
- 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, i.e.:
- 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 [in ms]
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 1x I?n, 150 ms for 2x I?n, and 40 ms for 5x I?n;
- 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 1x I?n, 60 ms for 2x I?n, and 50 ms for 5x I?n.
- Maximum break time: 500 ms for 1x I?n, 200 ms for 2x I?n, and 150 ms for 5x I?n;
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 3000 A peak current but limit the requirement to RCDs classified as Selective.
Important note regarding TN-C earthing system
According to IEC 60364:
- RCD should not be used in a TN-C system (per definition, RCD would not provide necessary protection in circuits with L-PEN wiring)!
- When an RCD is used in a TN-C-S system, a PEN conductor should never be used downstream (instead, separate PE and N wiring is required).
See also
External links
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