Residual Current Devices 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 live 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.
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.
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.
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.
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
IΔn . Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value.
High sensitivity (
HS): 10 – 30 mA (for direct-contact / life injury protection),
Medium sensitivity (
MS): 100 – 300 – 500mA (for fire protection),
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 ] 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:
Type A:
|
RCD for which tripping is ensured for residual sinusoidal alternating currents.
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.
1. three pulse star connection or six pulse bridge connection,
2. two pulse bridge connection line-to-line with or without phase-angle monitoring, independently of the polarity. |
