RCD (Residual current device) is a mechanical switch with the residual current tripping characteristic attached to it. So basically it breaks the circuit only when there is a leakage current flowing to the earth. It is not clear when and by whom the first RCD was developed, but it certainly appeared on the market in the 1950s. RCDs are now widely used in most of the world; in some cases, this is mandatory, while in others their use is optional.
The following devices are all RCDs, equipped with additional functions and features, offering cost-effective solutions in different applications.
The main types of RCDs are:
1. AC type
AC-type RCDs are suitable for all systems where users have sinusoidal earth currents. They are not sensitive to impulsive leakage currents up to a peak of 250 A (8/20 waveform) such as those which may occur due to overlapping voltage impulses on the mains (e.g.: switching of fluorescent bulbs, X-ray equipment, data processing systems, and SCR controls).
2. A type
A type RCDs are not sensitive to impulsive currents up to a peak of 250 A (8/20 waveform). They are particularly suitable for protecting systems in which the user equipment has electronic devices for rectifying the current or phase cutting adjustment of a physical quantity (speed temperature, light intensity, etc.) supplied directly by the mains without the insertion of transformers and class I insulated (class II is, by definition, free of faults to earth).
These devices may generate a pulsating fault current with DC components which the A-type RCD can recognize.
3. A-APR type
In product standards for RCD, tests against unwanted tripping are the same both for voltage-dependent and voltage-independent RCDs. All the principal manufacturers have developed special voltage-independent RCDs with extremely high immunity against unwanted tripping.
Typical causes of unwanted RCD tripping could be:
– the presence of leakage current with modest value but with the high level of harmonics or high frequency;
– the presence of transient impulsive currents (e.g. usually caused opening and closing of capacitive or inductive loads);
– overvoltages caused by lightning;
– transient impulsive currents added to permanent leakage currents already present (eg. caused by electronic devices).
APR is more than ten times more resistant to unwanted tripping than standard types (both AC and A). APR RCDs can eliminate 99.9% of unwanted tripping! Unfortunately, there is not a classification in IEC or EN standards for these RCDs (the only exception is in Austrian national standard: they are called “Type G”).
4. F Type
Type F RCDs are for loads with single-phase inverters and similar equipment (e.g. modern washing machines), as an extension of type A. Type F RCDs, additional tests have been added to those for type A, in order to simulate the ground fault in presence of a single-phase inverter. Type F is characterized by a strong immunity to unwanted tripping.
Type F RCDs are not sensitive to impulsive currents up to a peak of 3,000A (8/20 waveform). Type F RCDs give better protection with the spread of modern electronic appliances in domestic installations, where type A RCDs could not properly cover them. It effectively fixes, in an “official” way, the problem of unwanted tripping with a non-selective RCD.
5. B type
Residual current protective devices of type B are used to detect smooth DC residual currents. Type B RCDs are recommended for use with drives and inverters for supplying motors for pumps, lifts, textile machines, machine tools, etc. since they recognize a continuous fault current with a low-level ripple.
Tripping values defined up to 100 kHz.
6. B+ type
Like type B residual current protection devices, type B+ residual current protection devices are suitable for use in alternating current systems. Tripping conditions for type B+ residual current protection devices are defined at up to 20 kHz and lie within this frequency range below a tripping value of 420 mA.
At the moment, type B+ is only introduced in a DIN VDE 0664-100 specification applicable in Germany. Type B+ is mostly used to prevent fire protection risk as they are recommended by the Association of German Insurance Companies.
7. Selective type
Selective RCDs have a delayed tripping action and are installed upstream of other rapid residual current operated circuit-breakers to guarantee selectivity and limit the power out only to the portion of the system affected by a fault.
The tripping time is not adjustable. It is set according to a predetermined time-current characteristic with an intrinsic delay for small currents, tending to disappear as the current grows.
8. Voltage independent type
Voltage-independent RCDs use the energy of the earth fault current to trip the mechanism directly. In this type of RCD, the output from the sensing coil operates a specially constructed magnetic relay and so releases the RCD mechanism, independently of the mains voltage.
Voltage-independent RCDs normally use a polarised (field weakening) relay construction. This operates by the cancellation of the permanent magnetic flux (which holds the relay ON) by the excitation flux (produced by the fault current). This can only occur in one half-cycle of the a.c. supply because the magnetic flux will be reinforced in the other half cycle. Operating times can vary from 20 to 120 ms at rated tripping current.
9. Voltage dependent type
Voltage-dependent RCDs generally employ an electronic amplifier to provide an enhanced signal from the sensing coil to operate a trip solenoid or relay. RCDs of this type is defined as “voltage-dependent” because they rely on a voltage source, derived from the mains supply, or an auxiliary supply, the amplifier with power.
The basic principle of operation is, however, the same as voltage-independent RCDs.
10. 30mA type
High-sensitivity RCDs (IΔn = 30mA) provide both protection against indirect contact hazards and additional protection against the dangers of direct contact. Because when a current higher than 30 mA passes through a part of a human body, there is serious danger for people if the current is not interrupted in a very short time.
11. 300mA type
The usage of 300 mA sensitivity RCDs provides good protection against fire risk due to this type of fault. Investigations have shown that the cost of fire damage in industrial and tertiary buildings can be very high. Some tests have shown that a very low leakage current (few mA) can evolve and, from 300 mA, it may set a fire in humid and dusty environments.
RCDs are very effective devices in providing protection against fire risk caused by insulation faults because they can detect leakage currents (ex: 300 mA) that are too low for other protective devices, but strong enough to set fire (i.e. overcurrent protection devices).