What Is a Residual Current Device (RCD) and How Does it Work?

In this article we will look at what a RCD is, its purpose, principle of operation and construction features.

What Is a Residual Current Device?

Residual current device (RCD): a mechanical switching device or association of devices designed to make, carry and break currents under normal service conditions and to cause the opening of the contacts when the residual current attains a given value under specified conditions [defined in the IEC 62873-2-2016].

A residual current device can be a combination of different individual elements designed to detect and evaluate residual current as well as to switch on and off the electric current.

In the electrical installation of buildings for protection against electric shock are widely used various RCDs household purpose, which comply with the requirements of IEC 61008-1-2013 and IEC 61009-1-2013. Household portable residual current devices which comply with IEC 61540:1997+AMD1:1998 CSV are also used in the electrical installations of buildings.

There are two types of RCD:

  1. residual current operated circuit-breaker without integral overcurrent protection (RCCB);
  2. residual current operated circuit-breaker with integral overcurrent protection (RCBO).

These terms identify two types of residual current devices which are commercially available and used in the electrical installation of buildings to provide protection against electric shocks.

However, in the regulatory requirements for their use in the electrical installations of buildings, the generalized name of the protective device is usually used – “residual current device”.

When it is necessary to emphasize that the RCD is intended, among other things, for overcurrent protection of electric circuits connected to it, we speak of a residual current device with built-in overcurrent protection. Such a RCD performs the same functions as a RCBO. Otherwise we speak about a residual current device without built-in overcurrent protection, the equivalent of which is a RCCB.

Purpose

The term “residual current device” is used in international and national standards to refer to a generalized protective device or set of devices, each of which performs the following three operations:

  1. detection of residual current in its main circuit, which appears when the main insulation of any hazardous-live-part that is part of the electrical circuits it protects is damaged and short-circuited to earth;
  2. comparison of the detected residual current with the value of the residual tripping current;
  3. disconnection of the electric circuits it protects in case the residual current in the main circuit exceeds the value of the residual tripping current.

A residual current device should trip the circuits it protects only under single or multiple fault conditions when a earth-fault current begins to flow. A RCD should also trip when electrical equipment is used carelessly, when a person touches a live part and a earth-fault current flows through his or her body. The RCD should not trip under normal, non-earth-fault conditions.

Earth-fault current can result from a fault in the main insulation of a hazardous-live-part in electrical circuits connected after of the RCD. A fault in the main insulation of a hazardous-live-part is usually accompanied by a short circuit to an exposed-conductive-part of a class I electrical appliance.

From the hazardous-live-part, the earth-fault current flows to the exposed-conductive-part. This current then flows from the exposed-conductive-part of the electrical equipment into the protective conductor and then through the building installation earthing arrangement into the earth.

How Does an RCD Work?

In this article, for simplicity and understanding, I will discuss the construction and operation of the RCD in ideal electrical circuits, in which there are no leakage currents. However, in electrical circuits of the electrical installation of buildings, there are always leakage currents that can cause false tripping of the RCD. To reduce the probability of false tripping of residual current devices, their characteristics should be coordinated with the characteristics of electrical circuits which are connected to the RCD.

In electrical circuits with normal (intact) insulation of live parts, there is always a leakage current. In TN-C, TN-S, TN-C-S and TT systems the leakage current is negligible compared to the earth-fault current. With a large number of simultaneously connected class I appliances, however, their total leakage current may exceed the rated residual operating current of the residual current device, thus triggering its automatic operation.

In order to avoid false operation of the RCD, its rated tripping differential current IΔn must be greater than the total leakage current in the circuits connected to the RCD IEL.

So, any RCD has a differential (summation) transformer. With its help it determines the residual current IΔ, which is the effective value of the vector sum of electric currents flowing in the conductors of its main circuit and breaks this circuit when IΔ exceeds or equals the set value (the rated residual operating current IΔn).

That is, the tripping condition of the RCD is as follows: IΔ ≥ IΔn.

The differential transformer is thus the key element by means of which the occurrence of a earth-fault current, which creates a real danger to humans and animals, can be monitored.

It should be added that IΔn is set by the manufacturer of the device and is usually indicated on its case, for example, IΔn = 0.03 A for a household RCD.

Consider an example of the operation of a two-pole RCD used in single-phase electrical circuits.

The differential transformer in it has two primary windings, made by two conductors of its main circuit, and one secondary winding, to which the residual current release is connected, causing the RCD to trip with or without time delay when IΔ ≥ IΔn.

Figure 1 illustrates the operation of the RCD differential transformer under normal conditions and under faulty circuit conditions:

Operating principle of the RCD
Figure 1. Operating principle of the RCD

Figure 1 shows:

  • I1 and I2 – currents in the primary windings of the differential transformer;
  • Ip – current in the secondary winding of the differential transformer;
  • IH – load current;
  • IEF – earth-fault current;
  • Ф1 and Ф2 – magnetic fluxes in the core of the differential transformer;
  • DTR – RCD differential transformer release;
  • H – class I electrical equipment.

Normal Operating Conditions of an Electric Circuit

Consider normal circuit conditions in which there is no fault in the main insulation of hazardous-live-parts and no earth fault. That is, there is no earth-fault current flowing through the main circuit of the RCD because there is no earth fault in the electrical circuit.

In both conductors of the main circuit of the RCD flow electrical currents equal in their absolute value to the load current IH.

From the above we get:

| I1 |= | I2 |= | Iн |

Therefore, the vector sum of these electric currents is zero:

IΔ = | I1 – I2 | = 0

The magnetic fluxes Ф1 and Ф2, created by the electric currents I1 and I2 in the core of the differential transformer, are also directed towards each other and are equal to each other in absolute value: | Ф1 | = | Ф2 |.

The magnetic fluxes Ф1 and Ф2 cancel each other out. Therefore the total magnetic flux in the core of the differential transformer is zero: ФΔ = | Ф1 – Ф2 | = 0

As a result, the absolute value of the electric current that can flow in the circuit connected to the secondary of the differential transformer will also be zero: | Ip | = 0

Under these conditions the DBT, which is connected to the secondary of the differential transformer, cannot trip. Therefore, under normal circuit conditions the RCD does not open its main circuit contacts and therefore does not disconnect the external circuits connected to it.

As a result, under normal electrical circuit conditions, the RCD does not trip and therefore does not disconnect external electrical circuits connected to it.

Fault Conditions

Under fault conditions in an electrical circuit, the main insulation of the hazardous live part is faulted and shorted to earth.

In this situation, one of the conductors of the main circuit RCD is flowing a earth-fault current IEF in addition to the load current IH. Therefore the absolute value of the electric current flowing in one of the primary windings of the differential transformer exceeds the absolute value of the electric current flowing in its other primary winding: | I1 | > | I2 |.

The vector sum of electric currents in the main circuit conductors of the residual current device will be non-zero: IΔ=| I1 – I2 |=| IH + IEF – IH | = | IEF |.

That is, in fact, in this situation, the residual current will be equal in absolute value to the earth-fault current.

Consequently, the residual current is used to monitor the occurrence of earth-fault current, which poses a real danger to humans, especially when it flows through their body.

The magnetic fluxes Ф1 and Ф2 in the core of the differential transformer, directly proportional to the electric currents I1 and I2, are not equal to each other in absolute value: | Ф1 | > | Ф2 |.

They cannot compensate each other, so the total magnetic flux in the core of the differential transformer is different from zero: ФΔ = | Ф1 – Ф2 | > 0.

The absolute value of the electric current that flows in the circuit connected to the secondary of the differential transformer will also be greater than zero. | Ip | > 0.

Under these conditions, the residual current release can be actuated by an electric current Ip, causing the RCD to open its main contacts and disconnect the external electric circuits connected to it.

Thus, under conditions of single or multiple faults, the residual current device opens its main circuit contacts and disconnects the external electric circuits connected to it.

Three-pole and four-pole RCDs are used in three-phase electrical circuits and are equipped with differential transformers with three and four primary windings respectively. These differential transformers function in the same way as a two-pole RCD differential transformer. The vector sums of the electric currents flowing in the main circuits of the RCD are determined by taking into account the lag and phase advance of the electric currents in the conductors connected to the RCD.

Nuances of RCD Operation in Real Electrical Circuits

In electrical circuits with normal (intact) insulation of live parts, there is always a leakage current. In TN-C, TN-S, TN-C-S and TT systems the leakage current is negligible compared to the earth-fault current. However, with a large number of simultaneously connected class I appliances their total leakage current can exceed the tripping residual current of the residual current device, thus triggering its automatic tripping.

To ensure that no false operations are performed on the RCD, its residual operating current I must be greater than the total leakage current IEL of the circuits connected to the RCD.

RCD Construction

The construction of the residual current device (see Figure 2) is thus specifically oriented to detect and evaluate the earth-fault current IEF in conjunction with the leakage current IEL by determining the residual (total) current in the RCD main circuit conductors, which is done through its differential transformer placed between the input and output terminals of the RCD.

RCD diagram
Figure 2. RCD diagram

Figure 2 shows:

  • 1 – power supply neutral earthing arrangement;
  • 2 – earthing arrangement of the electrical installation of a building;
  • 3 – RCD main contacts;
  • 4 – RCD tripping mechanism;
  • 5 – RCD differential current release;
  • 6 – RCD differential transformer;
  • 7 – RCD terminals;
  • 8 – RCD control device electrical circuit;
  • 9 – Class I electrical appliance.

The residual current device mechanism compares the residual current in the main circuit RCD with the residual tripping current.

If the residual current exceeds or equals the tripping residual current of the RCD, it will trip the circuits to be protected. To perform the latter two operations the residual current device has a residual current release connected to the secondary winding of the differential transformer.

An RCD can be made as a product that is a combination of several devices. For example, a household RCBO, as shown in Fig. 3, may be assembled from a residual current unit according to IEC 61009-1-2013 Annex G and a circuit breaker according to IEC 60898-1-2015. In this RCBO, the circuit breaker disconnects the electrical circuits both when overcurrents are flowing in its main circuit and when a trip command is given by the differential current unit.

RCBO diagram
Figure 3. Schematic diagram of an RCBO assembled from a residual current unit and a circuit breaker

Figure 3 shows:

  • 1 – power supply neutral earthing arrangement;
  • 2 – earthing arrangement of the electrical installation of a building;
  • 3 – the main contacts of the circuit breaker;
  • 4 – the terminals of the circuit breaker;
  • 5 – residual current release of the residual current unit;
  • 6 – differential transformer of the residual current unit;
  • 7 – mechanism for breaking the residual current unit;
  • 8 – electrical circuit of the control device of the residual current unit;
  • 9 – terminals of the residual current unit;
  • 10 – class I electrical equipment.

Let us take a closer look at the design of household differential current devices, which are manufactured in accordance with the requirements of IEC 61008-1 and IEC 61009-1 standards.

A residual current device has a main circuit and may have a control circuit and an auxiliary circuit. The main circuit combines all conductive parts of the RCD included in the electrical circuit which it is designed to close and open.

The control circuit of the residual current device is designed to perform closing and opening or performing both operations. This circuit includes the conductive parts of the RCD which are used to control it, except for those parts which are part of the main circuit of the RCD. The control circuit includes a control device circuit by means of which periodic monitoring of the RCD is carried out.

An auxiliary circuit comprises all conductive parts of a residual current device which are intended to be included in an electrical circuit used, for example, for remote indication of its switching position. This circuit does not include the conductive parts of the RCD which are part of its main and control circuit.

To equip a residual current device with a control circuit (other than a control circuit) and an auxiliary circuit, one or more auxiliary devices, such as a auxiliary switch, an shunt release and a under-voltage release, must be attached to the residual current device.

A auxiliary switch is a switch with one or more control contacts and/or auxiliary contacts that is mechanically actuated by a residual current device.

Shunt release and under-voltage release are used to control a residual current device.

The main circuit of a residual current device usually consists of two, three or four poles. Pole means the part of the RCD connected exclusively to one electrically independent conductive path of its main circuit, equipped with contacts designed to close and open the main circuit, excluding those parts which provide the means to mount and operate all poles together.

Two-pole residual current devices are the most widely used in electrical installations of buildings, designed for use in single-phase two-wire electrical circuits, and four-pole RCDs, which are used in three-phase four-wire electrical circuits. Three-pole RCDs are available for three-phase three-wire electrical circuits and have a smaller application area than four-pole devices because such circuits are used much less frequently.

RCBOs are equipped with protected poles to perform the overcurrent protection function. The remaining pole of the RCBO, if any, can be an unprotected pole or a switched neutral pole. The protected pole is equipped with an overcurrent release the same as the circuit breaker. The unprotected pole does not have an overcurrent release, but is otherwise capable of the same operation as the protected pole of the same RCBO. The switched neutral pole is designed to switch the electrical circuit of the neutral conductor, but is not designed to have short-circuit switching capability.

There are main contacts in the main circuit of each pole of a residual current device. The main contact is a contact that is included in the main circuit of the RCD and is designed to conduct the electric current flowing in its main circuit in the closed position.

When opening the main circuit of a RCD, through which electric current flows (especially – overcurrent), arcing may occur between the disconnected parts of the main contacts. Therefore, RCBOs, and often RCCBs as well, are equipped with arcing contacts on which arcing is expected to occur.

The arcing contact may be the main contact, or it may be a separate contact that opens later and closes before the other main circuit contact it protects from arc fault. With RCBOs, just as with circuit breakers, the arcing contacts are usually the main contacts. The main contact has a special design of conductive parts, which ensures that the arc moves into the arc chute, where it is broken into several parts by metal plates and is intensively extinguished.

In a multi-pole residual current device, the moving contacts of all poles (except for the switched neutral pole) must make and break the main circuit almost simultaneously during both automatic and manual operation. The contacts of the switched neutral pole must open later and close before the contacts of the other poles of the RCD.

In the control circuit of the residual current device there are control contacts which are mechanically actuated by the same RCD. The auxiliary contacts, if used, are part of the auxiliary circuit of the residual current device and are mechanically actuated by the same RCD.

Each RCD is equipped with one or more releases that are designed to initiate:

  1. automatic opening of main contacts in case of earth fault current (RCCB and RCBO) or overcurrent (RCBO) in the main circuit;
  2. the RCCB or RCBO automatically opens when the voltage drops or other characteristics of the electrical circuits and equipment connected to it change;
  3. RCCB or RCBO remote shutdown.

The release is a device mechanically connected to or integrated into the residual current device, which releases the holding device in the RCD mechanism and initiates its automatic opening.

In order to compare the residual current in the main circuit of the residual current device with the tripping current and to issue a command to open the main contacts, a residual current release is installed in the RCD. The residual current release initiates an automatic tripping of the RCD if the residual current in its main circuit exceeds or equals the tripping current of the RCD.

RCD Types

The following residual current devices are manufactured according to IEC 61008-1 and IEC 61009-1:

  • AC-type RCDs, whose proper triggering occurs only with sinusoidal alternating residual currents, either applied spiking or slowly rising;
  • Type A RCDs, whose proper triggering occurs with both sinusoidal alternating residual currents and pulsating direct residual currents, either applied by leaps and bounds or slowly rising.

IEC 624231 establishes additional requirements to IEC 61008-1 and IEC 61009-1, according to which Type F and Type B RCDs are available. Type F RCDs are designed to protect electrical circuits to which frequency converters are connected. They operate in the same way as type A RCDs and additionally:

  • at complex residual currents either applied by jumping or slowly growing;
  • at a pulsating direct residual current superimposed on a smoothed direct current of 0.01 A.

Type B RCDs are operated in the same way as Type F RCDs, and additionally:

  • at sinusoidal alternating residual currents having a frequency up to and including 1000 Hz;
  • at pulsating direct residual currents appearing in two or more phases;
  • at smoothed direct residual currents either applied by leaps and bounds or slowly increasing.

Depending on the presence of a time delay (in the presence of a tripping residual current), there are residual current devices without time delay – type for general applications and RCDs with time delay – type S for selective operation. S-type residual current devices are specially designed for selective operation when connected in series with RCDs for general applications.

References

  1. IEC 62873-2-2016
  2. IEC 61008-1-2013
  3. IEC 61009-1-2013
  4. IEC 61540:1997+AMD1:1998 CSV
  5. IEC 60050-442:1998/AMD3:2019