Types of RCDs

From Electrical Installation Guide

The fault residual current could take various waveforms depending on the load characteristics. The following types of RCDs are defined in IEC 60755, for suitable protection of different forms of residual current:

Type AC DB431079.png

Type AC RCDs detect residual sinusoidal alternating currents. Type AC RCDs are suitable for general use and cover most of the applications in practice.

Type A DB431080.png

In addition to the detection characteristics of type AC RCDs, Type A RCDs detect pulsating DC residual current. Such waveforms can be caused by diode or thyristor rectifier circuit in electronic loads. Type A RCD are specifically intended to be used for single phase class 1 electronic loads.

Type F DB431081.png

Type F RCDs is a new RCD type recently introduced in IEC 62423 and in IEC60755. In addition to the detection characteristics of type A RCDs, type F RCDs are specially designed for circuit protection where single phase variable speed drives could be used. In these circuits, the waveform of residual current could be a composite of multi-frequencies including motor frequency, convertor switching frequency and line frequency. For the reason of energy efficiency, the use of frequency converters in certain loads (washing machine, air conditioner, …) is increasing, and type F RCD will cover those new applications.

Type F also has enhanced disturbance withstand characteristics (non-tripping on surge current). They are capable of tripping even if a pure direct current of 10 mA is superimposed on a sinusoidal or pulsed DC differential current.

Type B DB431082.png

Type B RCDs can detect sinusoidal AC, pulsating DC, composite of multi-frequency as well as smooth DC residual currents. In addition, tripping conditions are defined with different frequencies from 50Hz to 1kHz. In an AC electrical distribution network, a pure DC residual current can be mainly generated from three-phase rectifying circuits, but also from some specific single phase rectifiers.

Type B RCD are intended to be used for loads with three-phase rectifier, such as variable speed drives, PV system, EV charging station and medical equipment.

Fig. F51 summarizes the definition of different types of RCD, with their main application and waveforms. It has to be noted that the different types of RCDs (AC, A, F and B) are nested one within the other like Russian dolls: type B, for example, also complies with the requirements of type F, type A and type AC.

Fig. F51 – Different types of RCDs

Annex B of IEC 60755 gives, for different electronic load architectures, the possible load and residual current waveforms, and proposes the suitable types of RCD for each case. (see Fig. F52)

Fig. F52 – Possible fault currents in electronic loads and suitable RCD types - IEC 60755
Circuit diagram with fault location Shape of line current IL Shape of earth fault current IF RCD tripping characteristic
1 Phase control DB431040.png DB431041.png DB431042.png AC, A, F, B
2 Burst control DB431040.png DB431043.png DB431044.png AC, A, F, B
3 Single-phase DB431045.png DB431046.png DB431047.png A, F, B
4 Two-pulse bridge DB431048.png DB431049.png DB431053.png A, F, B
5 Two-pulse bridge, half controlled DB431050.png DB431041.png DB431051.png A, F, B
6 Frequency inverter with two-pulse bridge DB431052.png DB431049.png DB431053.png
F, B
7 Single-phase with smoothing DB431055.png DB431056.png DB431057.png B
8 Frequency inverter with two-pulse bridge and PFC DB431058.png DB431049.png DB431053.png
9 Two-pulse bridge between phases DB431060.png DB431049.png DB431061.png B
10 Frequency inverter with two-pulse bridge between phases DB431062.png DB431049.png DB431063.png
11 Three-phase star DB431065.png DB431066.png DB431067.png B
12 Six-pulse bridge DB431068.png DB431069.png DB431067.png B
13 Frequency inverter with six-pulse bridge DB431070.png DB431071.png DB431072.png