Residual Current Unit: What it Is, Characteristics, Features

A residual current unit (r.c. unit) is a device performing simultaneously the functions of detection of the residual current and of comparison of the value of this current with the residual operating value, and incorporating the means of operating the tripping mechanism of a circuit-breaker with which it is designed to be assembled [IEC 61009-1-2013].

General Features

The term “residual current unit” is used in IEC standards and other documents to refer to a unit which is a part of a residual current device. The residual current unit performs the following two operations:

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

The third operation is the disconnection of the electric circuits it protects in case the residual current in the main circuit exceeds the value of the residual current of tripping – the residual current unit itself can not perform, since it is not equipped with main contacts. The circuit breaker to which the r.c. unit is connected makes the tripping of the electrical circuits. The circuit breaker and the residual current unit put together form a residual current device called a residual current operated circuit-breaker with integral overcurrent protection (RCBO).

In such an RCBO, the circuit breaker disconnects electrical circuits not only when an overcurrent occurs in them, but also when its mechanism is commanded to open the main contacts by the residual current unit. For this purpose, the r.c. unit is equipped with a special actuator which acts on a holding device in the circuit breaker mechanism. The holding mechanism frees the main contacts of the circuit breaker and they begin to open on their own.

The RCBO under consideration is assembled from the residual current unit and the circuit breaker before installing it in the low-voltage switchgear. The mechanical attachment of the residual current unit to the circuit breaker is usually accomplished with special latches, and the electrical connection is made with single or stranded copper conductors that are part of the r.c. unit design. Since the above assembly can only be done once, disassembly of the RCBO must be accompanied by visible mechanical faults. Residual current units and circuit breakers that are intended to be assembled together must bear the same manufacturer’s name or trademark.

The degree of protection of the r.c. unit shall be no less than that of the circuit-breaker with which it is to be assembled.

The circuit-breaker and the r.c. unit shall fit together readily in the correct manner, and the design shall be such as to prevent an incorrect assembly.

The terminals of the r.c. unit shall be able to clamp the range of nominal cross-sections of conductors specified in Table 5 of IEC 60898-1:2019 for the rated currents of the circuit-breakers with which it is designed to be assembled.

The electrical interconnections between the r.c. unit and the circuit-breaker with which it is to be assembled shall form part of the r.c. unit.

lt shall not be possible to assemble a circuit-breaker of a given rated short-circuit capacity with an r.c. unit so as to result in a lower short-circuit performance.

Marking of the R.C. Unit

The r.c. unit shall be marked with the following items: a), b), c), e), f), g), k), m), n), q) and, if necessary, l). These items contain information that must be marked. These items are described in detail in the article: How to Read Residual Current Device (RCD) Markings?

In addition the r.c. unit shall be marked with:

  • the maximum rated current of the circuit-breaker with which it may be assembled (e.g. 63 A max.);
  • the symbol Special symbol of the residual current unit .

Constructional Features

Two-, three-, and four-pole household residual current units (IEC 61009-1 standard) are commercially available, which are connected respectively to two-, three-, and four-pole household circuit breakers (IEC 60898-1 standard) to form two-, three-, and four-pole RCBOs. Firms usually make two sizes of residual current units. Residual current units of the first size are used with circuit breakers up to and including 63 A, r.c. units of the second size up to 100 A. A generalized view of the residual current units manufactured and their approximate dimensions are shown in Figs. 1 and 2.

Residual current units with rated currents up to 63 A
Figure 1. Residual current units with rated currents up to 63 A: 1 – two-pole; 2 – three-pole; 3 – four-pole
Residual current units with rated currents up to 125 A
Figure 2. Residual current units with rated currents up to 100 A: 1 – two-pole; 2 – four-pole

The r.c. unit and compatible circuit-breakers as declared by the manufacturer have to be assembled and disassembled five times. After each assembly the correct operation of the combination shall be verified by using the test button. The RCBO shall trip each time.

Some Characteristics of Residual Current Units

The residual current units are designed for use in AC electrical circuits with a frequency of 50 and 60 Hz.

The rated voltage Un of two-pole r.c. units is 230 V, three-pole and four-pole units is 400 V. It shall not be possible to assemble a circuit-breaker of a given rated voltage with an r.c. unit of a lower rated voltage.

The rated current In of the residual current units is usually 25, 40, 63, 80 and 100 A. They are used together with circuit breakers having a rated current not exceeding the rated current of the r.c. units. It shall not be possible to assemble a circuit-breaker of a given rated current with an r.c. unit of a lower maximum current marking.

The residual current units are designed to work with circuit breakers which are usually of the instantaneous trip type B and C. Some companies allow the connection of r.c. units with circuit breakers of instantaneous trip type D.

Most of the residual current units are of the general purpose type. S-type residual current units are available in limited quantities. Some companies manufacture residual current units which have a short time delay, usually not exceeding 0.01 s. The use of such r.c. units can greatly reduce the number of false RCBOs due to large, but short-lived inrush currents.

The largest number of residual current units have a rated residual operating current (IΔn) of 0.03 A. Residual current units in general use with rated residual operating currents of 0.10 A, 0.30 A and 0.50 A are available in limited quantities.

S-type residual current units usually have a rated residual operating current of 0.30 A. Small quantities of r.c. units of type S are available with a rated residual operating current of 0.10 A, 0.50 A and 1.00 A.

Type AC residual current units, which operate with sinusoidal alternating residual currents, and type A residual current units, which also operate with pulsating direct residual currents, are manufactured in series. Some companies manufacture F-type residual current units, which operate in the same way as A-type residual current units and, additionally, with complex forms of residual currents.

General-purpose residual current units are resistant to current pulses with a peak value of up to 250 A. S-type residual current units have a higher resistance to electric current pulses. They do not trip with current pulses up to 3000-5000 A peak-to-peak.

An approximate nomenclature of the residual current units manufactured in series by different companies according to IEC 61009-1 is given in the table below. Modifications of r.c. units are marked with “+”. A “-” means that r.c. units with the specified characteristics are generally not manufactured.

Table 1: Approximate nomenclature of serially manufactured r.c. units of type AC and type A
In, А IΔn, mА Two-pole r.c. units Three-pole r.c. units Four-pole r.c. units
General use Type S General use Type S General use Type S
25 10 +
40, 63, 80, 100 30 + + +
100 + + + + + +
300 + + + + + +
500 + + + + + +

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