RCBO – Residual Current Operated Circuit-Breaker with Integral Overcurrent Protection

What Is an RCBO?

The term ‘residual current operated circuit-breaker with integral overcurrent protection, RCBO (abbreviation)’ is officially defined within the IEC 61009-1-2013 as:

residual current operated circuit-breaker designed to perform the functions of protection against overloads and/or short-circuits.

IEC 61009-1-2013

Currently, there continues to be confusion over terminology, as many incompetent persons often use the general term “RCD” instead of the correct and specific term “RCBO”.

A residual current operated circuit-breaker with integral overcurrent protection is a type of residual current device which has an integrated overcurrent protection. Therefore, RCBOs are used for overload and short-circuit protection along with overcurrent protection devices, which are circuit breakers and fuses. In all other respects, the RCBO functions in the same way as the RCD.

These devices are intended to protect people against indirect contact, the exposed conductive parts of the installation being connected to an appropriate earth electrode and to protect against overcurrents the wiring installations of buildings and similar applications. They may be used to provide protection against fire hazards due to a persistent earth fault current, without the operation of the overcurrent protective device.

RCBOs having a rated residual operating current not exceeding 30 mA are also used as a means for additional protection in the case of failure of the protective means against electric shock.

The RCBOs under consideration are intended for use by ordinary persons and do not require maintenance. They can be used as disconnectors.

Example of a three-pole RCBO with a rated current of 63 A
Figure 1 – Example of a three-pole RCBO with a rated current of 63 A

How Does an RCBO Work?

For detailed information on the principle of working of any residual current device (including the RCBO), read the article: Understanding Residual Current Devices (RCDs).

Requirements

International requirements for household RCBOs are described in IEC 61009-1. This International Standard applies to residual current operated circuit-breakers with integral overcurrent protection functionally independent of, or functionally dependent on, line voltage for household and similar uses (hereafter referred to as RCBOs), for rated voltages not exceeding 440 V AC with rated frequencies of 50 Hz, 60 Hz or 50/60 Hz and rated currents not exceeding 125 A and rated short-circuit capacities not exceeding 25 000 A for operation at 50 Hz or 60 Hz.

This standard applies to devices performing simultaneously the function of detection of the residual current, of comparison of the value of this current with the residual operating value and of opening of the protected circuit when the residual current exceeds this value, and also of performing the function of making, carrying and breaking overcurrents under specified conditions.

The content of the present standard related to operation under residual current conditions is based on IEC 61008-1. The content of the present standard related to protection against overcurrents is based on IEC 60898-1.

Installation and application rules of RCBOs are given in the IEC 60364 series.

For more severe overvoltage conditions, circuit-breakers complying with other standards (e.g. IEC 60947-2) should be used.

For environments with higher pollution degrees, enclosures giving the appropriate degree of protection should be used.

RCBOs of the general type are resistant to unwanted tripping, including the case where surge voltages (as a result of switching transients or induced by lightning) cause loading currents in the installation without occurrence of flashover.

RCBOs of type S are considered to be sufficiently proof against unwanted tripping even if the surge voltage causes a flashover and a follow-on current occurs.

For RCBOs having a degree of protection higher than IP20 special constructions may be required.

IEC 61009-1 also applies to RCBOs obtained by the assembly of an adaptable residual current device with a circuit-breaker. The mechanical assembly shall be effected in the factory by the manufacturer, or on site, in which case the requirements of Annex G shall apply. It also applies to RCBOs having more than one rated current, provided that the means for changing from one discrete rating to another is not accessible in normal service and that the rating cannot be changed without the use of a tool.

Supplementary requirements may be necessary for RCBOs of the plug-in type.

Particular requirements are necessary for RCBOs incorporated in or intended only for association with plugs and socket-outlets or with appliance couplers for household and similar general purposes and if intended to be used at frequencies other than 50 Hz or 60 Hz.

For RCBOs incorporated in, or intended only for association with socket-outlets, the requirements of IEC 61009-1 may be used, as far as applicable, in conjunction with the requirements of IEC 60884-1 or the national requirements of the country where the product is placed on the market.

NOTE 1 – Residual current-operated protective devices (RCDs) incorporated in, or intended only for association with socket-outlets, can either meet IEC 62640 or this standard.

NOTE 1 – In DK, plugs and socket-outlets shall be in accordance with the requirements of the heavy current regulations section 107.
NOTE 2 – In the UK, the plug part associated with an RCBO shall comply with BS 1363-1 and the socket-outlet(s) associated with an RCBO shall comply with BS 1363-2. In the UK, the plug part and the socket-outlet(s) associated with an RCBO need not comply with any IEC 60884-1 requirements.

The requirements of IEC 61009-1 apply for normal environmental conditions. Additional requirements may be necessary for RCBOs used in locations having severe environmental conditions

RCBOs including batteries are not covered by IEC 61009-1.

Construction Features

Constructively, residual current operated circuit breakers with integral overcurrent protection can be made as a single unit (Figures 2 and 3) or as a unit which is assembled from a residual current unit and a circuit-breaker before it is installed in a low-voltage switchgear (Figures 4 and 5). Below is a summary of RCBOs in a single enclosure or factory-assembled as a single unit.

The RCBOs under consideration are manufactured in two modifications. Early RCBOs were a single device consisting of a two-pole, three-pole or four-pole circuit-breaker, which is factory-connected to a two-pole, three-pole or four-pole residual current unit, respectively (Fig. 2). Modern modifications of two-pole RCBOs (Fig. 1) are originally designed so that they are manufactured in a single case, similar to the case of two-pole RCCBs. They have 1,5-2 times less width than the two-pole RCBOs developed earlier.

Two-pole RCBOs
Figure 2 – Two-pole RCBOs
RCBOs factory-assembled from circuit breakers and residual current units
Figure 3 – RCBOs factory-assembled from circuit breakers and residual current units: 1 – two-pole; 2 – three-pole; 3 – four-pole
RCBOs with rated current up to 63 A
Figure 4 – RCBOs with rated current up to 63 A, assembled from circuit-breakers and residual current units before installation in low-voltage switchgear: 1 – two-pole; 2 – three-pole; 3 – four-pole
RCBOs with rated current up to 100 A
Figure 5 – RCBOs with rated current up to 100 A, assembled from circuit breakers and residual current units before installation in low-voltage switchgear: 1 – two-pole; 2 – four-pole

In order to realise the overcurrent protection function, residual current circuit breakers with integral overcurrent protection are equipped with a direct overcurrent release, which usually includes:

  • A thermal overload release with inverse time delay, the tripping of which depends on the thermal effect of the electric current flowing through it;
  • An electromagnetic short-circuit release, causing the RCBO to open without time delay.

The overload release is designed to protect against small overload currents, and the short-circuit release against high overload currents and short-circuit currents.

Characteristics of RCBOs

The characteristics of an RCBO shall be stated in the following terms:

  • type of installation;
  • number of poles and current paths;
  • rated current In;
  • rated residual operating current IΔn;
  • rated residual non-operating current IΔno;
  • rated voltage Un;
  • rated frequency;
  • rated short-circuit capacity Icn;
  • rated residual making and breaking capacity IΔm;
  • time-delay, if applicable;
  • operating characteristics in case of residual currents with DC components;
  • method of mounting;
  • method of connection;
  • range of instantaneous tripping overcurrent;
  • I2t classification;
  • degree of protection (see IEC 60529).

For RCBOs functionally dependent on line voltage:

  • behaviour of the RCBO in case of failure of line voltage (see 4.1.2).

Type of Installation

According to the type of installation:

  • RCBO for fixed installation and fixed wiring;
  • RCBO for mobile installation and corded connection (of the device itself to the supply).

Number of Poles and Current Paths

According to the number of poles and current paths:

  • single-pole RCBO with one overcurrent protected pole and uninterrupted neutral (two current paths);
  • two-pole RCBO with one overcurrent protected pole;
  • two-pole RCBO with two overcurrent protected poles;
  • three-pole RCBO with three overcurrent protected poles;
  • three-pole RCBO with three overcurrent protected poles and uninterrupted neutral (four current paths);
  • four-pole RCBO with three overcurrent protected poles;
  • four-pole RCBO with four overcurrent protected poles.

The pole which is not an overcurrent protected pole may be:

  • “unprotected”, or
  • “switched neutral”.

Two-pole RCBOs used in single-phase electrical circuits are the most widely used in the electrical installations of buildings. Four-pole RCBOs used in three-phase four-wire electrical circuits have a smaller scope of application. Three-pole RCBOs used in three-phase three-wire circuits are even rarer.

Method of Mounting

According to the method of mounting:

  • surface-type RCBO;
  • flush-type RCBO;
  • panel board type RCBO, also referred to as distribution board type.

Note – These types may be intended to be mounted on rails.

Method of Connection

According to the method of connection:

  • RCBOs, the electrical connections which are not associated with the mechanical mounting;
  • RCBOs, the electrical connections which are associated with the mechanical mounting.

NOTE – Examples of this type are:

  • plug-in type;
  • bolt-on type;
  • screw-in type.

Some RCBOs may be of the plug-in type or bolt-on type on the line side only, the load terminals being usually suitable for wiring connection.

Range of Instantaneous Tripping Overcurrent

According to the instantaneous tripping current:

  • B-type RCBO;
  • C-type RCBO;
  • D-type RCBO.

RCBOs are usually of the instantaneous trip type B or C.

RCBOs with instantaneous trip type B are used for overcurrent protection of most final electrical circuits in the electrical installations of residential buildings, with type C – for overcurrent protection of those electrical circuits in which high inrush currents are possible when starting electrical equipment, such as electric motors, electric lights, etc.

Standard ranges of overcurrent instantaneous tripping are given in Table 1.

TypeRange
BAbove 3 In up to and including 5 In
CAbove 5 In up to and including 10 In
DAbove 10 In up to and including 20 Ina
Table 1 – Ranges of overcurrent instantaneous tripping

a) For special cases values up to 50 In may also be used.

I2t classification

According to the I2t characteristic:

In addition to the I2t characteristic to be provided by the manufacturer in accordance with Clause 5 IEC 61009-1, RCBOs may be classified according to their I2t characteristic.

Rated Current (In)

Rated current (In) is a current assigned by the manufacturer as the current which the RCBO can carry in uninterrupted duty, at a specified reference ambient air temperature.

The standard reference ambient air temperature is 30 °C. If a different reference ambient air temperature for the RCBO is used, the effect on the overload protection of cables shall be taken into account, since this is also based on a reference ambient air temperature of 30 °C, according to installation rules (see Clause 523 of IEC 60364-5-52:2001).

Preferred values of rated current are: 6 – 8 – 10 – 13 – 16 – 20 – 25 – 32 – 40 – 50 – 63 – 80 – 100 – 125 A.

The rated current of an In RCBO typically ranges from 6 to 63 A. Some RCBOs (especially those of the instantaneous trip type C) have a greater range of rated current.

The RCBOs manufactured (with rated currents up to 32 A) are usually current limiting overcurrent protection devices, which are characterized by very short time tripping of short-circuit currents. The effective value of the short-circuit current does not have time to reach its maximum value during such a short period of time.

Rated Residual Operating Current (IΔn)

Rated residual operating current IΔn is the value of residual operating current, assigned to the RCBO by the manufacturer, at which the RCBO shall operate under specified conditions.

For an RCBO having multiple settings of residual operating current, the highest setting is used to designate it.

RCBOs with continuously adjustable settings are not allowed.

Standard values of rated residual operating current are: 0,006 – 0,01 – 0,03 – 0,1 – 0,3 – 0,5 A.

NOTE – In KR and Japan, the values of 0,015 A, 0,2 A and 1 A are also considered as standard values.

RCBOs in most cases have a rated residual current (IΔn) equal to 0.01 or 0.03 A. RCBOs with IΔn = 0.03 A are the most common devices. They are used for additional protection against electric shock. RCBOs of general use with IΔn of 0.10 and 0.30 A are manufactured in smaller quantities.

Rated Residual Non-Operating Current (IΔno)

Rated residual non-operating current (IΔno) is the value of residual non-operating current, assigned to the RCBO by the manufacturer, at which the RCBO does not operate under specified conditions.

The standard value of residual non-operating current is 0,5 IΔn.

NOTE – For residual pulsating direct currents, residual non-operating currents depend on the current delay angle α.

Rated Voltage (Un)

The rated operational voltage (hereafter referred to as “rated voltage”) of an RCBO is the value of voltage, assigned by the manufacturer, to which its performance is referred.

NOTE – The same RCBO may be assigned a number of rated voltages and associated rated short-circuit capacities.

The rated voltage Un of two-pole RCBOs is usually 230 V, three- and four-pole is 400 V.

Rated Frequency

The rated frequency of an RCBO is the power frequency for which the RCBO is designed and to which the values of the other characteristics correspond.

NOTE – The same RCBO may be assigned a number of rated frequencies.

Preferred values of rated frequency are: 50 Hz, 60 Hz and 50/60 Hz.

Residual current operated circuit-breakers with integral overcurrent protection are designed for use in 50 and 60 Hz AC electrical circuits. Some special RCBOs can operate at higher rated frequencies, such as 400 Hz.

Rated Short-Circuit Capacity (Icn)

The rated short-circuit capacity of an RCBO is the value of the ultimate short-circuit breaking capacity assigned to that RCBO by the manufacturer.

NOTE – An RCBO having a given rated short-circuit capacity Icn has a corresponding service short-circuit capacity (Ics).

Standard values up to and including 10 000 A.

Standard values of rated short-circuit capacities up to and including 10 000 A are: 1 500 A – 3 000 A – 4 500 A – 6 000 A – 10 000 A.

NOTE – The values of 1 000 A, 2 000 A, 2 500 A, 7 500 A and 9 000 A are also considered as standard in some countries.

Values above 10 000 A up to and including 25 000 A.

For values above 10 000 A up to and including 25 000 A preferred values are 15 000 A and 20 000 A.

The rated short-circuit capacity of most RCBOs manufactured is typically 4,500, 6,000, or 10,000 A.

Rated Residual Making and Breaking Capacity (IΔm)

Rated residual making and breaking capacity (IΔm) is the r.m.s. value of the AC component of residual prospective current, assigned by the manufacturer, which an RCBO can make, carry and break under specified conditions.

The minimum value of the rated residual making and breaking capacity (IΔm) is 10 In or 500 A, whichever is greater.

Time-Delay (RCBO type S)

A time-delay RCBO which complies with the relevant part of Table 2 and Table 3 if applicable.

Limiting values of break time and non-actuating time for alternating residual currents
Table 2 – Limiting values of break time and non-actuating time for alternating residual currents (r.m.s. values) for type AC and A RCBO

NOTE – In the US, where the tripping times are specifically related to current, the following formulas apply:

T = (20 / I)1.43

for high-resistance faults and

T = 1,25 * (10 / V)1,43

for low resistance faults.

Maximum values of break time for half-wave residual currents for type A RCBO
Table 3 – Maximum values of break time for half-wave residual currents (r.m.s. values) for type A RCBO

Operating Characteristics in Case of Residual Currents with DC Components

RCBO Type AC

An RCBO for which tripping is ensured for residual sinusoidal alternating currents, whether suddenly applied or slowly rising.

RCBO Type A

An RCBO for which tripping is ensured for residual sinusoidal alternating currents and residual pulsating direct currents, whether suddenly applied or slowly rising.

RCBOs are manufactured of the AC type, which trip under sinusoidal AC residual currents, and of the A type, which also trip under pulsating DC residual currents.

Recently, RCBOs of type F and type B have started to be manufactured. These RCBOs are designed to operate with more complex forms of residual current (type F) and even with constant residual current (type B).

Classification of RCBOs

RCBOs are classified in the following 12 ways:

  1. According to the method of operation:

NOTE – The selection of the various types is made according to the requirements of IEC 60364-5-53.

  • RCBO functionally independent of line voltage
  • RCBO functionally dependent on line voltage

Opening automatically in case of failure of the line voltage, without or with delay:

Not opening automatically in case of failure of the line voltage:

  • a) able to trip in case of a hazardous situation (e.g. due to an earth fault), arising on failure of the line voltage (requirements under consideration);
  • b) not able to trip in case of a hazardous situation (e.g. due to an earth fault), arising on failure of line voltage.

NOTE – The selection of the RCBOs in b) is subject to the conditions of 531.2.2.2 of IEC 60364-5-53:2001.

2. According to the type of installation:

  • RCBO for fixed installation and fixed wiring;
  • RCBO for mobile installation and corded connection (of the device itself to the supply).

3. According to the possibility of adjusting the residual operating current:

  • RCBO with a single value of rated residual operating current;
  • RCBO with multiple settings of residual operating current by fixed steps.

4. According to resistance to unwanted tripping due to voltage surges:

  • RCBOs with normal resistance to unwanted tripping (general type as in Table 2, and Table 3 if applicable);
  • RCBOs with increased resistance to unwanted tripping (S type as in Table 2, and Table 3 if applicable).

RCBOs of general use are resistant to electric current surges with a peak value of up to 250 A. General purpose RCBOs are also manufactured with a higher resistance to surge currents of up to 3000 A.

5. According to behaviour in presence of DC components:

  • RCBOs of type AC;
  • RCBOs of type A.

6. According to time-delay (in presence of a residual current):

  • RCBO without time-delay: type for general use;
  • RCBO with time-delay: type S for selectivity.

Most RCBOs are general-purpose devices that trip without time delay when a residual current trip occurs in their main circuit.

7. According to the protection against external influences:

  • enclosed-type RCBO (not requiring an appropriate enclosure);
  • unenclosed-type RCBO (for use with an appropriate enclosure).

8. According to the method of mounting:

  • surface-type RCBO;
  • flush-type RCBO;
  • panel board type RCBO, also referred to as distribution board type.

9. According to the type of terminals:

  • RCBOs with screw-type terminals for external copper conductors;
  • RCBOs with screwless type terminals for external copper conductors;
  • RCBOs with flat quick-connect terminals for external copper conductors;
  • RCBOs with screw-type terminals for external aluminum conductors.

10. According to the instantaneous tripping current:

  • B-type RCBO;
  • C-type RCBO;
  • D-type RCBO.

11. According to the number of poles and current paths:

  • single-pole RCBO with one overcurrent protected pole and uninterrupted neutral (two current paths);
  • two-pole RCBO with one overcurrent protected pole;
  • two-pole RCBO with two overcurrent protected poles;
  • three-pole RCBO with three overcurrent protected poles;
  • three-pole RCBO with three overcurrent protected poles and uninterrupted neutral (four current paths);
  • four-pole RCBO with three overcurrent protected poles;
  • four-pole RCBO with four overcurrent protected poles.

The pole which is not an overcurrent protected pole may be:

  • “unprotected”, or
  • “switched neutral”.

Two-pole RCBOs used in single-phase electrical circuits are the most widely used in the electrical installations of buildings. Four-pole RCBOs used in three-phase four-wire electrical circuits have a smaller scope of application. Three-pole RCBOs used in three-phase three-wire circuits are even rarer.

12. According to the method of connection:

  • RCBOs, the electrical connections which are not associated with the mechanical mounting;
  • RCBOs, the electrical connections which are associated with the mechanical mounting.

NOTE – Examples of this type are:

  • plug-in type;
  • bolt-on type;
  • screw-in type.

Some RCBOs may be of the plug-in type or bolt-on type on the line side only, the load terminals being usually suitable for wiring connection.

13. According to the I²t characteristic:

In addition to the I2t characteristic to be provided by the manufacturer in accordance with Clause 5 IEC 61009-1, RCBOs may be classified according to their I2t characteristic.

An approximate nomenclature of RCBOs of general use, serially manufactured by various companies according to the requirements of IEC 61009-1, is given in the table below.

Table 4: Approximate nomenclature of RCBOs of general application of type AC and type A
In, A IΔn, mА Two-pole RCBOs Four-pole RCBOs
6, 10, 13, 16 10 +
6, 10, 13, 16, 20, 25, 32, 40, 50, 63 30 + +
100 + +
300 + +

Read also: How to Read Residual Current Device (RCD) Markings?

FAQ

What Is the Difference Between an RCD and RCBO?

RCBO is a type of RCD. 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 integral overcurrent protection. Such an RCD performs the same functions as an RCBO. Otherwise we speak about a residual current device without integral overcurrent protection, the equivalent of which is an RCCB.

What Is the Difference Between an RCCB and RCBO?

An RCCB is a device that monitors the current flowing in a circuit and quickly interrupts the power if it detects an imbalance, which is an indication of a potential fault, such as earth leakage. This is done to protect people from electric shock and to minimize the risk of electrical fires.

An RCBO, on the other hand, is a combination of an RCCB and a circuit breaker. It provides both earth leakage protection (like an RCCB) and overcurrent protection (like a circuit breaker) in a single device. An overcurrent protection device like a Circuit breaker is a switch that can be opened automatically to protect the electrical circuit from overloading or overcurrent.

In summary, an RCCB is a device that provides protection against earth leakage, and an RCBO is a device that provides protection against both earth leakage and overcurrents.

What Is the Difference Between an MCB and RCBO?

The MCB, or miniature circuit breaker, is an electrical switch integrated for the automatic protection of the electrical circuit from damage as a result of the overcurrent. It automatically stops the flow of electricity through the circuit upon detection of a fault. The main difference between an RCBO and MCB is that the MCB doesn’t provide protection against earth leakage.

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