MCBs and RCDs in the EU: characteristics, standards and selection

Selecting an MCB requires ensuring the condition Ib ≤ In ≤ Iz, where the breaker is matched to the conductor rather than the load power. This guide covers tripping characteristics (B, C, or D) and essential RCD types, such as Type A, which is the minimum requirement for EV charging points.

Which standard covers what

In the European electrical market, selecting the correct protective device requires strict adherence to specific technical standards that define the device's application, voltage limits, and user requirements. For domestic and similar installations, the primary standard for miniature circuit breakers (MCBs) is IEC/EN 60898-1. In Poland, this is implemented as PN-EN 60898-1:2019-02, which is available in a Polish version. These air circuit breakers are designed for AC operation at 50/60 Hz, with a rated voltage up to 440 V between phases, a rated current up to 125 A, and a breaking capacity of up to 25,000 A. Crucially, devices compliant with this standard are intended for operation by unskilled persons, are non-maintainable, and are designed to provide circuit isolation.

When dealing with DC applications or combined AC/DC requirements, the selection criteria shift to different parts of the IEC 60898 series. It is a common misconception that IEC/EN 60898-2 applies only to direct current; in fact, it covers devices for both AC and DC operation. For DC applications under this standard, the limits are up to 220 V DC for single-pole devices and up to 440 V DC for two-pole devices, with a rated current up to 125 A and a DC breaking capacity of up to 10,000 A. The Polish implementation is PN-EN 60898-2:2022-03.

For applications requiring exclusively direct current protection, IEC 60898-3 is the governing standard. This covers devices rated up to 440 V DC and 125 A, with a breaking capacity of up to 10,000 A. In Poland, this is standardized as PN-EN IEC 60898-3:2025-07. Finally, for industrial environments where equipment is intended to be operated by instructed or qualified personnel, the IEC/EN 60947-2 standard is applied instead of the domestic-focused 60898 series.

Summary of Standards for Circuit Breakers

Standard	Application Type	Voltage / Current / Breaking Capacity (Max)	User Profile
PN-EN 60898-1	Domestic/Similar (AC 50/60 Hz)	440 V (phase-to-phase) / 125 A / 25,000 A	Unskilled persons
PN-EN 60898-2	AC and DC	DC: 220 V (1P) or 440 V (2P) / 125 A / 10,000 A (DC)	Unskilled persons
PN-EN IEC 60898-3	DC only	440 V DC / 125 A / 10,000 A	Unskilled persons
IEC/EN 60947-2	Industrial	As specified by device type	Instructed or qualified persons

The B, C and D curves — what they actually mean

When selecting a Miniature Circuit Breaker (MCB), the most critical parameter for protecting against short circuits is the tripping characteristic. This is determined by the electromagnetic trip unit, which reacts to high-current faults. According to IEC/EN 60898-1, these characteristics are categorized into types B, C, and D, defined by the range of current relative to the rated current (In) required to trigger the electromagnetic trip.

While the electromagnetic trip unit varies between types, the thermal trip unit—responsible for protecting against sustained overloads—remains identical across all three characteristics. For all types, the non-tripping current is 1.13 In, and the tripping current is 1.45 In. The performance of this thermal unit is tested over a specific conventional time: 1 hour for rated currents up to and including 63 A, and 2 hours for rated currents exceeding 63 A.

In practical installation, the choice of curve depends on the inrush current of the connected loads. Type B is suitable for circuits with low inrush currents, such as general-purpose sockets or lighting. Type C is the standard choice for loads with moderate inrush currents, including LED luminaires, switching power supplies, and motors. Type D is reserved for heavy-duty applications with high initial current surges, such as large transformers or heavy drives.

Engineers should account for ambient conditions during selection. Catalog characteristics are provided for a reference temperature of 30 °C in accordance with PN-EN 60898-1. If the ambient temperature is higher than this reference, the thermal trip unit will operate at a lower current. Additionally, some modular devices may be dual-certified under both IEC/EN 60898-1 and IEC/EN 60947-2, allowing for the use of industrial-grade characteristics like K or Z in modular applications.

It is important to distinguish these from industrial characteristics defined by IEC/EN 60947-2. For example, the Z characteristic triggers between 2 and 3 In. The K characteristic range is not fixed by the standard; manufacturers declare it according to IEC/EN 60947-2, with different values provided by different brands (e.g., 8–12 In or 8–14 In).

Characteristic	Electromagnetic Trip Range (Short-circuit)	Typical Application
Type B	> 3 In to 5 In	Low inrush loads (lighting, general sockets)
Type C	> 5 In to 10 In	Moderate inrush loads (LEDs, power supplies, motors)
Type D	> 10 In to 20 In	High inrush loads (transformers, large drives)

Breaking capacity: Icn and Ics

When designing an electrical installation, selecting the correct breaking capacity is critical for ensuring the safety and longevity of the protective devices. The primary parameter to consider is the rated short-circuit capacity (Icn), which represents the maximum short-circuit current that the device can interrupt once. Beyond this value, the device may require replacement. In practice, the breaking capacity must be selected such that it is not lower than the expected short-circuit current at the specific point of installation. For instance, the short-circuit current is typically higher near a transformer station than at the end of a long circuit.

Standard values for Icn up to 10,000 A include 1500, 3000, 4500, 6000, and 10,000 A. For the range exceeding 10,000 A up to 25,000 A, a value of 20,000 A is preferred. In the Polish market, mass-produced residential series typically feature an Icn of 6000 A, while series with higher durability offer an Icn of 10,000 A.

A crucial distinction for engineers is the relationship between Icn and the rated service short-circuit capacity (Ics). While Icn defines the limit for a single interruption, Ics indicates the current level up to which the device remains fit for further operation after the fault has occurred. The relationship between these two values is defined by a coefficient (k), which varies depending on the Icn rating:

• For Icn up to 6000 A inclusive, Ics is 100% of Icn.
• For Icn above 6000 A up to 10,000 A, Ics is 75% of Icn.
• For Icn above 10,000 A, Ics is 50% of Icn.

Energy limiting class and rated current

According to IEC/EN 60898-1, miniature circuit breakers (MCBs) are classified based on their $I^2t$ characteristic, which defines their ability to limit the energy passed during a short-circuit event. In the current market, Class 3 is the standard for domestic applications; this is indicated on the device housing by a number in a square located next to the breaking capacity value. In contrast, Class 1 indicates that the device provides no limitation of the short-circuit current.

This energy limiting classification applies exclusively to Type B and Type C tripping characteristics. It does not apply to Type D characteristics, nor does it apply to circuit breakers with rated currents exceeding 63 A.

Regarding selection, IEC/EN 60898-1 provides a preferred series of rated currents: 6, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, and 125 A. While this series is preferred, it is not exhaustive; manufacturers often provide a wider range of devices, including those with lower rated currents starting from 0.5 A.

The basic rule: the breaker protects the cable

In electrical installation design, the primary function of a circuit breaker is to protect the conductor from thermal damage caused by overcurrent. To ensure adequate protection against overload, two fundamental conditions must be satisfied simultaneously: the design current of the circuit ($I_b$) must be less than or equal to the rated current of the protection device ($I_n$), which must in turn be less than or equal to the continuous current-carrying capacity of the cable ($I_z$). The effective operating current of the protection device ($I_2$) must not exceed 1.45 times the current-carrying capacity of the cable ($I_z$).

To apply these rules correctly, it is essential to understand the specific designations used in technical documentation. $I_b$ represents the design current of the circuit, while $I_n$ is the rated current of the protection device (or the set value for adjustable devices). $I_z$ denotes the continuous current-carrying capacity of the conductor. The term $I_2$ refers to the effective operating current of the protection device at the conventional time. In various product standards, this value ($I_2$) may also be designated as $I_t$ or $I_f$.

For designers using miniature circuit breakers (MCBs) manufactured according to IEC/EN 60898-1, the selection process is simplified by the device's inherent characteristics. For this specific group of devices, the effective operating current is defined as $1.45 \cdot I_n$. Consequently, if the condition $I_n \leq I_z$ is met, the second requirement ($I_2 \leq 1.45 \cdot I_z$) is automatically satisfied. This characteristic provides a significant practical advantage when ensuring coordination between the breaker and the cable.

From a practical engineering standpoint, the rated current of the circuit breaker must be selected based on the current-carrying capacity of the cable, rather than the power rating of the connected load. A common and dangerous error in installation is increasing the breaker rating to accommodate a load when the existing cable is too thin. While this may resolve the issue of nuisance tripping, it removes the protection mechanism and creates a significant fire hazard. Note that regarding the regulatory framework, in the IEC/HD 60364-4-43 (2023) edition, point 433.1 has been renumbered to 431.4.2; in Poland, the applicable standard is PN-HD 60364-4-43:2024-04.

Maximum disconnection time

The maximum automatic disconnection times for protective purposes are defined by Table 41.1 of the IEC/HD 60364-4-41 standard. In the Polish market, the applicable version is PN-HD 60364-4-41:2017-09. These time limits are critical for ensuring that electrical installations remain safe by disconnecting the power supply before touch voltages reach hazardous levels during a fault.

The required disconnection times depend on the earthing system (TN or TT) and the phase voltage ($U_0$). For AC phase voltages where $120\text{ V} < U_0 \le 230\text{ V}$, the maximum disconnection time is 0.4 s in a TN system and 0.2 s in a TT system. These short time limits apply specifically to final circuits, including socket-outlets with a rated current up to 63 A and fixed consumer circuits with a rated current up to 32 A. For distribution circuits and other final circuits, the permitted times are extended to 5 s in a TN system and 1 s in a TT system. However, in a TT system, if the disconnection is achieved by an overcurrent protective device and all extraneous conductive parts are covered by equipotential bonding, the time limits applicable to the TN system may be used.

Selecting the correct device characteristic is essential to meet these mandatory time constraints. The choice of characteristic is directly influenced by the loop impedance; for instance, a "harder" characteristic, such as Type D, may fail to provide disconnection within the required time if the fault loop impedance is too high. Therefore, the designer must ensure that the selected overcurrent protective device can clear the fault within the limits specified by the standard under the actual installation conditions.

RCDs: product standards and RCCB versus RCBO

Residual Current Devices (RCDs) are governed by specific international and European standards depending on their functional design. Residual Current Circuit Breakers (RCCBs), which do not include built-in overcurrent protection, are standardized under IEC/EN 61008-1 (in Poland: PN-EN 61008-1:2013-05). Conversely, Residual Current Breakers with Overcurrent protection (RCBOs) are regulated by IEC/EN 61009-1 (in Poland: PN-EN 61009-1:2013-06). Both standards cover devices rated for alternating current up to 440 V at 50/60 Hz and up to 125 A. The IEC/EN 61009-1 standard specifies a rated short-circuit capacity of up to 25,000 A.

The structural difference between these two devices significantly impacts installation and troubleshooting. An RCCB requires a preceding overcurrent protection device to ensure the circuit is protected against overloads and short circuits. An RCBO integrates both functions into a single unit, which allows for a more compact distribution board layout by saving space. However, from a maintenance perspective, identifying the exact cause of a trip can be more challenging with an RCBO, as the device combines both residual current and overcurrent tripping characteristics.

It is important for designers and installers to distinguish RCBOs from Arc Fault Detection Devices (AFDDs). While an RCBO provides protection against residual currents and overcurrents, an AFDD is a separate device with a different primary function: detecting arc faults. AFDDs are governed by a distinct standard, IEC/EN 62606.

RCD types: AC, A, F and B

Selecting the correct Residual Current Device (RCD) is critical for ensuring the protection of personnel and equipment. The classification of RCDs is governed by specific international standards. Types AC and A are defined within the IEC 61008 and IEC 61009 series. In contrast, Types F and B are defined by the IEC/EN 62423 standard, which is applied exclusively in conjunction with IEC 61008-1 and IEC 61009-1. In Poland, the relevant standard is PN-EN 62423:2013-06.

The practical necessity for specific RCD types arises from the presence of power electronics in modern circuits, such as inverters, frequency converters, and power supplies. A fundamental limitation is that Type AC RCDs cannot detect pulsating direct currents (DC). Therefore, the specific electronic components present in a circuit dictate the required RCD type to ensure reliable operation and safety.

Type F RCDs function similarly to Type A, but with added capability: they react to composite differential currents and to pulsating DC currents superimposed on a smooth DC component. This makes Type F suitable for circuits supplying frequency converters connected between a phase and the neutral conductor. Type B RCDs offer the highest level of detection, functioning like Type F but with additional sensitivity to sinusoidal AC currents up to 1000 Hz, AC currents superimposed on smooth DC, pulsating DC superimposed on smooth DC, rectified pulsating currents originating from two or more phases, and smooth DC currents regardless of polarity.

For electric vehicle (EV) charging installations, specific requirements are established in PN-HD 60364-7-722:2019-01. Each connection point must be protected by its own RCD, which must be at least Type A with a residual current not exceeding 30 mA. A measure against DC residual currents must be implemented: this can be achieved by using a Type B RCD, or a Type A RCD combined with a device that disconnects the power supply when a DC residual current exceeds 6 mA. The 6 mA threshold is selected to ensure the DC component does not interfere with the operation of a primary Type A or Type F RCD. An RCD equipped with 6 mA DC detection does not replace a Type B RCD in applications where Type B is specifically required. Devices designed to detect DC residual currents are standardized under IEC 62955.

RCD Type	Reaction to Currents	Typical Application
Type AC	Sinusoidal AC	Basic circuits without significant electronic components.
Type A	Sinusoidal AC and pulsating DC	Standard domestic and commercial circuits with basic electronics.
Type F	Sinusoidal AC, pulsating DC, and composite differential currents	Circuits with frequency converters connected phase-to-neutral.
Type B	Sinusoidal AC (up to 1000 Hz), pulsating DC, composite currents, and smooth DC (any polarity)	Complex electronic loads and specific EV charging requirements.

Selectivity of residual current devices

According to the IEC/EN 61008-1 standard, residual current devices (RCDs) are classified into two main categories: those without intentional time delay, intended for general use, and those with a time delay, designated as Type S for selectivity purposes. The designation "Type G" does not exist within the IEC/EN 61008-1 standard; the standard only recognizes general-purpose devices and Type S. Designations such as "G" or "KV" are manufacturer-specific labels for devices with a short delay that maintain standard tripping times and can serve as replacements for standard devices.

To achieve effective selectivity between devices, the rated residual current of the upstream selective RCD must be selected at least one step higher than that of the downstream device. A typical series of rated residual currents includes 30 mA, 100 mA, 300 mA, 500 mA, and 1 A. Based on manufacturer technical data, a selective RCD typically features a non-tripping time of 50 ms, a maximum tripping time of up to 500 ms at the rated residual current, and a maximum tripping time of up to 150 ms at five times the rated residual current.

While a selective RCD provides essential fire protection and protection against damage, it is not suitable for supplementary protection of persons. For the protection of individuals, a device without intentional time delay with a rated residual current of 30 mA must be used.

The most common selection mistakes

One of the most frequent and dangerous errors in circuit design is selecting the rated current of a circuit breaker based solely on the power of the consumer rather than the current-carrying capacity of the conductor. When the breaker's rating exceeds the capacity of the cable, the conductor loses its protection, creating a significant fire risk as the cable may overheat before the device trips.

Designers must also account for the specific tripping characteristics and fault conditions. A common mistake is overlooking the disconnection time requirement; for instance, a breaker with a Type D characteristic may fail to trip within the required timeframe if the loop impedance is too high. Furthermore, selecting a device with insufficient breaking capacity in locations where high short-circuit currents are expected—such as near a transformer station—can lead to equipment failure or catastrophic damage during a fault.

The selection of Residual Current Devices (RCDs) requires careful attention to the nature of the load. Using a Type AC RCD in circuits containing power electronics is a critical error, as these devices may fail to detect pulsating direct currents. Failing to ensure selectivity between upstream and downstream devices can result in a fault in a single branch circuit causing a total loss of power to the entire installation.

Finally, technical errors during installation can negate even the most precise selection. Tightening a terminal screw directly onto a multi-strand conductor without using a cable ferrule is a common mistake. This action can cut through several strands of the wire, reducing the effective cross-section and causing the contact point to overheat.

What is available on the Polish market

The Polish market offers a wide range of modular miniature circuit breakers (MCBs) from major international manufacturers. Large distributors typically stock high-performance series such as ABB (S200 series), Schneider Electric (Acti9 iC60), Legrand (TX³), Eaton (xPole Home), Siemens (SENTRON 5SL6), Schrack Technik (AMPARO), Noark (Ex9BN), and Kanlux (IDEAL TS). These products are designed to meet rigorous EU standards for residential and industrial installations.

In addition to global brands, local manufacturing is represented by companies such as Elektro-Plast Nasielsk, which produces its own modular apparatus, including EPS6 circuit breakers in B and C characteristics, EPRPN6 residual current breakers with an integrated overcurrent release, and EPRP6 type AC residual current breakers. When sourcing components, it is important to distinguish between manufacturers of protective devices and those specializing in electrical accessories; for example, Ospel is a manufacturer of installation fittings such as switches, sockets, and frames, rather than protective apparatus.