Selection, coordination and short-circuit protection of industrial automatic circuit breakers

Introduction

Industrial automatic circuit breakers (MCCB) are critical components of electrical production systems. They ensure safety, reliability, and efficiency of power supply, protecting equipment from overloads and short circuits. Choosing the correct MCCB that complies with standards DSTU, EN, ISO is the basis for the long-term reliability of electrical production systems. This article discusses the technical basis of MCCB selection, coordination, and protection, which is significant for technical experts and production managers.

Fundamental Principles

Automatic MCCB circuit breakers are based on the principles of electromagnetic and thermal operation. Electromagnetic response is triggered by high short-circuit current, which causes an increase in the magnetic field, thereby opening the circuit breaker. Thermal response occurs through the maintenance of current exceeding the nominal value, which causes heating and opening of the contact. These mechanisms provide protection against overload and short circuit.

For the correct selection of MCCB it is necessary to know: nominal current, sensing current, tripping current, short-circuit sensing current, tripping level, tripping time and endurance. Each of these parameters must comply with the requirements of standards such as IEC 60947-2, ISO 281, DIN 51825.

Technical specifications and standards

Selection of MCCB should be made considering the following standards:

• IEC 60947-2: Sets requirements for automatic switches, including tripping current, tripping time and endurance.
• ISO 281: Sets the standards for electrical switches, including requirements for current perception and endurance.
• DIN 51825: Defines the standard for measuring the durability of electrical switches.
• DSTU 3073-2017: Establishes requirements for electrical devices in Ukraine, including standards for MCCB.
• UkrSEPRO: Product Certification for Use in Ukraine.

Selection criteria include:

• Nominal current (In): 16, 25, 32, 40, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630 A.
• Current rating (Ics): 5, 10, 20, 30, 40, 50, 60, 70, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250 A.
• Disconnection current (Icu): 5, 10, 20, 30, 40, 50, 60, 70, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250 A.
• Turn-off time: 0.1–10 s.
• Ambient temperature: -25 to +55 °C.
• Voltage: 230/400 V, 400/690 V.
• Minimum durability: 20000 cycles.

Recommendations for Selection and Calculation

Selection of MCCB shall be carried out taking into account the total system power, overload currents, short-circuit currents and protection requirements. Below is a table for selection of MCCB considering different parameters:

The following formulas are used for selecting MCCB:

• Iₙ = (P / U) * 1.25 – calculation of nominal current for electrical receivers.
• Icu = 5 * In – calculation of the disconnecting current.
• Ics = 1.1 * In – calculation of perception current.

Best Practices for Installation and Implementation

Installation of MCCB shall be carried out in accordance with the following recommendations:

• Use high-precision tools for establishing contact.
• Perform coordination of MCCB with other components of the system.
• Perform shutdown and feel tests.
• Use additional devices for overvoltage protection.
• Perform endurance testing before use.

During installation of MCCB perform the following steps:

1. Check the installation requirements.
2. Prepare the location for installation.
3. Install the MCCB in the correct position.
4. Connect the wiring taking into account the insulation.
5. Perform the test.

Potential failure modes and cause analysis

General tripping modes of MCCB include:

• Overload: Occurs due to exceeding the nominal current. Relevant information on sensation, endurance.
• Short circuit: Occurs through high current, which may cause disconnection.
• Incorrect installation: Causes incorrect operation or failure.
• Зношування: Виникає через підвищений струм або високу температуру.
• Несумісність з іншими компонентами: Викликає нестабільність електричної системи.

Visual signs of failure:

• Increased contact temperature.
• Change of contact appearance.
• Incorrect operation according to requirements.
• Increased durability.
• Incorrect operation according to requirements.

Predicted increase in maintenance and condition monitoring

For monitoring the condition of MCCB, the following can be used:

• Thermal sensors: For temperature measurement.
• Мікропроцесорні системи: Для аналізу струмів та напруг.
• Current Monitoring: For detecting overloads.
• Voltage Monitoring: For detecting deviations.
• Fatigue Monitoring: For detection of wear.

Predicted increase in maintenance is performed by:

• Data measurement analysis.
• Computer models.
• Current and Voltage Simulation.
• Use of additional devices.
• Periodic inspections.

Comparison Table

Conclusion

Proper selection, coordination and short circuit protection of MCCB are key elements of an effective electrical system. In the production in Ukraine it is important to use components complying with standards DSTU, EN, ISO, as well as certified by UkrSEPRO. In our e-catalogue, on the site https://www.unitecd.com/e-catalog/, you will find a wide range of MCCB meeting all requirements.

List of references

• IEC 60947-2: Requirements for automatic switches.
• ISO 281: Standards for electrical switches.
• DIN 51825: Standards for fatigue measurement.
• DSTU 3073-2017: Standards for Electrical Equipment in Ukraine.
• UkrSEPRO: Product Certification for Use in Ukraine.