SPECIAL FEATURE
CIRCUIT PROTECTION & SWITCHGEAR
THE IMPORTANCE OF SELECTING THE CORRECT PROTECTIVE DEVICE Raphael Magnus, Managing Director of Safe Electric, looks at the importance of selecting the correct ACB, MCCB, MCB, RCBOs and bi-directional devices in electrical engineering design and adhering to electrical standards.
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here have been many systems over the decades where changes have been made in the selection of a protective device or the incorrect cable size has had a detrimental effect on the system, which can lead to a cascade failure and a lot of damage to those systems. When designing electrical systems, it is crucial to select the appropriate Air Circuit Breaker (ACB), Moulded Case Circuit Breaker (MCCB) and Miniature Circuit Breaker (MCB), to ensure safety and efficiency. MCBs are available in various types, such as B, C and D, and different breaking capacities, such as 6kA, 10kA and 15kA. This article will discuss the differences between these MCBs and the importance of using the correct one.
ACB and MCCB micrologic devices
ACBs and MCCBs are used in higher power applications and protect against overloads, short circuits and earth faults.
Differences between B, C and D Type MCBs
B, C and D-type MCBs differ in their tripping characteristics, which determine how quickly they will disconnect the circuit in response to an overload or short circuit.
Micrologic devices are electronic trip units used in ACBs and MCCBs to provide advanced protection and monitoring features. These devices can be customised to meet specific application requirements and ensure the safety and efficiency of electrical systems. Using the correct devices ensures selecting the appropriate MCB, ACB and MCCB type and breaking capacity is crucial for the following reasons: • That the circuit is protected against overloads, short circuits and earth faults, preventing fires and equipment damage. • Properly sized devices minimise nuisance tripping and ensure that the electrical system operates efficiently. • Adhering to the guidelines outlined in electrical standards, such as BS 7671 and the Electrical Installation Design Guide by the IET, ensures that the electrical installation meets the required safety standards. You should always check your source readings and adjust for the worst-case scenario. For example, wet-dry environmental conditions: just because it works in the wet season does not mean it will when the ground is baked dry and the resistivity of the soil increases, or the operational temperatures soar.
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• B Type MCBs: These MCBs are designed for general use in residential and light commercial applications. They trip between three to five times the rated current (In) and are sensitive to low-level short circuits. • C Type MCBs: Suitable for commercial and industrial applications, C-type MCBs trip between five to 10 times the rated current. They are less sensitive to short circuits than B-type MCBs, making them ideal for circuits with inductive loads, such as motors and transformers. • D Type MCBs: These MCBs are designed for circuits with high inrush currents, such as large motors, transformers and X-ray machines. They trip between 10 to 20 times the rated current and are the least sensitive to short circuits among the three types.
Differences between 6kA, 10kA and 15kA breaking capacities Breaking capacity refers to the maximum current that an MCB can safely interrupt without causing damage. The higher the breaking capacity, the more robust the MCB. • 6kA: Suitable for residential and light commercial applications with lower short-circuit currents. • 10kA: Commonly used in commercial and industrial settings with higher short-circuit currents. • 15kA: Designed for heavy-duty industrial applications with very high short-circuit currents.
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28/08/2024 09:48