Introduction

In electrical panel design, ensuring coordination between various components is crucial for safety and operational reliability. Type 2 coordination for motor starters, as defined by IEC 60947-4-1, ensures that, in the event of a short circuit, the motor starter will not suffer damage that would prevent it from being put back into service after clearing the fault. This is particularly important for maintaining system integrity and minimizing downtime [1].

Understanding Coordination Types

Coordination types are defined in terms of the level of protection and continuity they provide. IEC 60947-4-1 specifies two main types of coordination: Type 1 and Type 2.

• Type 1 Coordination: After a short circuit, the starter may be damaged and require replacement [2].
• Type 2 Coordination: After a short circuit, the starter should not have any damage that prevents its further use (except for minor contact welding which can be separated). This ensures that the system can be quickly put back into service [3].

Requirements for Type 2 Coordination

To achieve Type 2 coordination, the combination of the circuit breaker, contactor, and overload relay must be tested under short-circuit conditions. The relevant IEC standard outlining these requirements is IEC 61439, specifically focusing on low-voltage switchgear and controlgear assemblies [7].

According to IEC 61439, the assembly must be able to withstand the thermal and dynamic stresses of short-circuit currents up to a specified prospective short-circuit current [9].

Design Considerations

When designing a motor starter with Type 2 coordination, several factors need to be considered:

1. Selection of Components

The choice of components is crucial for achieving Type 2 coordination. This includes selecting the appropriate circuit breaker, contactor, and overload relay. Manufacturers typically provide coordination tables that specify compatible combinations for achieving Type 2 coordination [5].

2. Short-Circuit Current Calculation

Accurate calculation of the prospective short-circuit current at the motor starter's location is essential. The formula to calculate the short-circuit current \( I_{sc} \) is given by:

$$ I_{sc} = \frac{U_n}{Z_{total}} $$

where \( U_n \) is the nominal voltage and \( Z_{total} \) is the total impedance of the circuit.

3. Verification According to IEC 61439

IEC 61439 requires verification of the short-circuit withstand strength of assemblies. This can be achieved through testing or by using calculations to prove that the assembly can withstand the specified fault current [6].

Practical Example

Consider a motor starter designed for a motor with a full-load current of 50 A. The system voltage is 400 V, and the total impedance is calculated to be 0.1 ohms. The prospective short-circuit current is calculated as:

$$ I_{sc} = \frac{400 \text{ V}}{0.1 \, \Omega} = 4000 \text{ A} $$

Using the manufacturer's coordination tables, select a circuit breaker, contactor, and overload relay combination that is rated for at least 4000 A short-circuit current to achieve Type 2 coordination [4].

Conclusion

Type 2 coordination is a critical aspect of motor starter design, ensuring that systems can withstand short-circuit events without significant damage. By adhering to IEC 61439 and selecting the appropriate components, engineers can design motor control panels that maintain operational integrity and minimize downtime [3].