In the realm of electrical engineering, ensuring the safety and reliability of switchgear and controlgear assemblies is paramount. A Type-Tested Assembly (TTA) plays a crucial role in this regard. Previously defined under IEC 60439, TTAs are now governed by the IEC 61439 standard, which was published in 2009 and became effective in 2011. This standard outlines a flexible design verification framework applicable to all low-voltage assemblies, ensuring compliance with performance requirements such as temperature rise, short-circuit withstand, and protection [1][3].

Understanding Type-Tested Assemblies

According to IEC 61439, a TTA is an assembly verified through specific type tests or equivalent methods to ensure it meets the requirements of the standard. This involves subjecting the assembly to a series of type tests detailed in IEC 61439-1 and IEC 61439-2. These tests cover various aspects such as temperature rise, dielectric properties, short-circuit withstand strength, and mechanical operation [2][4].

The aim of type testing is to demonstrate that the assembly can operate safely and reliably under normal operating conditions and during short-circuit events, thus preventing electrical failures that could lead to safety hazards or equipment damage [3][5].

Type Tests in IEC 61439

The IEC 61439 standard specifies several type tests that an assembly must pass to be classified as a TTA. These include:

• Verification of temperature rise: Ensures that the assembly can dissipate heat effectively and operate without overheating. This is critical as excessive temperature rise can reduce the lifespan of components by 50% per 10°C rise above limits [1][5].
• Verification of dielectric properties: Confirms the insulation strength of the assembly under normal and abnormal conditions [5].
• Verification of short-circuit withstand strength: Demonstrates the assembly's ability to withstand electrical stresses during short-circuit events [4].
• Verification of protection against electric shock: Ensures that the assembly provides adequate protection to prevent accidental contact with live parts [1].
• Verification of mechanical operation: Confirms that mechanical components function correctly under operational stress [5].

Clause 10 of IEC 61439-1 provides detailed information about these tests and their acceptance criteria [2].

Design Considerations for TTAs

Designing a TTA involves careful consideration of several factors to ensure compliance with IEC 61439. Some critical design considerations include:

• Current Rating: The assembly's busbars and components must be rated to handle the expected load current without exceeding temperature limits [4].
• Short-Circuit Rating: Components must be capable of withstanding the maximum prospective short-circuit current at the installation site [3].
• Temperature Rise: Adequate ventilation or cooling must be provided to prevent overheating, especially in high-density assemblies [2].
• Clearance and Creepage Distances: Sufficient distances must be maintained to prevent electrical arcing between conductive parts [1].

Example Calculation: Temperature Rise

Consider an assembly with a rated current of 1000 A. We need to calculate the expected temperature rise using the formula:

$$ \Delta T = I^2 \times R \times t $$

Where:

• $\Delta T$ is the temperature rise in degrees Celsius.
• $I$ is the current in amperes (A).
• $R$ is the resistance in ohms ($\Omega$).
• $t$ is the time in seconds (s).

Assuming a resistance of 0.001 $\Omega$ and a duration of one hour (3600 seconds), the temperature rise is calculated as:

$$ \Delta T = (1000)^2 \times 0.001 \times 3600 = 3,600,000 \, \text{J} $$

This energy is dissipated as heat, and the design must ensure adequate cooling to manage this heat [1].

Practical Examples of TTAs

Consider a power distribution board in a commercial building. To qualify as a TTA, the board undergoes type testing to verify that its components, such as circuit breakers and busbars, can handle the electrical demands without failure. This includes testing for temperature rise, dielectric strength, and short-circuit withstand capability [4].

Another example is an industrial motor control center (MCC) designed as a TTA. It must demonstrate through testing that it can safely and reliably control multiple motors, even under fault conditions [5].

Conclusion

Type-Tested Assemblies are essential for ensuring the safety and reliability of electrical installations. By adhering to the rigorous testing requirements of IEC 61439, TTAs provide confidence that the assembly will perform as expected under normal and fault conditions. Understanding the type tests and design considerations helps engineers and designers create compliant and robust electrical assemblies [2][3].