Getting to grips with IEC 61439-2

Introduced in 1993, the IEC 60439 standard was intended to harmonise the low voltage industry with an objective to form one standard that offered protection for personnel and switchgear. It also tried to be a complete standard that covered the total performance of a low voltage assembly and that of individual products that are fitted within the assembly. 

This one standard regulated the different types of switchgear and classified them as TTA (type-tested switchgear assemblies) or PTTA (partially type-tested switchgear assemblies). So, what has changed under the new standard?

The new standard IEC 61439 is the successor to the IEC 60439 and comes into force from November 2014. However, it has already been implemented by the IEC and customers are now specifying low voltage switchgear with the IEC 61439 standard in mind.

Rittal’s Ri4Power modular low voltage switchgear system, for example, has been tested according to IEC 61439-2, which incorporates the testing and development of new products that previously were not required under IEC 60439-1. The outcome of the new testing procedures and results can be found in a dedicated design software package, Rittal Power Engineering and the technical system manual for Ri4Power.

Key changes
The terms for type-tested switchgear and controlgear assemblies (TTA) and partially type – tested switchgear and controlgear assemblies (PTTA) used in IEC 60439-1 have been removed. For new power switchgear and controlgear assemblies the type test reports have been replaced by design verification. The previous routine test report has been replaced with routine verification.

There are 13 design characteristics and, in total, 21 single verifications specified by IEC 61439 that need to be verified by testing and comparison with a reference design and assessment. Tests on individual devices to their respective product standards are not an alternative to the design verification in this standard for the assembly. 

Verification of temperature rise (part 10.10 of the standard) is the most time-consuming verification and can be expensive, regardless of which method is used. For verification of temperature rise, options available include testing, derivation of the rated values of similar variants and calculation methods.

Verification using the calculation method is confined to low voltage controlgear assemblies up to 630A and 1,600A. For higher current ratings, derivation and testing verification must be performed. It is a requirement to record all heat rise data and the method of verification used. Heat rise calculation and the effect of heat on the assembly is critical to the performance of components such as circuit breakers, fuses and controlgear.

For example, moulded case circuit breakers are compliant with IEC 60947, which tests the circuit breaker in free air. The performance of the circuit breaker is then recorded. Under IEC 61439 the circuit breaker is tested within an assembly. The maximum rated performance of the circuit breaker can deviate from the nominal rating because the environment has changed. Higher ambient conditions will apply due to environmental conditions such as the protection category (IP rating), size of functional unit (size of the enclosure where the circuit breaker is fitted) or assemblies with forced ventilation and so on. 

For verification of temperature rise, the actual achievable rated current and the rated diversity factor of the respective circuit should be indicated for both the manufacturer and the user. Merely stating the rated currents of the switchgear or individual components of the assembly is not sufficient, since this may not make allowance for environmental influences.

Rittal has undertaken temperature rise testing with a leading manufacturers of circuit breakers in various functional unit sizes and recorded all test data. The results, published in Rittal’s Ri4Power systems catalogue, show the de-rated current rating of the circuit breaker. This figure should be used by the design engineer when specifying circuit breakers.

Another feature of the new standard is the naming of Inc as the rated current of the circuit (operating current of the load and not the rating of the circuit breaker as listed under IEC 60947, free air testing).

The standard now also divides responsibility for the manufacture of a low voltage switchgear assembly between the original manufacturer and the assembly manufacturer. The original manufacturer is the organisation that originally developed the switchgear system and who is responsible for carrying out the design verification tests (Rittal, for example).

The assembly manufacturer is the organisation which builds the switchgear and controlgear assembly for a customer application (typically a panel builder or systems integrator) and is responsible for carrying out routine verification tests. These routine verification tests confirm that the assembly has been manufactured in line with the original manufacturer’s recommendations and design rules.