Guidelines for Calculation and Design of Low Voltage Switchgear

IEC 61439 replaces IEC60439 and introduces the principle of verifying switchgear. These replace the existing Type Test Assembly (TTA) categories and some Type Test Assembly categories (PTTA) in IEC 60439.

IEC 61439 recognizes three equivalent and alternative verification methods that manufacturers may use to ensure compliance with standard requirements. These are:

• Testing

• Calculation

Design rules

Testing includes independent testing conducted by approved testing agencies or testing conducted by individual manufacturers according to approved procedures.

If verification methods other than testing are used to demonstrate compliance with standards, degradation factors must be included.

The strength, corrosion resistance, and insulation properties of materials and components: thermal stability, heat resistance of insulation materials, resistance to abnormal heat and fire caused by internal electrical effects, resistance to ultraviolet (UV) radiation, lifting, mechanical, impact, mechanical operation, power frequency withstand voltage, anti electric shock, and protection circuit integrity can only be verified through testing.

The protection level, impulse withstand voltage, and electromagnetic compatibility can be verified through testing or design guidelines, that is, if they comply with the design guidelines, they also meet the standard requirements.

The net creepage distance, effective continuity between exposed conductors, effectiveness of components and protective circuit components for external faults, temperature rise, and short endurance can be verified through testing, calculation, or design guidelines.

The merging of switchgear and components, internal circuits and connections, and external conductor terminals can only be verified through design guidelines.

Comparison of Verification Methods

Test

Testing remains the best method for verifying compliance. If tested by an independent testing agency or manufacturer according to approved procedures, the test is valid. This will be a method for manufacturers to optimize their products, minimize cabinet size, etc. without affecting performance, reliability, or safety.

For standardized switchgear, testing provides manufacturers and end-users with verified compliance, but not all customized switchgear can be tested due to high costs. The other verification methods detailed below can be used for these customized complete sets of equipment.

calculate

If allowed, established conventions can be used for calculations to predict performance. The disadvantage of this method is that it lowers the level of all published performance parameters and increases the margin for parameters such as temperature rise to ensure expected performance, reliability, and safety when operating at maximum load.

For example, if the manufacturer has not conducted testing to demonstrate that a 250A MCCB of a specific compartment size complies with temperature rise limits, the manufacturer can only rate the 250A equipment of that compartment size as 80% of its rated frame, i.e. 200A.

This usually leads to an increase in component ratings and housing dimensions, but it will never affect performance, despite reduced space utilization and increased costs in the data center.

The switchgear needs to undergo indoor temperature rise testing on all MCCBs and ACBs with standardized compartment sizes, so there is no need to lower its equipment rating.

Design rules

Where permitted, this enables manufacturers to use data validated through testing to determine the parameters of variables in the tested configuration. This can be replacing functional units from other manufacturers, if their performance values are equal to or better than the replaced unit, or using data published by the supplier to calculate consistency and replace units of different grades within the same enclosure.

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