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Forms of Internal Separation in Low-Voltage Switchgear Assemblies

The design and construction of low-voltage switchgear and controlgear assemblies are governed by the IEC 61439 series of standards. A critical aspect of these assemblies is the internal separation, which enhances safety and reliability by limiting the spread of faults and facilitating maintenance. This guide explores the forms of internal separation as outlined in IEC 61439, focusing on Forms 1 to 4b.

Understanding Internal Separation

Internal separation in switchgear assemblies involves dividing the assembly into different compartments to achieve specific objectives such as:

• Protection against contact with live parts
• Reduction of the risk of arc faults spreading
• Facilitation of maintenance and operational tasks

According to IEC 61439-2, the forms of separation are classified into four main categories, each with subtypes, ranging from Form 1 (no separation) to Form 4b (advanced separation) [1][2][3]. These forms are detailed in Annex D of IEC 61439-2.

Form 1: No Internal Separation

Form 1 provides no separation within the assembly. All equipment, including busbars, connections, and functional units, are housed in a single compartment. This form requires full upstream isolation for any internal work and is typically used in small shops or temporary panels with low-fault-current systems [1][2][4].

Form 2: Separation of Busbars from Functional Units

Form 2 introduces separation between busbars and functional units, with two subtypes:

• Form 2a: Terminals are in the same compartment as their functional unit.
• Form 2b: Terminals are in a separate common compartment.

Form 2 improves safety over Form 1 but limits access to live functional units, suitable for installations requiring moderate protection against faults [2][3][5].

Form 3: Separation of Busbars from Functional Units, Plus Separation Between Functional Units

Form 3 provides further separation, which includes:

• Form 3a: Terminals are in the same compartment as their functional unit.
• Form 3b: Terminals are in separate compartments.

This form is ideal for industrial panels needing unit isolation without full terminal segregation. Type examples include Type 1, which uses insulated busbar coverings [1][3][6].

Form 4: Full Separation (Busbars, Functional Units, and Terminals)

Form 4 offers the highest level of separation, designed for enhanced safety and operational security:

• Form 4a: Terminals are in the same compartment as their functional unit; busbar separation often by insulated coverings [1].
• Form 4b: Terminals are in dedicated, enclosed compartments, enabling maintenance on one unit while others remain energized [6].

Form 4b is particularly suitable for critical installations where operational continuity and safety are paramount, such as in data centers and hospitals. Type examples include Type 7, which offers integral glanding per unit [1][6][7].

Design Considerations and Calculations

When designing a switchgear assembly, engineers must consider not only the form of internal separation but also the electrical and mechanical constraints. Here are some design considerations:

Current Rating and Busbar Sizing

The current rating of busbars is critical. It can be calculated using the formula:

$$ I = \frac{P}{V \cdot \eta} $$

Where:

• I is the current in amperes (A)
• P is the power in watts (W)
• V is the voltage in volts (V)
• \eta is the efficiency factor

Busbars must also withstand mechanical forces during short circuits, which can be calculated using:

$$ F = \frac{I^2 \cdot L}{b} $$

Where:

• F is the force in newtons (N)
• I is the peak short-circuit current in amperes (A)
• L is the length of the busbar in meters (m)
• b is the distance between supports in meters (m)

Insulation Coordination

Insulation levels must be coordinated to ensure safety and reliability. IEC 61439 provides guidelines for verifying insulation by dielectric strength testing, often conducted at 2.5 times the rated operational voltage for one minute [6].

Practical Example

Consider a data center requiring high reliability and safety. The use of Form 4b separation ensures that each server rack can be isolated, minimizing downtime during maintenance or in case of a fault.

Suppose the system operates at 400 V with a total power requirement of 500 kW. Assuming an efficiency of 95%, the current calculation is:

$$ I = \frac{500,000}{400 \times 0.95} \approx 1316 \, \text{A} $$

This current determines the sizing of busbars and the need for adequate separation to manage heat dissipation and fault isolation.

Conclusion

The selection of the appropriate form of internal separation is a crucial step in the design of low-voltage switchgear assemblies. It impacts safety, reliability, and operational efficiency. By understanding the requirements set forth in IEC 61439, engineers can make informed decisions tailored to the specific needs of their projects [7].

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