Rated Voltage of Solid Insulated Vacuum Circuit Breaker Fuse Types

Introduction

Solid insulated vacuum circuit breaker (VCB) fuse types are widely used in modern medium-voltage (MV) power distribution systems, especially in compact substations, ring main units (RMUs), and transformer protection feeders. One of the most critical technical parameters of this equipment is its rated voltage, which directly determines insulation performance, system compatibility, and long-term operational reliability.

Understanding the rated voltage of solid insulated VCB fuse types is essential for correct equipment selection, protection coordination, and compliance with IEC standards. An incorrect voltage rating can result in insulation stress, partial discharge, premature aging, or even catastrophic failure.

This article provides a detailed explanation of rated voltage definitions, common voltage classes, insulation coordination, fuse–VCB matching principles, and practical selection guidance.

What Is Rated Voltage in Solid Insulated VCB Fuse Types

Rated voltage refers to the highest system voltage for which the solid insulated VCB fuse combination is designed to operate continuously under normal conditions. It represents the maximum RMS line-to-line voltage that the equipment insulation system can withstand without degradation.

In solid insulated VCB fuse types, rated voltage applies to:

• The vacuum circuit breaker

• The high voltage fuse link

• The solid insulation system (epoxy resin)

• The overall assembly inside the switchgear

All these elements must be designed and tested for the same voltage class.

Typical Rated Voltage Classes

Solid insulated VCB fuse types are mainly used within standard medium-voltage ranges defined by IEC standards.

Among these, 12 kV and 24 kV are the most widely used voltage classes for solid insulated VCB fuse combinations.

12 kV Solid Insulated VCB Fuse Types

The 12 kV voltage class is the most mature and widely deployed.

Key characteristics:

• Designed for 10 kV and 11 kV distribution systems

• Common in urban RMUs and compact substations

• Compact size due to lower insulation requirements

• Broad availability of matching fuses and VCB components

12 kV solid insulated VCB fuse types are typically used for distribution transformer protection up to several MVA, offering an optimal balance between size, cost, and performance.

24 kV Solid Insulated VCB Fuse Types

The 24 kV voltage class is increasingly important as utilities expand higher-voltage distribution networks.

Key characteristics:

• Designed for 20 kV and 22 kV systems

• Requires thicker epoxy insulation and improved field control

• Larger physical dimensions compared to 12 kV units

• Higher insulation test requirements

24 kV solid insulated VCB fuse types are widely applied in regional utility networks, renewable energy substations, and SF₆-free switchgear projects.

36 kV Solid Insulated VCB Fuse Types

36 kV solid insulated designs exist but are less common.

Reasons include:

• Much higher insulation stress

• Larger epoxy volumes

• Increased manufacturing complexity

• Higher cost

They are mainly used in special industrial applications or compact 33 kV substations where gas insulation is not preferred.

IEC Standards Governing Rated Voltage

Rated voltage is defined and verified according to IEC 62271 series standards, mainly:

• IEC 62271-1 (Common specifications)

• IEC 62271-100 (AC circuit breakers)

• IEC 62271-105 (Fuse-switch combinations)

These standards define not only rated voltage, but also associated insulation levels.

Insulation Levels Associated with Rated Voltage

Each rated voltage class corresponds to specific withstand voltage requirements.

The solid insulation system must withstand these stresses without partial discharge or breakdown.

Role of Solid Insulation in Voltage Rating

Unlike air-insulated or gas-insulated switchgear, solid insulated VCB fuse types rely on epoxy resin as the primary insulation medium.

Key implications:

• Electric field distribution must be precisely controlled

• Sharp edges and air voids must be eliminated

• Partial discharge limits are stricter

• Manufacturing quality directly affects voltage endurance

As rated voltage increases, the insulation design must include field grading layers, embedded conductive shields, and thicker insulation sections.

Fuse Rated Voltage vs VCB Rated Voltage

In a solid insulated VCB fuse type, fuse rated voltage and VCB rated voltage must match the system voltage.

Key rules:

• Fuse rated voltage ≥ system highest voltage

• VCB rated voltage = system rated voltage

• Fuse, VCB, and insulation must be coordinated

For example:

• 12 kV system → 12 kV VCB + 12 kV fuse

• 24 kV system → 24 kV VCB + 24 kV fuse

Using a lower-rated fuse is not acceptable and can lead to insulation failure.

Impact of Installation Conditions on Rated Voltage

Although rated voltage is defined under standard conditions, real installations may require derating.

Factors include:

• High altitude (reduced air pressure)

• High ambient temperature

• Severe pollution levels

• Frequent switching overvoltages

Manufacturers may specify correction factors to maintain insulation safety margins.

Rated Voltage Selection for Typical Applications

Transformer Protection Feeders

• Small to medium transformers → 12 kV

• Larger distribution transformers → 24 kV

Ring Main Units

• Urban networks → 12 kV

• Utility distribution loops → 24 kV

Industrial Power Systems

• Internal MV distribution → 6–12 kV

• Utility connection point → 24 kV

Correct voltage selection ensures coordination between transformer insulation, fuse melting characteristics, and breaker performance.

Common Misunderstandings About Rated Voltage

• Rated voltage is not the same as operating voltage margin

• A higher rated voltage does not automatically mean higher current capacity

• Fuse interrupting capacity is independent of rated voltage

• Over-specifying voltage increases size and cost without benefit

Understanding these points helps avoid incorrect equipment selection.

Key Advantages of Correct Voltage Rating

Proper rated voltage selection provides:

• Stable insulation performance

• Reduced partial discharge risk

• Long service life

• Compliance with IEC and utility standards

• Safe coordination between fuse and breaker

It is one of the most important parameters in MV switchgear design.

Conclusion

The rated voltage of solid insulated vacuum circuit breaker fuse types typically ranges from 7.2 kV to 36 kV, with 12 kV and 24 kV being the dominant voltage classes in modern medium-voltage distribution systems. Rated voltage defines not only the insulation strength of the VCB and fuse, but also the reliability, safety, and longevity of the entire switchgear assembly.

Correct selection of rated voltage—based on system voltage, insulation requirements, and IEC standards—is essential to ensure safe operation, effective protection, and long-term performance in solid insulated MV switchgear applications.