What Does the Letter Designation on Solid Insulation Type Switchgear Mean?

Solid insulation type switchgear is becoming a dominant trend across global medium-voltage distribution systems. As utilities, industrial plants, and renewable-energy developers shift away from SF6-dependent equipment, solid-insulated switchgear (SIS) provides an environmentally friendly, maintenance-reduced, compact, and safety-enhanced alternative.

Yet one of the first challenges buyers face is understanding the letter-based model designation system used in manufacturing catalogs. These codes appear simple, but they actually convey extensive technical details about configuration, insulation, structure, and functional capabilities.

This article explains what the letter designations truly mean, how manufacturers use them to classify equipment, and how engineers can interpret them when selecting the correct solid insulation type switchgear for a project.

1. Introduction to Solid Insulation Type Switchgear

Solid insulation type switchgear uses epoxy resin, silicone rubber, or composite polymers to insulate live parts instead of traditional sulfur hexafluoride (SF6) gas. Manufacturers encapsulate conductors, vacuum interrupters, and busbar sections inside solid-dielectric components, creating a hermetically sealed modular system.

Why the naming system matters

Because many configurations exist—ring main units, modular panels, extensible nodes, and compact block-type units—manufacturers adopt letter designations to classify:

• Switchgear insulation structure

• Functional unit type

• Switching mechanism

• Extensibility

• Voltage class

• Application category

• Installation environment

• Protection level

• Operation mode

Understanding these letters is critical for comparing products across suppliers.

chinadegatech-Solid Insulation Type Switchgear

2. Why Manufacturers Use Letter Designations

Across the medium-voltage distribution market, letter designations serve several essential purposes:

1. Universal recognition

Even though each manufacturer has its own naming conventions, many letters overlap with international design language (such as IEC standards).

2. Technical clarity

A single model name may encode 10+ separate attributes.

3. Cross-comparison

Engineers can compare similar units from different suppliers.

4. Quick specification

Procurement teams can decode features without opening a full datasheet.

5. Standardized engineering workflows

Large EPC firms and utility companies rely on consistent naming for documentation, system modeling, and maintenance planning.

3. A Typical Letter Designation Format for Solid Insulation Type Switchgear

Different brands may vary, but most solid-insulated switchgear naming follows a structure similar to:

SIS – M – R – V – 12 – E

Where each block represents a category:

The following chapters break down the most common meanings for each letter used across the industry.

4. Meaning of the Most Common Letters Used in Solid Insulation Switchgear Names

Below is a unified interpretation used by leading MV manufacturers worldwide.

4.1 S — Solid Insulation / Sealed Structure

“S” is the most common indicator of solid insulation:

• S = Solid insulated

• S = Sealed (fully encapsulated)

• S = Safe (touch-proof) in some naming conventions

Frequently used forms:

• SIS = Solid Insulated Switchgear

• SIS-RMU = Solid Insulated Ring Main Unit

• S-M = Modular solid-insulated panel

Whenever an “S” appears, it refers either to the insulation technology or a sealed construction.

4.2 I — Insulated / Indoor Type

“I” often follows “S” as part of:

• SI = Solid Insulated

• I = Indoor installation

• I = Insulation category

Many switchgears use “I” to clarify that the product is designed for indoor substations and enclosed structures.

4.3 V — Vacuum Interrupter / Vacuum Switch

One of the most universal letters used in switchgear models:

• V = Vacuum circuit breaker

• V = Vacuum load break switch

• V = Vacuum interrupter module

In solid insulated switchgear, the main switching component is nearly always vacuum-based, so “V” is a key identifier.

Examples:

• SIS-V12 — solid-insulated, vacuum switch, 12 kV

• SVR — vacuum ring-main configuration

4.4 R — Ring Main Unit (RMU)

“R” appears whenever the switchgear is built as part of a ring network:

• R = Ring Main Unit

• R = Ring configuration

• R = Radial operation mode (rare cases)

If your project includes distribution automation, loop networks, or multi-directional supply, “R” is a critical letter to look for.

4.5 M — Modular / Metal-Enclosed / Metering

Different manufacturers use this letter differently:

Most common meanings

• M = Modular

• M = Metal-enclosed

• M = Metering unit (when used later in the code)

Example structure:

• SIS-MV-12-E

  • M = modular

  • V = vacuum

  • 12 = voltage

4.6 C — Cable Compartment / Compact / Control Unit

“C” appears widely in models for compact systems:

• C = Compact structure

• C = Cable switch / cable connection compartment

• C = Control section (auxiliary unit)

Single-block solid-insulated units often include “C” in their designation.

4.7 E — Extensible / Expansion Ready

“E” is one of the most important letters for future-proof engineering design:

• E = Extensible

• E = Expansion-ready busbar

• E = Extension modules supported

Example:

• SIS-RV12-E = 12 kV solid insulated switchgear with vacuum switch and extensible busbar.

4.8 T — Transformer Protection / Terminal Unit

“T” indicates a transformer-related function:

• T = Transformer protection panel

• T = Terminal feeder

Common examples:

• SIS-RT = Ring-main switchgear with transformer protection

• VT = Voltage transformer (when used together)

4.9 P — Protection / Pole Unit

“P” is often used in protection-oriented modules:

• P = Protection unit

• P = Pole (as in three-pole encapsulation)

• P = Plug-in type cable termination

You see variants like:

• SISP (solid insulated switchgear, protection type)

• V/P (vacuum protection device)

4.10 L — Load Break Switch

“L” appears in units where the switching device is not a full VCB:

• L = Load break switch

• L = Load feeder

• L = Loop switch (partial usage)

If the designation includes both L and V, it shows the panel includes multiple switching devices.

4.11 F — Fuse Combination / Feeder

Two common meanings:

• F = Fuse protection

• F = Feeder panel

Solid insulated RMUs frequently use:

• SF = solid insulated + fuse

• RF = ring main + fuse

4.12 Additional Letters Found in Advanced Models

These appear less frequently but are essential in some manufacturer catalogs.

5. What the Number Designations Mean

Just like letters, numbers also encode critical performance parameters.

Most common number meanings

A typical full designation may look like:

SIS-RV-12-630A-E

Which means:
Solid insulated switchgear, ring-main with vacuum switch, 12 kV, 630A, extensible design.

6. How These Designations Help in Product Selection

Understanding these designations allows engineers to quickly determine:

1. Insulation class

Solid vs gas vs air.

2. Feeder type

Incoming, outgoing, transformer, measurement, automation.

3. Switching device type

VCB vs LBS vs fuse.

4. Network topology

Radial, ring-main, dual-supply, or looped.

5. Future scalability

Whether the system is extensible.

6. Environmental suitability

Indoor vs outdoor, compact vs modular.

7. Operational features

Grounding, interlocking, safety shutters, smart automation.

This helps reduce procurement errors and ensures compatibility with existing networks.

chinadegatech-Solid Insulation Type Switchgear

7. Comparison With Other Switchgear Naming Systems

Solid insulated switchgear naming resembles other common MV naming systems:

However, solid insulated switchgear adds unique letters for encapsulation and polymer technology.

8. Frequently Asked Questions (FAQ)

Q1: Are letter designations standardized across all manufacturers?

Not completely. Many letters are industry-common (such as V for Vacuum), but each manufacturer maintains proprietary naming logic.

Q2: What is the most important letter to identify in a solid insulation switchgear model?

“S” (solid), “I” (insulated or indoor), and “V” (vacuum) are the core identifiers.

Q3: Is “E” always used for extensible switchgear?

Most brands use “E”, but some use “X” or “EXT”.

Q4: Does the letter affect the performance of the switchgear?

No. Letters describe features but do not directly determine performance. Performance depends on design, materials, and quality.

Q5: Where can I find the manufacturer’s official interpretation of letter designations?

In the switchgear datasheet or technical manual. But cross-brand understanding (provided in this article) helps greatly.

Q6: Do letters also indicate environmental sustainability?

Sometimes. Certain manufacturers use symbols like “ECO”, “G”, or “N” to identify eco-friendly or SF6-free products.

Q7: Are solid-insulated switchgear always SF6-free?

Yes. By definition, SIS uses solid dielectric materials instead of SF6 gas.

Conclusion

The letter designations on solid insulation type switchgear provide a powerful shorthand system for understanding product structure, switching components, configuration, insulation method, rating, protection level, and extensibility.

Whether you are selecting equipment for utilities, industrial plants, renewable energy installations, or urban distribution networks, decoding these letters allows more accurate, efficient, and reliable engineering decisions.