Introduction
For decades, the choice of insulation medium in medium voltage switchgear was effectively binary: air or SF6 gas1. Air-insulated switchgear demanded large physical footprints and regular maintenance. SF6 gas-insulated switchgear delivered compactness and performance but introduced a potent greenhouse gas with a global warming potential 23,500 times that of CO₂ — a liability that grows heavier with every tightening of environmental regulation.
Solid insulation switchgear technology replaces both air gaps and SF6 gas with cast epoxy resin2 as the primary insulation medium — encapsulating live conductors, busbars, and switching elements in a solid dielectric material that provides superior pollution resistance, eliminates gas management requirements, reduces installation footprint by up to 50% compared to AIS, and delivers a maintenance-free insulation system rated for 30-year service life.
For electrical engineers designing secondary substations, industrial power systems, and renewable energy MV infrastructure, SIS technology represents a fundamental shift in how medium voltage insulation is engineered — not an incremental improvement on existing gas or air technology, but a different insulation philosophy with distinct performance characteristics, environmental credentials, and lifecycle economics. Understanding what solid insulation switchgear technology is, how it works, and where it outperforms alternatives is the foundation of every well-specified modern MV switchgear procurement.
This article provides a complete technical reference for solid insulation switchgear technology — from insulation physics and material science to system architecture, application selection, and maintenance requirements across the full MV distribution range.
Table of Contents
- What Is Solid Insulation Technology and How Does It Work in MV Switchgear?
- How Does SIS Switchgear Performance Compare to AIS and GIS Across Key Parameters?
- How to Specify and Select Solid Insulation Switchgear for Your Application?
- What Are the Installation, Maintenance, and Lifecycle Requirements of SIS Switchgear?
What Is Solid Insulation Technology and How Does It Work in MV Switchgear?
Solid insulation switchgear technology is the application of cast solid dielectric materials — primarily epoxy resin compounds — as the primary insulation medium surrounding all live MV conductors, busbars, and switching element interfaces within a switchgear assembly. Unlike air insulation (which relies on physical clearance distances) or gas insulation (which relies on pressurized SF6 to achieve dielectric strength), solid insulation achieves its dielectric performance through the intrinsic molecular structure of the encapsulating material itself.
The Physics of Solid Dielectric Insulation
In any insulation system, dielectric strength is the maximum electric field the material can withstand before breakdown — the point at which charge carriers accelerate through the material, creating a conductive path and catastrophic failure. The dielectric strength of the insulation medium determines how close live conductors can be positioned to grounded structures and to each other, which directly governs equipment physical size.
Comparative Dielectric Strengths:
- Air (1 bar, uniform field): 30 kV/cm
- SF6 (3 bar): ~220 kV/cm
- Cast epoxy resin (APG): 180–200 kV/cm (bulk); effectively unlimited at surfaces with proper field grading
The bulk dielectric strength of cast epoxy resin approaches that of pressurized SF6 — which is why SIS switchgear achieves comparable compactness to GIS without requiring any pressurized gas system. More importantly, solid insulation eliminates the surface flashover failure mode that limits air-insulated equipment in polluted environments: a solid epoxy surface cannot be contaminated by airborne particles, moisture, or condensation in the way that air-gap insulation surfaces can.
Automatic Pressure Gelation (APG) — The Manufacturing Technology
The solid insulation in SIS switchgear is produced by Automatic Pressure Gelation (APG) — a precision casting process that injects liquid epoxy resin compound under controlled pressure into a heated mold containing the conductor assembly, then cures the resin under precise temperature and pressure profiles to produce a void-free, bubble-free solid insulation body.
APG Process Critical Parameters:
- Resin system: Cycloaliphatic epoxy resin with anhydride hardener and alumina trihydrate (ATH) filler for enhanced arc resistance and thermal stability
- Mold temperature: 130–160°C during gelation; controlled to prevent thermal stress cracking
- Injection pressure: 3–8 bar to eliminate voids and ensure complete conductor encapsulation
- Cure cycle: 4–8 hours at elevated temperature; followed by post-cure at 140°C for dimensional stability
- Quality control: Each cast component undergoes partial discharge3 testing (< 5 pC at 1.5 × Um) to verify void-free insulation
Voids in cast epoxy insulation are the primary quality failure mode — a void as small as 0.1mm diameter creates a partial discharge inception point that progressively erodes the surrounding insulation under operating voltage, eventually causing insulation failure. The APG process, properly controlled, eliminates voids by maintaining positive pressure throughout gelation, preventing the formation of shrinkage cavities as the resin cures.
Electric Field Grading in Solid Insulation Systems
At geometric discontinuities — conductor edges, connection interfaces, and insulation boundaries — the electric field concentrates to levels that can exceed the local dielectric strength even when the average field is well within limits. Solid insulation SIS design uses two techniques to manage field concentration:
Geometric Field Grading:
Conductor edges and termination interfaces are designed with controlled radii (minimum 3–5mm for MV applications) to distribute the electric field over a larger surface area, reducing peak field intensity below the partial discharge inception threshold.
Resistive or Capacitive Field Grading Layers:
At the interfaces between solid insulation components — busbar joints, cable terminations, and interrupter connections — field grading layers of semi-conductive or capacitively graded material are applied to redistribute the electric field gradient uniformly across the interface, preventing field concentration at the junction boundary.
SIS Switchgear System Architecture
A complete SIS switchgear panel integrates solid insulation technology across all primary insulation functions:
- Epoxy-encapsulated busbars: Three-phase busbars fully encapsulated in cast epoxy, eliminating phase-to-earth air clearance requirements
- Solid insulation current transformers (CTs): Toroidal CTs cast directly onto the encapsulated busbar — no separate CT mounting or air clearance required
- Epoxy-encapsulated cable terminations: Plug-in or bolted cable interfaces with pre-moulded stress cones providing field-graded solid insulation continuity from cable to busbar
- Vacuum interrupter4 assembly: The switching element — a vacuum interrupter per phase — mounted within the solid insulation structure, with epoxy encapsulation providing both mechanical support and primary insulation to earth
- Magnetic actuator mechanism: Permanent magnet actuator (PMA) operating mechanism providing M2 mechanical endurance with sealed, maintenance-free construction
Key Solid Insulation Material Properties
| Property | Cast Epoxy (APG) | Air (Reference) | SF6 (3 bar) |
|---|---|---|---|
| Dielectric Strength (bulk) | 180–200 kV/cm | 30 kV/cm | ~220 kV/cm |
| Relative Permittivity (εr) | 3.5–4.5 | 1.0 | 1.006 |
| Thermal Class | F (155°C) | — | — |
| Pollution Resistance | Excellent (sealed surface) | Poor (surface contamination) | Excellent (sealed) |
| Partial Discharge Inception | > 1.5 × Um (void-free) | N/A | > 1.5 × Um |
| Thermal Conductivity | 0.2–0.8 W/m·K | 0.026 W/m·K | 0.014 W/m·K |
| Arc Resistance (IEC 61621) | > 180 seconds | N/A | N/A |
| GHG Impact | None | None | GWP 23,500 |
How Does SIS Switchgear Performance Compare to AIS and GIS Across Key Parameters?
Solid insulation switchgear occupies a distinct performance position relative to AIS and GIS — combining the environmental credentials and maintenance simplicity of vacuum technology with a compactness approaching GIS, at a lifecycle cost that typically undercuts both alternatives for MV distribution applications in the 12–40.5kV range.
Footprint and Space Efficiency
SIS switchgear achieves its compact footprint through the elimination of air clearance distances. In AIS, the minimum phase-to-phase and phase-to-earth clearances required by IEC 62271-1 at 12kV are:
- Phase-to-earth clearance (air): 120mm minimum
- Phase-to-phase clearance (air): 160mm minimum
In SIS, these clearances are replaced by solid epoxy insulation with dielectric strength of 180–200 kV/cm — reducing the required insulation thickness to 8–15mm at 12kV. The result is a panel width reduction of 40–60% compared to equivalent AIS, and a depth reduction of 30–50%.
Typical Panel Dimensions Comparison (12kV, 630A, 25kA):
| Parameter | AIS | GIS | SIS |
|---|---|---|---|
| Panel Width | 800–1,000mm | 500–650mm | 400–550mm |
| Panel Depth | 1,200–1,600mm | 800–1,000mm | 600–800mm |
| Panel Height | 2,200mm | 2,000mm | 1,800–2,000mm |
| Floor Area per Panel | 0.96–1.60 m² | 0.40–0.65 m² | 0.24–0.44 m² |
| Relative Footprint | 100% (reference) | ~45% | ~30% |
Maintenance Requirements
The sealed construction of SIS switchgear — solid epoxy insulation with no air gaps to contaminate, no SF6 gas to monitor, and vacuum interrupters with no internal maintenance access — produces a maintenance profile fundamentally different from AIS or GIS:
AIS Maintenance Requirements:
- Annual: Insulation surface cleaning; contact resistance measurement
- 3 years: Arc chute inspection and cleaning; mechanism lubrication
- 5 years: Full overhaul; contact replacement assessment
- Post-fault: Immediate arc chute inspection; insulation surface decontamination
GIS Maintenance Requirements:
- 6 months: SF6 pressure check; leak inspection
- 1 year: Gas moisture and purity analysis
- 3 years: Full gas analysis; contact resistance check
- Post-fault: Gas quality analysis; decomposition product check before re-energization
SIS Maintenance Requirements:
- Annual: Contact resistance measurement; operating time check; visual inspection
- 3 years: Power frequency hi-pot test; partial discharge measurement
- 5 years: Contact travel measurement; full electrical verification
- Post-fault: Hi-pot test + PD measurement + contact resistance
The elimination of arc chute maintenance, SF6 gas management, and insulation surface cleaning reduces SIS annual maintenance cost by 60–75% compared to AIS and 40–55% compared to GIS over a 25-year service life.
Environmental Performance
SIS switchgear’s environmental credentials are a direct consequence of its technology choices:
- Zero SF6: No greenhouse gas content, no F-Gas regulation obligations, no certified gas handling personnel requirement, no end-of-life gas recovery cost
- No arc gases: Vacuum arc extinction produces no toxic decomposition products — no SOF₂, SO₂F₂, or HF generation during switching operations
- Reduced material volume: Compact design uses less steel, copper, and insulation material per rated MVA than AIS
- Recyclable at end of life: Epoxy resin encapsulation can be mechanically separated from copper conductors for material recovery; no hazardous gas disposal required
Full Performance Comparison: SIS vs. AIS vs. GIS
| Parameter | AIS | GIS (SF6) | SIS (Vacuum) |
|---|---|---|---|
| Voltage Range | 12–40.5kV | 12–1,100kV | 12–40.5kV |
| Relative Footprint | 100% | ~45% | ~30% |
| Arc Quenching Medium | Air | SF6 | Vacuum |
| Insulation Medium | Air | SF6 | Solid Epoxy |
| Pollution Resistance | Poor | Excellent | Excellent |
| Maintenance Frequency | High | Medium | Low |
| SF6 GHG Content | None | Yes (GWP 23,500) | None |
| Electrical Endurance | E1 standard | E1–E2 | E2 standard |
| Mechanical Endurance | M1 standard | M1–M2 | M2 standard |
| Lifecycle Cost (25yr) | Medium | Medium-High | Low |
| Suitable Environments | Indoor clean | Indoor/Outdoor | Indoor/harsh |
Customer Case: SIS Switchgear Solving a Space and Environment Compliance Challenge
A procurement manager overseeing a 24kV secondary substation upgrade for a pharmaceutical manufacturing campus in Western Europe contacted Bepto with two simultaneous constraints: the available substation room was 35% smaller than the footprint of the existing AIS equipment being replaced, and the campus environmental policy prohibited any SF6-containing equipment in new installations — eliminating GIS as an option.
After specifying Bepto’s SIS switchgear with solid epoxy insulation and vacuum interrupters, the engineering team installed a complete 24kV switchgear lineup — eight feeder panels plus bus section — within the available room footprint, with 15% clearance to spare. The zero-SF6 design satisfied the campus environmental policy without compromise, and the sealed solid insulation construction was specified as requiring no annual maintenance interventions beyond contact resistance measurement — a significant operational benefit for a pharmaceutical facility where substation access requires clean-room protocols.
How to Specify and Select Solid Insulation Switchgear for Your Application?
Specifying SIS switchgear correctly requires systematic evaluation of electrical requirements, environmental conditions, space constraints, maintenance capability, and regulatory obligations — with particular attention to the insulation system verification requirements that distinguish genuine solid insulation performance from marketing claims.
Step 1: Define Electrical Requirements
- Rated Voltage: 12kV, 24kV, or 40.5kV — confirm BIL (75 / 125 / 185kV) matches system insulation coordination
- Rated Normal Current: 630A, 1250A, or 2500A — verify thermal rating at maximum ambient temperature (standard 40°C; derated above)
- Short-Circuit Rating: 16kA, 20kA, 25kA, or 31.5kA — confirm both short-circuit breaking current (vacuum interrupter) and short-time withstand current (busbar and enclosure)
- Endurance Classes: Specify M2/E2 for all automatic or frequently switched applications; verify both classes in type test certificate
- Special Switching Duties: Identify capacitive, inductive, or motor switching requirements; confirm vacuum interrupter special duty ratings
Step 2: Verify Insulation System Quality
- Partial Discharge Test: Require factory PD test certificate for every cast epoxy component at 1.5 × Um/√3; PD < 5 pC confirms void-free insulation
- Dielectric Type Test: Confirm power frequency and lightning impulse withstand tests per IEC 62271-1 were conducted on the complete panel assembly, not individual components
- Insulation Resistance: Require IR measurement > 1,000 MΩ at 2.5kV DC between phases and phase-to-earth at factory acceptance
- Thermal Cycling Test: For installations with wide temperature variation, confirm insulation system has been qualified for the specified temperature range without cracking or delamination
Step 3: Match Standards and Certifications
- IEC 62271-2005: Metal-enclosed MV switchgear — primary standard for complete SIS panel assembly
- IEC 62271-100: Vacuum circuit breaker type test — short-circuit breaking, load-break, and endurance
- IEC 62271-1: Common specifications — dielectric withstand, temperature rise, mechanical endurance
- IEC 61641: Internal arc testing — specify IAC classification (AFL / AFLR) for personnel safety
- IEC 60270: Partial discharge measurement — specify PD acceptance level for insulation quality verification
- GB/T 11022 / GB/T 3906: China national standards for HV switchgear assemblies
Application Scenarios
- Urban Secondary Substations: SIS for compact footprint in space-constrained city-center installations; zero SF6 for environmental compliance
- Industrial MV Substations: SIS for chemical, pharmaceutical, food processing, and cement plants — sealed insulation immune to aggressive atmospheres
- Renewable Energy MV Collection: SIS for solar and wind farm feeder switching — 25-year maintenance-free design life matching renewable asset lifecycle
- Data Center MV Distribution: SIS for critical power infrastructure — highest reliability, zero unplanned maintenance, no gas management complexity
- Marine and Offshore: SIS with IP65+ enclosure for platform power distribution — salt fog and humidity immunity without SF6 environmental risk
- Building-Integrated Substations: SIS for substations inside commercial buildings, hospitals, and airports — compact, silent, zero gas emission
What Are the Installation, Maintenance, and Lifecycle Requirements of SIS Switchgear?
The sealed, solid insulation construction of SIS switchgear simplifies installation and maintenance compared to AIS and GIS — but it introduces specific requirements for insulation system verification, busbar joint quality, and condition monitoring that must be understood and implemented to realize the technology’s full lifecycle performance.
Pre-Commissioning Installation Checklist
- Busbar Joint Torque Verification — All busbar bolted connections must be torqued to manufacturer specification using a calibrated torque wrench; under-torqued joints cause resistive heating and insulation thermal stress; over-torqued joints crack epoxy encapsulation
- Cable Termination Stress Cone Inspection — Pre-moulded stress cones at cable interfaces must be correctly seated and free of contamination; improper installation creates field concentration at the cable-to-busbar interface
- Panel Alignment and Levelling — SIS panels must be aligned and levelled to manufacturer tolerance before busbar coupling; misalignment stresses epoxy busbar joints and can cause cracking under thermal expansion
- Partial Discharge Acceptance Test — Conduct PD measurement on the complete installed panel at 1.2 × Um/√3 per IEC 60270 before energization; PD > 10 pC on the installed assembly indicates a joint or termination defect requiring investigation
- Insulation Resistance Test — Measure IR at 2.5kV DC between phases and phase-to-earth; IR > 1,000 MΩ required before energization
- Vacuum Interrupter Hi-Pot Test — Apply power frequency test voltage across open contacts per IEC 62271-100; confirms vacuum integrity of all interrupters after transport and installation
SIS Switchgear Maintenance Schedule
| Interval | Action | Acceptance Criterion |
|---|---|---|
| Annual | Contact resistance; operating time; visual inspection | < 100 μΩ; ±20% of baseline; no damage |
| 3 years | Power frequency hi-pot (open contacts); PD measurement | No flashover; PD < 10 pC installed |
| 5 years | Contact travel measurement; full electrical verification | Stroke > minimum wear limit; all parameters in spec |
| 10 years | Comprehensive assessment; mechanism inspection | Per manufacturer protocol |
| Post-fault | Hi-pot + PD + contact resistance; insulation thermal scan | Full acceptance criteria |
Common SIS Installation and Operational Mistakes
- Incorrect busbar joint torque — the single most common SIS installation defect; under-torqued joints cause progressive contact resistance increase and thermal runaway; always use calibrated torque tools and verify with thermal imaging at first load
- Omitting post-installation PD test — transport vibration and installation handling can damage epoxy components or disturb cable stress cones; PD testing is the only reliable method to detect installation-induced insulation defects before energization
- Applying thermal spray or paint to epoxy surfaces — field-applied coatings on epoxy insulation surfaces alter the surface resistivity and can create partial discharge inception points; never apply any coating to factory-finished epoxy insulation
- Exceeding rated short-circuit making current — vacuum interrupters are rated for a specific peak making current (2.5 × Isc); exceeding this value risks contact welding that prevents subsequent trip operation
Conclusion
Solid insulation switchgear technology represents the convergence of three independent engineering advances — cast epoxy insulation, vacuum arc extinction, and permanent magnet actuation — into a switchgear system architecture that simultaneously addresses the space constraints, maintenance burdens, environmental obligations, and reliability demands of modern MV power distribution. For the 12–40.5kV application range where SIS technology operates, it delivers a compelling combination of compact footprint, zero SF6 environmental impact, E2/M2 endurance class performance, and 25-year maintenance-minimized service life that neither AIS nor GIS can match across all parameters simultaneously.
Specify solid insulation switchgear where space is constrained, environments are harsh, maintenance access is limited, or environmental compliance prohibits SF6 — and verify insulation quality through partial discharge testing, not just voltage rating, because in solid insulation technology, the quality of the cast epoxy is the quality of the switchgear.
FAQs About Solid Insulation Switchgear Technology
Q: What is the fundamental difference between solid insulation switchgear and conventional air-insulated switchgear in terms of insulation principle?
A: AIS relies on physical air clearance distances (120–160mm at 12kV) to achieve dielectric withstand. SIS replaces air gaps with cast epoxy resin (dielectric strength 180–200 kV/cm), reducing insulation thickness to 8–15mm at 12kV — enabling 40–60% panel width reduction while eliminating surface contamination failure modes.
Q: Why does solid insulation switchgear achieve better pollution resistance than air-insulated switchgear in industrial environments?
A: AIS insulation surfaces are exposed to airborne contamination — dust, moisture, and chemical vapors — that progressively reduce surface resistivity and creepage withstand, eventually causing flashover. SIS epoxy encapsulation seals all live conductors in solid dielectric with no exposed air-gap surfaces, making contamination ingress physically impossible.
Q: What manufacturing process ensures void-free solid insulation quality in SIS switchgear components?
A: Automatic Pressure Gelation (APG) injects liquid epoxy resin under 3–8 bar pressure into heated molds containing conductor assemblies, curing under controlled temperature and pressure to eliminate shrinkage voids. Each component is verified by partial discharge testing at 1.5 × Um — PD < 5 pC confirms void-free insulation quality.
Q: How does solid insulation switchgear compare to SF6 gas-insulated switchgear for environmental compliance in new installations?
A: SIS contains zero SF6 — eliminating GWP 23,500 greenhouse gas content, F-Gas regulation obligations, certified gas handling requirements, and end-of-life gas recovery costs. For projects with environmental policies prohibiting SF6 or subject to EU F-Gas regulation phase-down, SIS is the technically equivalent zero-emission alternative for the 12–40.5kV range.
Q: What is the correct method to verify solid insulation quality in an installed SIS switchgear panel before energization?
A: Conduct partial discharge measurement on the complete installed assembly at 1.2 × Um/√3 per IEC 60270 — PD < 10 pC confirms installation integrity. Supplement with insulation resistance measurement (> 1,000 MΩ at 2.5kV DC) and power frequency hi-pot test on vacuum interrupter open contacts per IEC 62271-100.
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technical insights into the high global warming potential of SF6 gas compared to CO2 ↩
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material science data on dielectric strength and thermal stability of cast epoxy resin ↩
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diagnostic methods for detecting insulation voids and ensuring long-term dielectric reliability ↩
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engineering details on arc quenching technology and electrical endurance in vacuum environments ↩
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official safety and performance requirements for medium voltage metal-enclosed switchgear assemblies ↩
