Solid Insulation Switchgear Technology


Our discussion here will be limited to Medium Voltage Switchgear up to 33kV. Before we understand solid insulation switchgear technology, we will first go into the history of medium voltage switchgear solutions. If we talk of evolution of medium voltage solution, then we can broadly categorize them under two sections as given below:

  • Enclosure design
  • Interruption technology.

On the enclosure side it was started with masonry enclosures and later evolved into metallic enclosures. Whereas interruption technology has seen many transformations in the past few decades. It started with following sequence:

  • Bulk Oil
  • Minimum Oil
  • SF6 and then
  • Vacuum

In all forms of technology “Air” is being used as insulation medium between live parts of the switchgear. Now a day’s most prevalent technology across the globe is Air Insulated Switchgear (AIS) in 11kV and 33KV categories.

(Air Insulated Switchgear Panel)

The latest evolution in insulation technology for medium voltage devices is “Solid Insulation Switchgear” while interruption technology remains in Vacuum.

Solid Insulation System

Solid Insulation Switchgear
(Solid Insulation Switchgear, Source: Schneider Electric)

Solid insulation means “Air” is being replaced with “Solid Insulating Material”. We all know that in any Air Insulated Medium voltage switchgear panel all live components like Bus Bar, CT, PT and Cable terminations are surrounded by Air with required creepage and clearance to sustain power frequency & impulse voltages.

Epoxy is being evolved as a very good medium for insulation and Solid Insulation Switchgear Technology is based on excellent insulation property of epoxy. Suppose if all the live components mentioned above are being encapsulated inside epoxy and electrical connections are also insulated once connection is done then there is no need of Air as an insulating material. We can understand the same with below picture:

Solid Insulation Switchgear - Parts description
(Source: Schneider Electric)

We can see all electrical parts are insulated with an insulation level suitable for operating voltage. So the air insulation is not needed therefore the air gap can be reduced to a level of assembly requirement for fitment. This advantage of epoxy insulation helps in drastic reduction in overall size of the Solid Insulation Switchgear panel. The width got reduced to approximately around half of width of conventional AIS Panels.

Advantage of Solid Insulation Switchgear (SIS) over AIS

  • Low footprint saves on space required for installation of Solid Insulation Switchgear Panel.
  • SIS does not have impact of harsh environment like
    • high humidity
    • high degree of pollution
    • corrosive coastal atmosphere because of neutrality of epoxy to these conditions.
    • All three above factors could lead to condensation and subsequent tracking leading to flashover, degradation of conductive and metallic parts due to corrosive acid formation and corrosion of conductive & metallic enclosure in AIS. All above conditions may lead to failure of AIS Panel.
  • Epoxy is also neutral to conductive dust which may lead to tracking and subsequent flashover in AIS. This kind of environment is prevalent in metal, mineral and mining industry.
  • Due to low density of AIR at high altitudes, BIL of AIS Panels gets de-rated but SIS can work up to a height of 3000Meters.
  • SF6 free solution gives SIS edge over GIS Panels in spite of the fact that GIS also has most of the features similar to it. SF6 is not an environmental friendly gas and its replacement in future is inevitable with some suitable alternative.
  • SIS is accidently touchable, so even if a reptile enters inside the Panel enclosure there will not be any flashover.
  • Owing to all the above advantages we can say Solid Insulation Switchgear Panel is maintenance free and with a life span of around 40 years.
  • The OPEX will be negligible compared to AIS Panel.


Cost of purchase of Solid Insulation Switchgear is very high compared to conventional solution of Air Insulation Switchgear Panel. In special application cases like high altitude, high humidity, highly corrosive atmosphere, atmosphere with conductive dust, it is the best available solution to date. Benefits of Solid Insulation Switchgear overweight’s its high cost of acquisition for special application situations.

Frequently Asked Questions

Q- What is the material of SSIS?

A- Epoxy

Q- Is it shielded?

A- Yes with some manufacturer.

Q- What material is used for Shielding?

A- Epoxy surface is coated with Silicon layer which works as electrical shield.

Q- Is it available with earth switch?

A- Yes, it comes by default.

Q- Is it accidently touchable?

A- Yes

External References

Internal Reference

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