Looking to specify or buy an epoxy insulator? This article explains what they are — our product pages give the technical detail and a route to a quotation.
What Are Epoxy Insulators?
An epoxy insulator is an electrical insulator cast from epoxy resin and a hardener. What separates it from porcelain or glass is not the insulating ability — it is that cast resin is formed in a mould. The geometry is chosen by the designer instead of being imposed by the material, and metal parts can be encapsulated inside the casting rather than bolted on afterwards.
On this page
1. What an epoxy insulator actually is
An electrical insulator is a material that does not allow internal electric charges to flow freely, which inhibits the conduction of an electric current under the influence of an electric field. Insulators are made from a range of materials — glass, porcelain, glass-fibre, silicones and epoxy resins among them.
How well any of them insulates depends on three things together: the material itself, the distance between the live conductor and earth, and the voltage applied. That relationship is what the terms breakdown voltage and breakdown strength describe.
Epoxy insulators are made by mixing an epoxy resin with a hardener and curing it in a mould. Because the part is cast rather than fired or machined, two things follow that matter in practice:
- The geometry is free. Ribs, creepage profiles, mounting features and fixing centres are moulded in one piece, with no assembly and no fasteners inside the insulating path.
- Metal can be encapsulated. Inserts, electrodes and conductors can be cast into the resin, producing a monolithic part instead of an assembly.
2. The two resin families, and which one you need
This is the decision that matters most, and it is decided by the environment the part will live in, not by the voltage.
| Aromatic systems | Cycloaliphatic systems | |
|---|---|---|
| Typical chemistry | Bisphenol A-epichlorohydrin resin with an anhydride hardener | Cycloaliphatic epoxy resin and hardener, filled with pre-treated silica |
| Belongs | Indoors, inside enclosed equipment | Outdoors, and indoors with high humidity or pollution |
| Why | Excellent dielectric performance where the surface is protected from weather | Resists UV, tracking and weathering without the surface degradation that limits general-purpose resins |
| Voltage classes | Low, medium, high and very high | Low, medium, high and very high |
Both families cover the whole voltage range, so the choice is about exposure. A part destined for a sealed indoor panel and a part bolted to a trackside structure are the same insulator in function and a different resin in specification.
Two options beyond the standard families.
Resin systems can be developed to specific electrical, thermal and mechanical requirements when neither standard family fits. And where a cast part needs extra surface protection, a dedicated silicone line lets us overmould the epoxy insulator — useful in heavily polluted or coastal environments. Read more on our epoxy resin technology.
3. How much voltage epoxy withstands
Cast epoxy is a robust polymer with breakdown strengths ranging from 20 kV/mm to 40 kV/mm, depending on the system used. That figure is a property of the material, and it is worth being clear about what it does and does not tell you.
It tells you the resin has a large margin: a few millimetres of sound epoxy withstand tens of kilovolts. It does not, on its own, tell you the withstand voltage of a finished part. That depends on the geometry — the creepage path along the surface, the clearances through air, the field distribution around any embedded metal — and on the surface condition in service. Which is why a part is dimensioned to the system voltage rather than calculated from the material figure alone.
The real enemy is a void, not the voltage
A gas-filled void inside a casting sits in the electric field and ionises at a fraction of the voltage the surrounding resin would take. That is a partial discharge: a small, local breakdown that does not fail the part immediately but erodes it from the inside over years. It is why resin is prepared under vacuum before casting, and why finished parts are tested for partial discharges rather than only for withstand voltage.
Where this lands in practice
Indisol casts insulators across low, medium, high and very high voltage. As a concrete example of what a finished, characterised product looks like, our medium voltage sensor range is built for 12, 24, 36 and 72 kV with accuracy classes of 0.2% or 0.5%.
4. The forms an epoxy insulator takes
“Epoxy insulator” is a family name, not a part number. These are the forms it takes, with the names buyers and datasheets actually use.
| Form | What it does | Also called |
|---|---|---|
| Support and post insulators | Hold conductors at different potentials apart while carrying mechanical load — traction, bending or torsion | Post insulators, standoffs |
| Bushings | Carry a live conductor through a wall, an enclosure or a tank while keeping it insulated from the metal around it | Epoxy bushings, MV bushings |
| Epoxy housings | Enclose switches in medium voltage equipment, in substations and transformer stations. Cast to almost any shape and size, so they suit tailored solutions | Pole-housings |
| Epoxy terminals and bushing plates | Round or square resin plates, threaded with bolts or cables, that manage and distribute voltage in instrument transformers and oil-filled transformers. High voltage versions exist for HV equipment | Epoxy terminal bushings, terminal blocks |
| Parts for gas insulated switchgear | Busbar supports, fuse tubes and circuit breaker chambers inside SF6 and gas insulated equipment, where compact spacing raises the electrical stress | GIS insulators |
| Third rail insulators | Hold the conductor rail that powers a metro or light-rail vehicle, outdoors or in tunnel | Conductor rail, power rail insulators |
| Divider insulators | Insulate and, at the same time, produce a low voltage signal that reproduces the voltage on the conductor | Resistive, capacitive or RC dividers |
Beyond insulators proper, epoxy resin compositions are used to insulate motors, generators, transformers, switchgear and bushings, protecting components against short circuits, water and dust. In electronics the same chemistry overmoulds integrated circuits, transistors and hybrid circuits, and goes into printed circuit boards.
5. How a cast epoxy part is made
The process decides whether the part performs, because most of what can go wrong electrically goes wrong during casting.
- Resin prepared under vacuum, so the mix goes into the mould without entrained air — the voids that later become partial discharge sites.
- Gelation under pressure at temperature for medium and large series. This is what holds both dielectric reliability and dimensional accuracy across a whole production run, not just on the first parts.
- Vacuum or atmospheric pressure casting for specialised needs and smaller series.
- Metallization and sand blasting in house, which keeps the electrode and creepage surfaces under our own control rather than a subcontractor’s.
- 29 clamping machines and a continuous mixing line, covering pieces above 50 kg and one metre down to small, intricate components.
- A dedicated silicone line for overmoulding epoxy insulators where a design needs an additional protective layer.
Total epoxy processing capacity is around 2 000 tonnes a year. The technology page sets out the equipment in more detail.
6. How they are tested
An insulator that has not been tested is a casting with an opinion attached. For gas insulated applications, where the electrical stress is highest, every piece is tested — 100%, not a sample: industrial frequency, partial discharges, and X-ray examination of the casting. Bushings additionally go through serial electric tests, gas-leakage testing and customer-specific inspections.
| Electrical | Mechanical |
|---|---|
| Industrial frequency to 200 kV Partial discharges to 200 kV High voltage and current metrology, including amplitude and phase accuracy on sensors Contact resistance |
Flexion Traction Compression Twist Impact Roughness |
Testing is done both in house and in external laboratories, and production runs under ISO 9001 and ISO 14001.
7. Epoxy or porcelain?
Porcelain has a century of service behind it and remains a sound choice in plenty of installations. Cast epoxy wins where you need a shape porcelain cannot easily give you, where metal has to be encapsulated inside the insulator, or where weight and brittleness are a problem. It loses nothing in insulating ability — the trade-offs are mechanical, geometric and industrial rather than dielectric.
We wrote a longer comparison of the two: epoxy bushing versus porcelain bushing.
8. Glossary
A resin poured into a mould and cured to its final shape, as opposed to a material that is fired, extruded or machined. Cast resin insulators are formed rather than shaped.
The indoor family, typically a Bisphenol A-epichlorohydrin resin with an anhydride hardener. Used where the insulator sits inside enclosed equipment.
The outdoor family, filled with pre-treated silica. Resists UV, tracking and weathering, so it is specified for exposed installations and for indoor environments with high humidity or pollution.
The voltage at which an insulator stops insulating and conducts. Expressed for a material in kV per millimetre — 20 to 40 kV/mm for cast epoxy — and for a finished part as a withstand voltage that also depends on geometry.
A small, localised electrical breakdown, usually inside a void in the casting, that does not fail the part at once but erodes it over years. The reason resin is degassed under vacuum and parts are PD-tested.
The shortest path along the insulator’s surface between live metal and earth. Ribs and skirts exist to lengthen it, which is why outdoor insulators have a deeper profile than indoor ones.
A casting process for medium and large series in which the resin gels under pressure in a heated mould. It is what keeps dielectric reliability and dimensional accuracy consistent across a run.
A cast resin enclosure for switches in medium voltage equipment, found in substations and transformer stations.
A round or square resin plate threaded with bolts or cables, used to manage and distribute voltage in instrument transformers and oil-filled transformers. Sometimes called a terminal block.
What to send us for a quotation
Every part we cast is engineered to the customer’s specification rather than selected from a fixed catalogue, so a useful first message contains whatever you already know of these:
- The system voltage, and whether the part sits indoors, outdoors, or in gas
- The mechanical duty — load, direction, and any deflection limit
- Fixing centres and interfaces, or a drawing if you have one
- Service temperature, and anything unusual about the environment
- Quantity, and whether this is a prototype or a series
We will come back with a design proposal rather than a catalogue page.
