A resin cast transformer is only as reliable as the epoxy around its windings. If the resin has voids, cracks or poor heat resistance, the coil fails long before the design life. Power, solar and commercial building projects are closing purchase orders before March, so now is a good time to review your resin choice. This guide covers what to check in epoxy for a resin cast transformer, in plain terms.

How a Resin Cast Transformer Uses Epoxy

In a resin cast transformer, the high-voltage winding is cast in epoxy under vacuum. The resin insulates the coil, holds the turns in place, and protects it from dust, moisture and humidity. This is why a cast resin transformer can be installed indoors in buildings, hospitals and plants without oil. The resin is a structural part of the design, not just a coating.

Many buyers also search for the cast resin type transformer when they mean the same product. All of these names point to dry-type units where the winding is embedded in a solid insulation system.

Key Epoxy Properties for a Resin Cast Transformer

1. Dielectric strength

Dielectric strength shows how much electric field the resin can take before it breaks down. Filled epoxy systems for transformers often fall in the range of 15 to 25 kV/mm. A higher number is good, but a resin cast transformer only benefits if the casting is free of voids. A void can fail at a fraction of the rated value, so process quality matters as much as the data sheet.

2. Glass transition temperature (Tg)

Tg is the temperature at which the cured resin starts to soften. For a resin cast transformer, it must sit comfortably above the winding's operating temperature, including hot spots. For insulation class F (155°C) and class H (180°C) designs, a low Tg leaves little safety margin. Ask for Tg figures measured after the full cure cycle, not just the best-case number.

3. Viscosity and pot life

In a resin cast current transformer coil, low viscosity helps the resin flow between tight turns and fill every gap under vacuum. If it is too thick, you get trapped air. Check the mixed viscosity at your casting temperature, and confirm that the pot life gives you enough working time for large coils.

4. Fillers

Most systems use silica or alumina fillers, often between 50% and 70% by weight. Fillers reduce cost and shrinkage, improve thermal conductivity, and help match thermal expansion with copper or aluminium windings. Too much filler raises viscosity, and poor mixing causes settling. Flame-retardant fillers are also common because many projects need fire performance classes.

5. Thermal expansion and crack resistance

If the resin expands differently from the winding, repeated heating and cooling can cause cracks. Look for a system with low shrinkage, and consider flexible or toughened grades to handle load cycling.

Potting Epoxy vs Epoxy Encapsulation

These two terms are often mixed up, so here is the difference.

Potting epoxy fills a case or housing completely. The case stays in place after curing. This suits small components and electronics.

Epoxy encapsulation coats or surrounds a part with resin, often using a mould that is removed afterwards. Large transformer coils usually fall in this group, with the resin forming the outer insulation layer.

Feature

Potting epoxy

Epoxy encapsulation

Container

Stays as part of the product

Mould removed after cure

Typical use

Small parts, sensors, electronics

Coils and windings

Main concern

Heat dissipation, stress on parts

Voids, cracking, uniform thickness

Resin Cast Current Transformer: A Special Case

A resin cast current transformer is smaller, but the same rules apply. The resin must protect the core and secondary winding, hold dimensions tightly, and stay stable under temperature changes. For these parts, low shrinkage and a suitable Tg count more than raw volume. Epoxy is often chosen for indoor medium-voltage metering and protection units because it gives a solid, moisture-resistant body.

Process Tips for a Reliable Resin Cast Transformer

  • Dry the filler and preheat the mould to remove moisture before casting.

  • Degas the mixed resin under vacuum to pull out trapped air.

  • Cast slowly to let the resin wet every layer of the winding.

  • Follow a stepped cure cycle instead of rushing it, to reduce internal stress.

  • Run partial discharge tests on every resin cast transformer coil, since small voids may not be visible from outside.

  • Keep records of batch numbers, mixing ratios and cure temperatures for traceability.

Standards and Environmental Classes

Dry-type transformers are usually assessed against IEC 60076-11. This standard defines environmental (E0 to E2), climatic (C1 and C2) and fire behaviour (F0 and F1) classes. If your end customers need a resin cast transformer with E2/C2/F1 performance, share that with your resin supplier early so that the filler and resin system are chosen to match.

Questions to Ask a Supplier of Resin for a Resin Cast Transformer

  • What are the dielectric strength, Tg and viscosity values for this exact grade and filler system?

  • Under which cure schedule were they measured?

  • What are the mix ratio, pot life and gel time at my casting temperature?

  • Is there a data sheet for thermal expansion and flame retardancy?

  • Has this grade been used in a resin cast transformer of my rating and voltage class?

  • Can I get a trial sample for a test casting?

Always confirm values against the current technical data sheet before you finalize a design, because figures vary by grade.

Request a Data Sheet and Sample

If you are building or upgrading a resin cast transformer line, ask for the technical data sheet and a trial sample before approving a resin. MB Enterprises can share product data and arrange a sample so that your team can test it on a real coil. Use the "Request Technical Data Sheet and Sample" button.

FAQs

1. What is a cast resin transformer? 

It is a dry-type transformer where the winding is cast in epoxy resin instead of being immersed in oil. It is widely used indoors and in places where fire safety matters. Buyers also call it a resin cast transformer or a cast resin type transformer.

2. What is the difference between potting and encapsulation? 

Potting epoxy fills a case that stays on the product. Epoxy encapsulation surrounds the part with resin, often using a mould that is removed later.

3. What should I look for in epoxy for a resin cast transformer? 

It should sit well above the maximum winding temperature, including hot spots. Check the value against your insulation class.

4. Why is vacuum casting used for a resin cast transformer? 

It removes air from the resin and the winding so that no voids remain. Voids lower insulation strength and cause partial discharge.

5. Can the same epoxy be used for a resin cast current transformer? 

Often a similar family is used, but grade, filler and cure schedule should be matched to the part. Test a sample first.

6. How do fillers change the resin in a resin cast transformer? 

They reduce shrinkage and cost, improve heat transfer and help match expansion with the winding, but too much raises viscosity.