Epoxy resin boasts excellent electrical insulation properties and high mechanical strength, and can be easily cast into various shapes. Cast epoxy resin current transformers pose little fire risk as long as they avoid contact with oil. Featuring compact structure, they are convenient to use and maintain. They are widely applied in power distribution equipment ranging from 3kV to 35kV. Nevertheless, they have drawbacks. Conductive dust or oil stains on clean epoxy resin surfaces will sharply increase conductivity. Absorption of carbon dioxide or saline moisture will also impair insulation performance. Accordingly, such transformers are not suitable for circuits above 35kV and only applicable to indoor 10kV facilities. Transformers for voltages over 35kV are mostly installed outdoors, where epoxy resin casting is not a proper choice. Outdoor power distribution facilities prefer oil-immersed current transformers with stable insulation barely affected by external conditions. Thick insulation layers cannot be fully saturated via conventional oil filling, and residual air bubbles may trigger partial insulation breakdown. For this reason, vacuum pumping is adopted to remove internal air before oil injection, which proceeds under vacuum environment. This method eliminates bubbles and enhances the overall insulation strength of the transformer.