Specifying or buying a bushing? This article explains the types and where each one is used — our product page gives the technical detail and a route to a quotation.
Epoxy Bushings: Types, Applications and How to Specify One
A bushing carries a live conductor through a barrier — a transformer tank, a switchgear enclosure, a wall — and insulates it from that barrier along the way. Cast epoxy is the material of choice for medium and high voltage bushings because the insulation, the mounting interface and any embedded metal can all be formed in a single piece. This guide covers the seven types in common use, where each one belongs, and what we need from you to dimension one.
On this page
1. What a bushing actually has to do
Three jobs at once, and they pull against each other. It has to carry the current through the barrier without heating up. It has to hold off the voltage between that conductor and the earthed barrier it passes through, across a much shorter distance than open air would need. And it has to seal and support — take the mechanical load of the connection and, inside a tank or a gas enclosure, form part of the boundary itself.
The reason those three fight each other is geometry. Shortening the part to fit the equipment raises the electrical stress; lengthening it to relieve the stress makes it heavier and harder to mount. A bushing is always a compromise dimensioned to one specific piece of equipment, which is why it is engineered to the application rather than picked from a shelf.
2. The seven types, and when each is used
The names describe how the bushing is installed, not what it is made of. Most enquiries turn out to be one of these seven.
| Type | What distinguishes it | Typical use |
|---|---|---|
| Solid | Cast from a single piece of epoxy resin — excellent electrical insulation together with mechanical strength. | High voltage equipment |
| Split | Installed around an existing conductor without removing it. | Upgrading transmission and distribution systems already in service |
| Flange mount | Has a flange that is bolted or welded in place, for surface mounting. | High voltage transformers and similar equipment |
| Threaded | Installed into a threaded hole or onto a stud. | High voltage equipment such as circuit breakers |
| Tapered | Fits a tapered hole, giving a tight, secure seat. | High voltage transformers and similar equipment |
| Radial | Supports and insulates bent or curved conductors. | Equipment where conductors have to fit into tight spaces |
| Elbow | Sits at the junction of two conductors, insulating and supporting the connection. | Transformers and switchgear |
Radial and elbow types are the ones that most often end up inside gas insulated equipment, where space is the binding constraint — those are covered on our GIS products page.
3. Where epoxy bushings are used
They appear wherever a conductor has to cross an insulating boundary, which in practice means four sectors:
- Equipment — transformers, switchgear, circuit breakers and capacitors.
- Transmission and distribution — insulating and supporting high voltage conductors across the network.
- Industrial installations — motor control centres, variable frequency drives and power distribution units.
- Railways — electrification of railway lines and the construction of locomotives.
Demand has risen with the growth of the power sector and with the requirement for equipment that stays in service, which is the same pressure that pushed bushings from porcelain towards cast resin in the first place.
4. Why epoxy, and which of the two resins
Cast epoxy has a breakdown strength of 20 to 40 kV/mm depending on the system. That is a material figure, not the withstand voltage of a finished bushing — that also depends on the creepage path along the surface, the clearances through air and the field distribution around the embedded conductor. But it is what gives the designer room to make the part short enough to fit.
The choice between the two resin families is settled by where the bushing lives, not by its voltage. Both cover low, medium, high and very high voltage.
| Resin | Where it belongs | Why |
|---|---|---|
| Aromatic | Indoors — inside tanks, panels and enclosures | Bisphenol A-epichlorohydrin resin with an anhydride hardener. Excellent dielectric performance where the surface is protected from weather. |
| Cycloaliphatic | Outdoors, or indoors with humidity or pollution | Filled with pre-treated silica. Resists UV, tracking and weathering without the surface degradation that limits general-purpose resins. |
Where the environment is aggressive, we can overmould a cast bushing with silicone on our own line, adding surface protection without changing the body of the part. More on the materials in our reference guide to epoxy insulators.
5. How a bushing is cast
Most of what can go wrong electrically in a bushing is decided before the part leaves the mould.
- Resin prepared under vacuum, so the casting is free of the voids that become partial discharge sites. In a bushing the enemy is the void, not the voltage.
- Gelation under pressure at temperature for medium and large series, which is what holds dielectric reliability and dimensional accuracy across a whole production run.
- Metal encapsulated during casting — the conductor, inserts and electrodes are cast in rather than assembled afterwards, so there is no joint inside the insulating path.
- Metallization and sand blasting in house, which keeps the electrode and creepage surfaces under our own control.
We run 29 clamping machines and cast parts above 50 kg and one metre as well as small, intricate shapes. The process detail is on our technology page.
6. How it is tested before it ships
Our laboratory tests to 200 kV at industrial frequency and to 200 kV for partial discharges, and also measures contact resistance and the amplitude and phase accuracy of sensors. Production runs under ISO 9001 and ISO 14001.
| Electrical | Mechanical |
|---|---|
| Industrial frequency to 200 kV | Flexion |
| Partial discharges to 200 kV | Traction |
| Contact resistance | Compression |
| High voltage and current metrology | Twist |
| Impact | |
| Roughness |
For bushings destined for gas insulated applications, where the electrical stress is highest, every single piece is tested rather than a sample: industrial frequency, partial discharges and X-ray examination of the casting.
7. Epoxy or porcelain?
Porcelain remains a sound choice in many installations. What cast epoxy gives you is the ability to encapsulate metal during casting and to shape the part to the equipment — which is what makes a split, radial or elbow bushing possible at all. We set the two side by side in a separate article: epoxy bushing vs porcelain bushing.
If your existing bushings are failing rather than being specified new, the failure modes are worth reading first: why transformer bushings fail.
8. What to send us for a quotation
Every bushing we cast is dimensioned to a specification rather than taken off a shelf, so the more of this you can tell us, the closer the first proposal will be:
- The system voltage, and the withstand levels you have to prove.
- The current the conductor has to carry.
- The barrier it passes through — tank wall, panel, gas enclosure — and the mounting interface, or a drawing if you have one.
- Indoors or outdoors, and anything unusual about the environment: humidity, pollution, coastal air.
- Quantity, and whether this is a prototype or a series.
Send us the equipment, the voltage and the interface, and we will come back with a design proposal.
