There are several common types of bushings.

Simple Hollow Porcelain Bushings: These bushings are insulated by an air space between the conductor and the porcelain and are used primarily on small generating units. Many date back 40 to 60 years. The most common problem with these bushings is deteriorated gasket materials. Almost all require replacement of the neat cement between the metal flange and the porcelain. Years of heat and vibration have deteriorated the strength of the neat cement.

Asphalt Filled Bushings: As generator units became larger; it was desirable to transfer the heat from the conductor to the porcelain by filling the porcelain with asphalt, reducing the temperature. The asphalt also became a liquid seal when hot, to prevent hydrogen leakage. The asphalt is not to provide dielectric integrity to the bushing, but is primarily for heat transfer. Asphalt leaking from the bushing is easily observed. Although it is not always a danger sign, it should be checked carefully. Asphalt leakage is usually caused by old gaskets shrinking, or overheating of the bushing that causes excessive pressure inside the bushing, overcoming the spring pressure and forcing the asphalt out. The primary concern here is to determine why the bushing is overheating. The most common cause is a loose connector or a connector that is not making sufficient area contact.

Forced Hydrogen-Cooled Bushings: Hydrogen-cooled bushings are constructed similar to the “simple” and “asphalt filled” bushings already mentioned. They are slightly more complex internally, however, having internal piping to carry hydrogen around the conductor for cooling. There are three chambers inside the bushing. The inner chamber is a hollow pipe in the center of the bushing, inside the hollow conductor. Hydrogen is directed under pressure inside the generator end of the hollow pipe. The pipe has exit holes near the air end of the bushing, but still internal to the bushing, where the hydrogen flow is reversed. The hydrogen flows back along the outside of the pipe and in contact with the internal diameter of the porcelain housing (or in some instances along the inside of the conductor if asphalt is being used) where it exits at the generator end of the bushing.

There is a possibility of trouble in this design. Since the bushing is mounted near vertical and since the air end is pointed down, oil can possibly accumulate in the internal air end of the bushing. If sufficient oil accumulates, it will plug the holes in the pipe inside the bushing and stop the cooling hydrogen flow. The external symptom of this happening is overheating and asphalt running out of the end of the bushing.

Solid Cast Epoxy Bushings: These bushings are simple in design. The solid cast epoxy bushings use a cast epoxy flange and insulating body as one continuous material. It has the advantage of placing the body in direct contact with the conductor for better heat dissipation. It also eliminates the compression springs necessary with porcelain and eliminates the need for gaskets to seal around the conductor.

While many of the problems associated with porcelain insulated bushings have been eliminated, other problems can occur. Plasticisers in epoxy give it flexibility while new, but they tend to dissipate with time and temperature. The epoxy becomes brittle and no longer follows the thermal expansion and contraction of the copper conductor. This results in either leakage between the conductor and epoxy or cracks appearing in the epoxy. Care must be taken when tightening down the epoxy flanges onto uneven surfaces cracking of the epoxy flange can occur. Another difficulty is that the epoxy dielectrics may be adequate, but the heat transfer in epoxy is marginal, resulting in rapid aging of the epoxy.