Skip to content

The Seal-pac Transformer Story

May 21, 2013

This is the story of a bold attempt by GE to manufacture a compellingly different distribution transformer.

Pole type distribution transformers are ubiquitous, seen hanging near the top of distribution, i.e., telephone poles. They are connected to power lines, commonly ranging in voltage from 4800 to 7300 volts, and convert this higher distribution voltage to 120 and 240 volts for use in homes, stores and offices.

They consist of a cylindrical container, commonly referred to as a tank, with a core and coil assembly bolted to the tank’s interior. The core and coil assembly does the real work of converting voltages from, in this case, higher to lower voltages.

There is a cover that’s clamped to the top of the tank, with high voltage bushings extending through the cover. The high voltage power lines are attached to these bushings when the transformer is mounted on the pole. Low voltage bushings extend from the side of the tank. The bushings are an inexpensive ceramic, with skirts to increase the surface distance electricity must “creep” before reaching ground and creating a short circuit.

The tank is partially filled with insulating oil that covers the core and coil assembly. The oil acts as an insulator and as a means for transferring heat from the core and coil to the tank, and then, by conduction, to the air outside.

In some cases the covers are insulated in an attempt to keep squirrels from shorting out the units when they climbed on the cover and reached for the electrical connection at the top of the bushing.

This was, and still is, the basic configuration of pole type distribution transformers1.

Pole type transformers had several inherent deficiencies:

Air and moisture could leak into the tank through the seals around the cover and bushings, and cause the oil to oxidize.

They were large and cumbersome, and difficult for the lineman to handle when installing the units. There had been ongoing efforts to make them smaller and able to operate at higher temperatures.

At the time, there was considerable hue and cry to make transformers less unsightly. Utilities wanted their transformers painted “sky blue”.

In 1964, GE decided to resolve these deficiencies with a radically new design, called the Seal-pac transformer.

Photo from GE promotional brochure.

Photo from GE promotional brochure.


This photo illustrated the smaller size and more stylish looks of the Seal-pac transformer. Note that the 1959 design, of the same 25 KVA rating, required hollow circulating fins to improve the cooling effect of the oil.

The Seal-pac’s core and coil was mounted in a steel container completely filled with oil and welded shut to exclude any possibility of air and moisture entering the unit.

The casing was designed to expand, bellows-like, to accommodate the slight expansion and contraction of the oil as temperatures varied with variations in load.

The coils were made using precision winding where each succeeding layer of wire was laid into the valleys, interstices, of the preceding layer. This made the core and coil assembly smaller.

The Seal-pac was considerably smaller, and less obtrusive when mounted on the pole.

New equipment and materials were needed for the manufacture of this transformer.

Utilities greeted the Seal-pac with enthusiasm.

Competitors greeted the unit with some disparaging comments. One, for example, was that the flat surface at the top would be a good place for birds to build a nest.

It was thought that GE had a winner with the Seal-pac.

Then a few units failed.

The failure was due to a hot spot in the coil that led to thermal runaway.

It also developed that manufacturing costs were too high.

Eventually the Seal-pac was discontinued.

Whether it was the thermal failures or high cost that led to the Seal-pac’s demise isn’t clear.

It’s interesting to note that, to my knowledge, no manufacturer has introduced a dramatically new distribution transformer over the past 50 years.

As a result, pole type distribution transformers have become a commodity, where the lowest price prevails.

  1. Pad mounted distribution transformers, mounted on the ground, are essentially the same, except the core and coil assembly is mounted in a rectangular tank with a metal compartment covering the high and low voltage bushings that extend from the side of the tank. The high and low voltage sections of the compartment are separated by an insulated barrier. The closed compartment is locked to prevent unauthorized people from coming in contact with the electrical connections.


 *  *  *  *  *  *


To find earlier articles, click on the name of the preceding month below the calendar to display a list of articles published in that month. Continue clicking on the name of the preceding month to display articles published in prior months.

© Power For USA, 2010 – 2013. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author, Donn Dears, LLC, is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Power For USA with appropriate and specific direction to the original content.

One Comment leave one →
  1. May 23, 2013 3:33 am

    In this blog you are sharing a very nice story. Many things that you share in this article about distribution transformer, is very important for people.Thank you So much for this useful and informative article.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s