Many electric utilities are facing substation insulator failures due to aging pin and cap insulators. In the early to mid-1900s, these insulators were the only choice because the technology to form one large station post did not yet exist.

Companies manufactured these insulators by taking a steel pin and then cementing on layers of porcelain. Depending on the voltage, the insulators normally have two or three layers of porcelain. The assembly is then completed by cementing a steel cap on top. The design causes the pin to be relatively close to the cap, as far as distance is concerned, which is appropriate, provided the porcelain layers are not cracked.

Distribution voltage insulators are made up of one unit usually consisting of two layers of porcelain. As you move up to transmission voltages, multiple units are bolted together. For example, a 69-kV insulator would have two units, each consisting of two layers of porcelain in each unit. As you move up to 115 kV, the units are made up of three layers of porcelain each, and three units are stacked together. At 230 kV, five of these units are needed, and so on.

Look for Cracks in Insulators

A high failure rate often occurs among these types of insulators after they hit more than 25 years of service when mounted horizontally. Fewer failures occur when mounted vertically.

The failure mode appears to occur when the cement between the layers of porcelain cracks and water collects in the cracks. After a freeze event, the water expands and cracks the porcelain. This crack nearly eliminates the insulation value of that layer of porcelain. Since the layers are only about an inch thick, the electrical creep distance now has been reduced from several inches to about an inch.

The risk that a crack will result in a catastrophic bus fault is much higher with lower-voltage insulators. To explain this, consider percentages. A 15-kV pin and cap insulator is composed of two layers of porcelain and is a single unit. If one layer cracks, the insulation value is down 50%.

In the case of a 69-kV pin and cap insulator, each unit is made up of two layers, but the total insulator has two units. That way, if one layer cracks, the insulation value is down 25%. Since the 115-kV units are made up of three layers, and a 115-kV insulator is made up of three units, one cracked layer only causes an 11% reduction. As a result, the odds of one crack causing a catastrophic failure are much greater at the lower voltages.

Focus on Preventive Maintenance

Many of these catastrophic failures happen at 12 kV through 69 kV. To solve this problem, a utility can get ahead of this issue and prevent the failures. First, focus on replacing all 12-kV through 69-kV pin and cap insulators that are mounted horizontally. Also, change all the 15-kV pin and cap insulators regardless of orientation because they are inexpensive and easy to change; normally, there is no reason to leave any behind.

Now prioritize the remaining vertically mounted insulators. For the 69-kV insulators, give priority to those that are vertically mounted and associated with mechanical loads of switches before those that just held up bus.

If it is necessary to prioritize 15 kV, any hook-operated disconnect pivot end should be first. On occasion, these have broken off during switching, leaving the switch person holding the switch blade along with the bottom live parts of the switch and the jumper at the end of a hot stick. This definitely presented a problem.

For the higher-voltage pin and cap insulators, 115 kV and above, technicians should be able to handle maintenance concerns if they follow certain procedures. From an economic standpoint, these higher-voltage insulators may be more expensive to change out. The chance of a catastrophic failure is low, particularly if technicians follow some of the easy maintenance procedures. It's rare for a 115-kV or higher-voltage pin and cap insulator to explode.

A single crack will cause only an 11% decrease in insulation value on a 115-kV insulator. These insulators share the same design as the lower-voltage insulators and these layers do crack. You will find some cracks on vertically mounted insulators and a considerable number of cracks on horizontally mounted insulators. Replace horizontally mounted units as soon as possible.

Be Aware of Issues

To reduce the likelihood of a failure, there are several things you can do during maintenance and inspection of the substation:

  • Include all pin and cap insulators in your station's infrared scan. If any insulators show up with an elevated temperature, replace that unit as soon as possible.

  • Have substation inspectors walk under the bus and listen to the insulators. Often, if there is a crack in one of the layers, it will give off a corona sound.

  • Keep the insulators clean. Pressure wash every three to five years, and if the bolts are rusting and the rust is staining the insulators, replace with hot-dipped galvanized or stainless-steel bolts.

  • “Ring” the insulators on a three- to five-year maintenance cycle. These transmission voltage pin and cap insulators have large-diameter layers of porcelain that will ring like a dinner plate if there are no cracks. If the layer is cracked, it will sound dead. While this is not a perfect test, one can discover cracks in insulators with this method. The cracks are so fine that you might not located them with a visual inspection, but once it rings dead, you usually can find the crack. To ring the insulator, take anything metal (key, coin, wrench) and tap each layer.

If you find cracked units, replace the entire insulator set with a modern station post type. It can be tricky to just change one because they are a different length. If you want to keep using your pin and cap insulators, you should consider replacing the defective unit with another used unit in good condition or with a new replacement. The new unit will look different, but electrically and mechanically, it will be a match.

Change Out Insulators

Once one understands the insulator issues and knows how to operate with the higher-voltage units in the system, one must be aware of the challenges of changing from pin and cap to station post style insulators.

First of all, consider the insulator length. The only voltages where the pin and cap and station post are the same length are 15 kV and below. At 23 kV, the post is 2 inches longer. In every case for every voltage above this, the station post is at least an inch longer.

Second, there is the issue of the bottom of the insulator. In the case of the pin and cap, the bottom is a flange that can be bolted down through into a threaded hub of a switch. It also can be slid over studs similar to a wheel on a car. In the case of the station post, the bottom looks like the top, which has threaded holes to accept bolts. In most cases, this is not a problem, but there are cases where it is. Examples are switches that have a threaded hub that must be bolted into or switches with studs that need nuts. Another classic example is insulator pedestals that have threaded holes and are made to have a flange bolted down to them.

Insulator length sometimes can be made up by just flexing a long bus run. In the case of 115 kV, it is only about 1.5 inches longer, and with this voltage, runs between supports are normally long enough to have enough flex. If not, you will have to reconfigure the bus or go to plan “B.”

If you are attaching to switches or bus pedestals that require a flange bottom insulator, there are a couple things you can do. Assuming you are going to change all insulators in the whole area, and you can stand to gain several inches in length, the best answer is to purchase risers. They are hot-dipped galvanized spacers that can be bolted to the bottom of the station post insulator. This gives you a flange to bolt down through just like the pin and cap installation. These are not too expensive and can be bought in any height from about 3 inches to 4 inches and up. So with the extra insulator length plus the spacer, you will gain about 5.5 inches. If you can stand the height gain, this is the best answer. If you cannot stand the height gain, see plan “B.”

It's normally advisable to avoid plan “B.” First, it requires you to stock and keep track of special insulators for spares. Second, there is a premium for the special insulators from companies such as Lapp Insulators, which makes a line of insulators called Cap and Pin Replacement Insulators. Lapp offers three different types:

  • The first type is a station post with the physical and electrical specs of a pin and cap unit and can be bolted right in the stack.

  • The second type is a single-piece station post with threaded bolt holes top and bottom, just like a normal station post. The only exception is that the length is selected to match the length of the pin and cap insulator.

  • The third type is a single-piece station post with the length matched to the pin and cap insulator. This one also has a flange bottom so, physically, it exactly matches a pin and cap insulator; therefore, it can be bolted right in when the same height and flange bottom are required with no modifications. This is an option if you don't mind the premium for the special insulators and can deal with keeping additional spare insulators around.

In summary, don't wait for a bus fault to start thinking about your pin and cap insulators. It's not that hard to get ahead of it.


Gary Wright (gwright@forestgrove-or.gov) consults for McMinnville Water & Light and Forest Grove Light & Power in Oregon. He started out in the power industry in 1977, and first worked at Cowlitz County PUD in substation design and distribution system planning. He also worked in the operations department in charge of substation construction and maintenance and covered the dispatch center, wiremen, switchmen, PCB compliance, meter department, relay department and transformer department. Wright recently worked for Clark Public Utilities in charge of substations, metering and relaying and is now retired.

Company mentioned:

Lapp Insulators, LLC | www.lappinsulator.com