Porcelain and non-ceramic insulators (NCIs) have developed by following similar paths. In the early stages, various problems with both types sometimes occurred because of the learning curve on proper handling and use. More often, however, problems arose from the lack of understanding of the effects of the electrical, mechanical and environmental stresses on the materials, interfaces and insulator constructions. Through service experience and the manufacturers' resulting material and design modifications, both insulator types have become viable products.

The weaknesses of porcelain insulators are well known, largely because of the longer period of development, use and standardization. Apart from acts of vandalism, and occasional problems from poor quality control and misuse, porcelain insulators have served the industry well, and users have attained a significant level of confidence in their long-term reliability. Maintenance methods are well established, and porcelain insulators commonly outlive their 40- to 50-year life expectancy.

NCIs, however, have not yet attained the same level of experience or standardization, and their weaknesses are still being discovered.

NCI Damage

At one time, weathering was considered likely to become the NCIs' predominant cause of failure. This has not proven to be the case. The majority of today's problems stem from the relative ease of damage to the insulator from handling, vandals, birds and animals, flashover, higher operating temperatures, or dry band arcing in contaminated environments. These failure modes have caused heightened concern and increased field inspections.

Furthermore, it is difficult to assess the electrical integrity of NCI insulators while in service, thereby affecting maintenance methods. Although they cost less to install, their maintenance cost, particularly at transmission voltage levels, is becoming noticeable. Increased inspection and maintenance costs will result in a higher anticipated life-cycle cost. In certain applications, the longevity of these insulators is also being questioned.

Porcelain Damage

Porcelain insulators can certainly be damaged by rough handling , acts of vandalism or from flashover. Under normal fault conditions, the glaze sustains only superficial damage. The electrical or mechanical strengths usually are not affected under normal conditions, so change-out is not required. However, under extreme or sustained fault conditions, damage often occurs to the porcelain dielectric and sometimes to the hardware (Fig. 1). The extent of damage is easily spotted from the ground using binoculars, and the need for change-out is easily determined.

Vandal-damaged porcelain insulators also are easily spotted from the ground. Apart from extreme damage, vandal-damaged insulators do not impose immediate operational risks, either mechanically or electrically.

Field Inspection of NCIs

NCIs are more easily damaged than porcelain insulators, and the damage can easily occur during handling, shipping and construction. Vandal damage often requires inspection at close range to assess the extent of the damage because of the NCIs' smaller size. Insulators damaged by vandals may require change-out or monitoring.

Because of the smaller mass of the hardware, flashover (followed by power arc) often affects the insulator attachment hardware or corona ring, requiring change-out. Whether change-out is needed immediately or can be deferred requires inspection of the damage from close range by knowledgeable personnel.

To inspect the insulator, an outage may be necessary to safely assess the damage at close range. Often, change-out can be deferred, but the insulator needs to be monitored.

A sustained fault may cause separation failure (Fig. 2). Sometimes, monitoring for corona on transmission insulators requires night-vision equipment or the use of expensive daytime monitoring equipment. Corona rings on transmission insulators are generally not interchangeable. Even though another ring may fit physically, there is always the uncertainty of whether the ring grades the electric field in the same way as the original ring. This means that corona rings and insulators of the type installed on the system must be kept in stock. For some systems with three or four manufacturers' types, the extra stock of winsulators and corona rings can lead to confusion and extra cost.

Working Live

When manufactured, porcelain suspension insulators are tested to confirm their electrical and mechanical strengths, whereas NCIs are only tested mechanically. Porcelain insulators can be installed live, while NCIs should not, unless they are tested separately and identified as suitable for live installation. Porcelain suspension insulators can be tested through various methods to determine their electrical integrity prior to installing them live.

What Constitutes Damage

Various tests can determine the electrical integrity of a porcelain insulator, while the mechanical integrity can be determined visually. Cracks in posts and severe erosion of hardware, particularly the pin on suspension insulators, will lead to a severe loss of mechanical strength. A complete loss of the porcelain shell of a suspension insulator will result in a reduced strength but not low enough to jeopardize the insulator's ability to support the line.

What constitutes damage to NCIs and whether change-out is required immediately or can be deferred is not always so clear-cut. Most often field personnel knowledgeable in NCI designs must examine the damage at close range. The ease of damage to the thin rubber sheath on suspension, post types and bushings, exposing the fiberglass core to moisture, is a major concern. It can lead to a tracking failure or fracture of the insulator core. This type of damage can result from installation, birds or vandals (Fig. 3). Flashover of NCIs causing rupture of the rubber end seals also clearly constitutes a potential failure mode (Fig. 4).

Damage to the insulator housing, either by corona cutting or dry band arcing, can present a serious failure mechanism. Still, uncertainty and confusion exists at this time as to whether a loss of hydrophobicity of the NCI housing constitutes failure.

Latent Defects

The use of unsuitable cement in the assembly of porcelain insulators, in particular, suspension, pin-type and some designs of line posts, can cause cracks in the shells of these insulators. This latent defect could take 10 or more years to manifest itself in the field (Fig. 5). This volume expansion mechanism, generally referred to as cement growth, will lead to catastrophic failures of pin-type and line post insulators.

However, in ceramic suspension insulators, the shells will crack causing dielectric failure, but the mechanical strength is hardly affected. Of course the ensuing arc of a flashover passing through the head of a suspension insulator may result in the head blowing apart, leading to string separation and line drop. This latter failure mechanism is a rare occurrence as more than one cracked insulator in a string must be present for the arc to penetrate an insulator.

Porcelain also contains micro-cracks, which sometimes are introduced into the body during manufacturing. These micro-cracks may lead to dielectric failure with time, a phenomenon called aging. No tests are performed after assembly to detect for possible cracks in the dielectric.

NCIs also may contain latent defects. When the hardware is swaged to the core or if the compression dies are worn (or when over-crimping happens), a crack may develop in the fiberglass rod that can lead to puncture under lightning (Fig. 6) or even brittle fracture failure. Another serious, possible latent defect in NCIs is the bonding between the rubber housing and the fiberglass core. A slight change in rubber housing injection machine parameters, such as temperature or time, may result in poor or no bonding between the fiberglass core and the rubber housing. As only a statistical sampling of the moulded housings are examined for bonding, and no in-process tests are performed, this type of defect manifests itself in service by dielectric failure along the interface because of moisture ingress. Brittle fracture of the fiberglass core also may occur.

Performance Under Contamination

The performance of porcelain insulators varies considerably under contamination depending on factors that include: insulator type, design and leakage distance; type and severity of the contaminant; nature and frequency of the precipitation; and the degree of natural cleaning. In all but a few locations, the contaminant simply overwhelms the insulators and develops leakage current and channelled dry band arcing. This leads to porcelain dielectric shattering. Porcelain can withstand the heat produced by leakage current, dry band arcing, and flashover without damage. Change-out is not necessary, and the insulators can be restored to service by cleaning.

Routine maintenance, such as water washing or dry cleaning, removes contamination and restores insulators to their original insulation strength, thereby preventing flashover. Greasing and silicone-rubber coatings will reduce the required maintenance, and in many instances, particularly silicone-rubber-coated insulators, maintenance cleaning may be delayed for more than 10 years. The maintenance methods for porcelain insulators are well developed, and various industry application guides are available.

NCIs' flashover performance is considerably better than porcelain insulators, as confirmed through various laboratory tests. However, they cannot withstand the heat produced from leakage current and dry band arcing as well as porcelain. So the rubber housing erodes, exposing the fiberglass core to moisture and voltage, leading to tracking of the core and insulator failure.

Although NCIs can be routinely cleaned using the methods developed for porcelain, damage will occur if, for example, the water pressure is too high or if abrasive dry cleaning techniques are used. Both cleaning methods are still under development for NCI's. The same techniques should not be used for all NCIs because of their different materials and constructions. Any one method might effectively clean some insulators while damaging others.

After monitoring the erosion of NCI housings, or after experiencing flashovers in severe contamination, a few utilities have returned to porcelain, along with their required maintenance, to obtain greater reliability.

Conductor Temperature

The fiberglass core in NCIs is subject to creep while under tension, a phenomenon of plastic flow that occurs at elevated temperatures. In addition, the chemical bond of the primer layer between the rubber housing and the metal hardware that forms the end seals in NCIs may not be stable under elevated hardware temperatures.

Today's conductors have emergency ratings of 200°C (392°F), thereby producing hardware temperatures in excess of 100°C (212°F). This is a concern because the effects of high temperature on the compression attachment of the hardware to the core and the long-term stability of the end seals are unknown.

Final Evaluation

As with any new technology, service experience is often needed to uncover problems and sometimes it takes a long time to uncover the weaknesses. NCIs provide significant advantages because they are lightweight, easier to install and lower in cost.

Possible Sustained Damage to Porcelain and Non-Ceramic nsulators

Condition	Porcelain	Nonceramic
Handling &Shipping	Possible damage	Easily damaged
Storage	Damage unlikely	Damage possible
Construction	Possible damage	Easily damaged
Lightning	Possible puncture Unlikely housing damage No hardware damage Change-out not required	Possible puncture Unlikely housing damage Possible hardware damage Change-out may be required
Sustained fault	Separation unlikely	Separation likely
Maintenance procedures	Live methods	De-energized
High conductor temperature	Unknown	Unknown
Monitoring	Not required	Required
Contamination	Cleaning, grease, RTV coating methods standardized	Damage during cleaning possible, failure likely
Latent defects	Cement growth Micro-cracks in porcelain	Swaging damage to core Bonding of housing to core
Vandalism	Easily spotted from the ground	Close-up inspection required

NCIs can provide utilities real benefits over porcelain. They are initially lower in cost and easier to install. However, it is likely that NCIs may ultimately cost more to maintain, resulting in a higher life-cycle cost. Expenses related to monitoring, de-energized maintenance, and more frequent change-outs in problem areas must be taken into account when comparing the true cost and reliability of an insulator strategy.

Although NCIs have been on the marketplace for decades and the insulating compounds and end fitting methods have improved dramatically, we still have more to learn. The key to a successful insulator strategy requires a good grasp of the advantages and disadvantages for both porcelain and NCIs for utility-specific conditions and environment. Utilities now have two viable choices in insulator types, and sound engineering and business decisions will ensure the utility obtains the desired service life.

Edward A. Cherney received a BS degree in physics and chemistry from the University of Waterloo in 1967; an MS degree in physics from McMaster University in 1969; and a Ph.D. in electrical engineering from the University of Waterloo in 1974. In 1968, he began his career in the electrical insulation field working in the research division of Ontario Hydro. Cherney later worked for a manufacturer of insulators and then with a manufacturer of silicone materials. He is an adjunct professor at the University of Waterloo. He is involved in several IEEE working groups on insulators, is a registered engineer in the province of Ontario, and is a Fellow of the IEEE.
edward@primus.ca