There has been growing concern about failures of porcelain cutouts in service, which are both an operational and a safety issue. As a result, many manufacturers have developed composite cutouts and there has been a requirement for standards to address their electrical and mechanical characteristics. This 2013 contribution by John Vandermaar of Powertech Labs reviewed this issue and summarized such a standard developed by the Canadian Standards Association (CSA C310, “Distribution class polymeric cutouts) with main focus on test requirements.
A cutout is defined by Canadian Standards Association (CSA) as “an assembly that consists of a fuse support and either a fuseholder or a solid blade capable of being operated with the use of a switch stick”. They are mounted on poles and are used to disconnect or reconnect equipment from the power source. Historically, porcelain was the material of choice for the cutout body as well as most other equipment that required high voltage electrical insulators. Porcelain has very good electrical properties, is durable and relatively inexpensive to manufacture. However, it is not without its problems. For example, during the early 1980’s large numbers of porcelain cutouts began failing, with reported cutout failures by BC Hydro, the major electric utility in the province of British Columbia, Canada. In addition to associated power outages, some of these failures reportedly caused pole top fires, while others resulted in either a ‘near miss’ or injury to personnel. In one case, a line worker was struck on the neck by broken cutout debris weighing approximately 0.5 kg. Since this problem first started to appear, BC Hydro had an alarming recurrence of cutout failures. Particularly disturbing was the fact that failures were occurring even in relatively new cutouts (<10 years old). Figs. 1 and 2 show examples of such failed cutouts.
The failures were attributed to cement growth and to expansion of the cement grout due to moisture ingress and freeze/thaw cycling. This causes the porcelain to crack resulting in a dielectric short between the mounting bracket and the lower fuse bracket. Indeed, many utilities throughout North America have had similar experience with porcelain cutouts and in 2002 the Canadian Electricity Association (CEA) commissioned a study on the issue. The objective was to determine if a method was available or could be developed to effectively evaluate the in situ condition of a cutout prior to operation. The research concluded that was no reliable way to detect incipient failure of a porcelain cutout.
As a result of the problem, utilities began looking for alternatives to conventional porcelain cutouts and manufacturers began introducing new designs using polymeric materials such as used in suspension insulators. One of the advantages of polymeric material is that it is not brittle like porcelain and therefore will not develop cracks and shatter. This reduces risk to line workers. The insulating body of these cutouts typically consists of a fibreglass rod as the structural member with a polymeric housing or are a one-piece body made of resin. Figs. 3 and 4 show examples of both types.
At the distribution level, most porcelain suspension insulators and lightning arresters have been replaced with polymeric designs. This success would not have been possible without the parallel development of standards for these products. Just as standards were necessary to address the unique characteristics of polymeric suspension insulators, Canadian utilities recognized that standards were also required for polymer cutouts. As a result, the Canadian Standards Association developed standard CSA C310-09, “Distribution class polymeric cutouts”.
The standards most commonly specified for cutouts are ANSI/IEEE C37.41 and C37.42 and these were developed based on porcelain being used as the insulating material. They specify requirements such as dielectric test requirements; interrupting, short time current, time current requirements; manual operation, thermal-cycle and bolt-torque requirements etc. However, they do not contain any specifications to deal with the unique requirements of composite cutouts. CSA standard C310-09, “Distribution class polymeric cutouts” was issued in 2009 to establish standards that address not only the conventional cutout requirements but also issues that are unique to polymeric cutouts. The new standard was prepared by a technical committee with representation from both the user community and manufacturers.
CSA Standard C310-09
CSA standard C310-09 is structured as follows:
2. Reference publications
5. Mechanical characteristics
6. Electrical characteristics
7. Design tests: This section details tests intended to verify the suitability of the design, materials, and method of manufacture of the cutout body. Most of these tests generally follow the same requirements as required for polymer insulators.
8. Type tests: Type tests are intended to verify the main characteristics of a polymeric cutout. None of these tests are unique to polymeric cutouts. They are the same as required on porcelain cutouts.
9. Sample tests: Sample tests are intended to verify the characteristics of the cutout, which depend on the quality of manufacture and materials used.
10. Routine tests: Routine tests are intended to detect manufacturing defects and are carried out on every cutout to be supplied. These tests focus on characteristics unique to polymeric cutouts.
11. Test report
12. Marking and product identification
14. Optional Items
The main difference between the CSA standard and ANSI/IEEE C37.41 and C37.42 is that the former specifies test requirements that are applicable only to polymeric cutouts. These tests provide assurance that the products that pass these test requirements will meet the user’s service requirements.
CSA Standard C310-09 Design Tests
The following design tests are specified in the standard:
a. Water penetration test
This test checks for possible penetration of a moisture into the interface between the core and housing. The procedure is the same as would be used for polymeric insulators and includes a steep-front impulse test (minimum steepness of 1000 kV/μs). The main purpose of the steep-front impulse test is to detect water that may have penetrated between core and sheath. When this test is conducted on insulators, the voltage is essentially applied to a device that is shaped like an insulated cylinder with metal electrodes at the ends that cover the surround the sheath. This results in minimal stress concentrations along the insulation during the test. When failures occur they tend to not be at any particular location. In the case of cutouts, the test is done with voltage applied between mounting pin and the two line terminals simultaneously (line terminals connected together). This results in stress concentration around the mounting pin. It was found that cutouts tested would often fail near the mounting pin area and Fig. 5 shows an example of a typical pinhole failure at that location.
When this same test was performed on new polymeric cutouts, the same failure would occur and this showed that failures were not due to water ingress. To reduce the stress in the area of the pin, i.e. to the equivalent of a conventional composite insulator, a copper strip was placed around the cutout body at the pin. Now, failures at the mounting pin no longer occurred. There was a great deal of debate within the technical committee whether to do the steep front test with a metallic strap around the mounting pin or without the strap. If the purpose is only to detect water ingress, then testing with the strap in place is preferable. By contrast, if the purpose is also to test susceptibility of cutouts to steep front voltage in service, then the test should be done without metallic strap. One eastern Canadian utility reported that they had seen pinhole failure of polymeric cutouts in service. As a result, it was decided that this test was also to verify susceptibility of cutouts to steep front voltage in service and therefore it was to be done without metallic strap.
b. Water diffusion test
This test is intended to check resistance of the core material to water attack.
c. Thermal cycle, manual operation & torque test
This test subjects the cutout to four cycles of thermal variations, ranging from –50°C to 50°C. During the test, the cutout is subjected to 50 open/close operations twice at –50°C and twice at 50°C. This ensures the cutout will not fail during operation such as has occurred with cracked porcelain cutouts. This procedure is different from that specified for this same test in ANSI/IEEE C37.41.
d. Accelerated weathering test
This test evaluates the ability of the cutout material to withstand exposure to UV.
e. Tracking & erosion test
This is an accelerated material ageing test and tests the ability of the cutout to resist surface tracking and/or erosion over its service life. Fig. 6 shows a polymeric cutout on the tracking wheel used for this test.
This is also a test required by many standards for polymeric insulators. The test level required by the CSA standard is the same as for polymeric insulators (35 kV/mm of leakage distance). This test also has stress concentration around mounting pin area. Due to this stress concentration, erosion failure at the pin area may occur on a polymeric cutout when it would not have occurred on an equivalent polymeric insulator. This test also requires a steep-front test on completion of the the tracking wheel test cycle.
f. Flammability test
This test is intended to check the shed housing material for ignition and self-extinguishing properties.
g. Closing-in test
This test evaluates the force required to close the switch blade.
A three-point bend test is done on one cutout with a force of 1 kN. Fig. 7 shows this test on a polymeric cutout.
CSA Standard C310-09 Type Tests
The following type tests are specified in the standard. These tests are the equivalent to those required by the ANSI/IEEE standards on conventional cutouts:
a. Dielectric tests
b. Interrupting tests
c. Radio-influence tests
d. Short-time current tests for disconnecting cutouts
e. Temperature rise test
CSA Standard C310-09 Sample Tests
The following sample tests are specified in the standard:
a. Visual observation & galvanization
The eccentricity of the core rod relative to sheath is measured and must not exceed manufacturer specifications. Fig. 8 shows an example of this measurement.
c. Dielectric test
d. Tensile load test
CSA Standard C310-09 Routine Tests
The following routine tests are specified in the standard:
a. Visual inspection: Polymeric cutouts shall be visually inspected and shall be free of the following:
• superficial defects
• defective area exceeding 0.2% of the total insulating surface or a depth greater than 1 mm (0.04 in);
• polymeric shed tearing;
• visible polymeric housing cracks;
• visible cracks or fractures in any of the metal fittings;
• separation or de-bonding at the polymeric shed or sheath interface or joint;
• moulding flash protruding greater than 1 mm (0.04 in) above the polymeric shed or sheath surface;
• rotational misalignment of the upper metal fitting to the lower fitting
• incorrect or missing identification markings;
• missing galvanization in the form of flaking or cracking on metal fittings.
b. Positive engagement of the fuse tube is to be verified.
As a result of problems with porcelain cutouts, manufacturers have introduced new cutout designs using polymeric materials. The Canadian Standards Association has developed a standard that focuses specifically on this new class of cutouts. This is one of the only standards that covers the unique characteristic of polymeric cutouts and details a test program that provides utilities with assurance that the products they purchase will meet service requirements.