Damage limit proof test and test of the tightness of the interface between end fittings and insulator housing,these two tests are two of the most important test in inspection process of composite insulator. Here is how international standard IEC 61109:2008 “Suspension and tension insulators for AC system with a nominal voltage greater than 1000V” defines these 2 tests in section 11 page 17:
11.2 Damage limit proof test and test of the tightness of the interface between end
fittings and insulator housing
11.2.1 Test specimens
Four insulators taken from the production line shall be tested. In the case of long insulators,specimens may be manufactured, assembled on the production line, with an insulation length (metal to metal spacing) not less than 800 mm. Both end fittings shall be the same as on standard production insulators. The fittings shall be assembled such that the insulating part from the fitting to the closest shed is identical to that of the production line insulator. The insulators shall be examined visually and checked to see that the dimensions conform with the drawing; they shall then be subjected to the mechanical routine test according to 13.1.
NOTE If the manufacturer only has facilities to produce insulators shorter than 800 mm, the design tests may be performed on insulators of those lengths available to him, but the results are only valid for up to the lengths tested.
11.2.2 Performance of the test
- a) The four specimens are subjected to a tensile load applied between the couplings at ambient temperature. The tensile load shall be increased rapidly but smoothly from zero up to 70 % of the SML and then maintained at this value for 96 h.
- b) Both ends of one of the four specimens shall, at the end of the 96 h test, be subjected to crack indication by dye penetration, in accordance with ISO 3452, on the housing in the zone embracing the complete length of the interface between the housing and metal fitting and including an additional area, sufficiently extended, beyond the end of the metal part.
The indication shall be performed in the following way:
– the surface shall be properly pre-cleaned with the cleaner;
– the penetrant shall be applied on the cleaned surface and left to act for 20 min;
– the surface shall be cleaned of the excess penetrant and dried;
– the developer shall be applied, if necessary;
– the surface shall be inspected.
Some housing materials may be penetrated by the penetrant. In such cases, evidence shall be provided to validate the interpretation of the results. After the penetration test the specimen shall be inspected. If any cracks are visible, the housing and, if necessary, the metal fittings and the core shall be cut perpendicular to the crack in the middle of the widest of the indicated cracks, into two halves. The surface of the two halves shall then be investigated to measure the depth of the cracks.
- c) The three remaining specimens are then again subjected to a tensile load applied between
the couplings at ambient temperature. The tensile load shall be increased rapidly but smoothly from zero to approximately 75 % of the SMS and then gradually increased in a time between 30 s to 90 s to the SMS. If 100 % of the SML is reached in less than 90 s, the load (100 % of SML) shall be maintained for the remainder of the 90 s (this test is considered to be equivalent to a 1 min 100 % withstand test at SML). In order to obtain more information from the test, unless special reasons apply (for instance the maximum tensile load of the test machine), the load may be increased until the failing load is reached and its value recorded.
11.2.3 Evaluation of the test
The test is passed if
– no failure (breakage or complete pull-out of the core, or fracture of the metal fitting) occurs either during the 96 h test at 70 % of the SML (11.2.2 a)) or during the 1 min 100 % withstand test at SML (11.2.2 c)),
– no cracks are indicated by the dye penetration method described in 18.104.22.168 b),
– the investigation of the halves described in 22.214.171.124 b) shows clearly that the cracks do not
reach the core.
The applicable testing machine related to this test is called “Horizontal tensile testing machine”. It’s produced by Contune International Corporation and ,as we previously metioned, this testing machine is widely used in toutine test , type test and sample test by dozens of insulator manufacturers around the world. It is driven by hydraulic power to provide stable tensile strength source, and controlled with computer to customize testing process and test report is generated in later stage for operator’s review and evaluation.