Sample test is a major method of insulator acceptance inspection for mass production. It is widely used by inspectors of insulators from manufacturers, power girds and electrical laboratories. Here we have definition, rules, test method and acceptance criteria of composite insulator of suspension type and tension type of international standard IEC 61109:2008 “spension and tension insulators for a.c. systems with a nominal voltage greater than 1 000”
12 Sample tests 12.1 General rules For the sample tests, two samples are used, E1 and E2. The sizes of these samples are indicated in Table 4 below. If more than 10 000 insulators are concerned, they shall be divided into an optimum number of lots comprising between 2 000 and 10 000 insulators. The results of the tests shall be evaluated separately for each lot.
The insulators shall be selected from the lot at random. The purchaser has the right to make the selection. The samples shall be subjected to the applicable sampling tests.
The sampling tests are as follows:
- a) verification of dimensions ………………………………………………. (E1 + E2)
- b) verification of the locking system ……………………………………… (E2)
- c) verification of the tightness of the interface between
end fittings and insulator housing …………………………………….. (E2)
- d) verification of the specified mechanical load, SML ………………… (E1)
- e) galvanizing test …………………………………………………………… (E2)
In the event of a failure of the sample to satisfy a test, the re-testing procedure shall be applied as prescribed in 12.6.
Insulators of sample E2 only can be used in service and only if the galvanizing test is performed with the magnetic method.
Table 4 – Sample sizes
Lot size
N
Sample size E1 E2
N ≤ 300 300 < N ≤ 2 000
2 000 < N ≤ 5 000
5 000 < N ≤ 10 000
Subject to agreement 4
8
12
Subject to agreement 3
4
6
– 18 – 61109 ¤ IEC:2008
12.2 Verification of dimensions (E1 + E2)
The dimensions given in the drawings shall be verified. The tolerances given in the drawings are valid. If no tolerances are given in the drawings the values mentioned in Clause 8 shall be used.
12.3 Verification of the end fittings (E2) The dimensions and gauges for end fittings are given in IEC 61466-1. The appropriate verification shall be made for the types of fitting used including, if applicable, verification of the locking system in accordance with IEC 60383-1.
12.4 Verification of tightness of the interface between end fittings and insulator housing (E2) and of the specified mechanical load, SML (E1)
- a) One insulator, selected randomly from the sample E2, shall 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, which shall act during 20 min, shall be applied on the cleaned surface;
– within 5 min after the application of the penetrant, the insulator shall be subjected, at the ambient temperature, to a tensile load of 70 % of the SML, applied between the metal fittings; the tensile load shall be increased rapidly but smoothly from zero up to 70 % of the SML, and then maintained at this value for 1 min;
– the surface shall be cleaned with the excess penetrant removed, 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 1 min test at 70 % of the SML, if any cracks occur, 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.
- b) The insulators of the sample E1 shall be subjected at ambient temperature to a tensile load, applied between the couplings. The tensile load shall be increased rapidly but smoothly from zero to approximately 75 % of the SML and then gradually increased to the SML in a time between 30 s to 90 s.
If 100 % of the SML is reached in less than 90 s, the load (100 % of the SML) shall be maintained for the remainder of the 90 s (this test is considered to be equivalent to a 1 min withstand test at the 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.
The insulators have passed this test if
– no failure (breakage or complete pull-out of the core, or fracture of the metal fitting) occurs either during the 1 min 70 % withstand test (a)) or during the 1 min 100 % withstand test (b)),
– no cracks are indicated after the dye penetration method described in 12.4 a),
– the investigation of the halves described in 12.4 a) shows clearly that the cracks do not reach the core.
12.5 Galvanizing test (E2)
This test shall be performed on all galvanized parts in accordance with IEC 60383-1.
12.6 Re-testing procedure
If only one insulator or end fitting fails to comply with the sampling tests, re-testing shall be performed using a new sample size equal to twice the quantity originally submitted to the tests.
The re-testing shall comprise the test in which failure occurred.
If two or more insulators or metal parts fail to comply with any of the sampling tests, or if any failure occurs during the re-testing, the complete lot is considered as not complying with this standard and shall be withdrawn by the manufacturer.
Provided the cause of the failure can be clearly identified, the manufacturer may sort the lot to eliminate all the insulators with this defect. The sorted lot may then be re-submitted for testing.
The number then selected shall be three times the first quantity chosen for tests. If any insulator fails during this re-testing, the complete lot is considered as not complying with this standard and shall be withdrawn by the manufacturer.
To comply with requirement of test in this standard and avoid insufficient of tensile load of test machine as mentioned in 12.4 b), an applicable test machine of good quality and sufficient tensile load is recommended, such as horizontal tensile test machine produced by Chinese manufacturer Contune International Corporation, which has computer control to customize the time of test for withstand test and the way tensile strength increase gradually. The length of the test machine can be customized according to user’s demand to adopt different length of suspension and tension insulator, as well as the tensile strength from 10KN, 30KN,50KN up to 5000KN.
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