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Insulator retest and rejection

Insulator retest and rejection is to ensure the insulator quality and insulator performance, insulator material and the insulator mechanical load and so on.

Insulator retest and rejection

Sample Procedure for testing of insulators shall be as per clause 7.1 to 7.6 of IEC 61109 for Acceptance & Routine Tests.
For the sampling tests, two samples are used, E1 and E2. The sizes of these samples are indicated in the table below.

Lot Size (N) Sample Size
E1 E2
N < 300 Subject to agreement
300 < N < 2000 4 3
2000 < N < 5000 8 4
5000 < N < 10000 12 6

If more than 10000 insulators are concerned, they shall be divided into an optimum number of lots comprising between 2000 and 10000 insulators. The results of the tests shall be evaluated separately for each lot. The insulators shall be selected by the purchaser’s representative from the lost at random.
The samples shall be subjected to the applicable sampling tests.
The sampling tests are:
Verification of dimensions - (E1+E2)
Verification of the locking system - (E2)
Verification of tightness of the interface between end fittings & - (E2)
Insulators housing
Verification of the specified mechanical load SML - (E1)
Galvanizing test - (E2)
In the event of a failure of the sample to satisfy a test, the retesting procedure shall be as follows:
If only one insulator or metal part fails to comply with the sampling tests, a new sample equal to twice the quantity originally submitted to the tests shall be subjected to retesting. The retesting shall comprise the test in which failure occurs. If two or insulator or metal parts fail to comply with any of the sampling tests or if any failure occurs during the retesting, the complete lot is considered as not complying with this standard and shall be withdrawn by the manufacturer.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 these defects. The sorted lot then be resubmitted for testing. The number then selected shall be three times the first chosen quantity for tests.
If any insulators fail during this retesting, the complete lot is considered as not complying with this standard and shall be withdrawn by the manufacturer.

Verification of dimensions ( E1 + E2)
The dimensions given in the drawings shall be verified. The tolerances given in the drawing are valid. If no tolerances are given in the drawings the values mentioned in these specifications shall hold good.

Verification of the locking system (E2)
This test applied only to the insulators equipped with socket coupling as specified by IEC-120 and is performed according to IEC 383.

Verification of tightness of the interface between end fittings and insulator housing (E2).
One insulator selected randomly from the sample E2, shall be subjected to crack indication by due 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 metal 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 minutes, shall be applied on the cleaned surface. Within 5 minutes 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 upto 70% f the SML, and then maintained at this value for 1 minute;
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 crack occur, the housing and, if necessary, the metal fittings and the core shall be cut, perpendicularly 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 for the depth of the cracks.

Verification of the specified mechanical load SML
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 be gradually increased to the SML in a time between 30 sec to 90 sec.
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).
These insulators have passed the test at 13.4 & 13.5 above 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 13.4 above.
The investigation of the halves described in 13.4 above shows clearly that the cracks do not reach the core.

Galvanizing Test :
This test shall be performed according to IS : 2633/IS : 6745 on galvanized parts.


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