IEC 60422 Mineral Insulating Oils in Electrical Equipment
IEC 60422 Mineral Insulating Oils in Electrical Equipment - Supervision and Maintenance Guidance
9.3 Testing procedures
9.3.1 General
The venue for testing and the number and type of tests that can be carried out on a given sample of oil may vary depending on local circumstances and economic considerations.
Oil in service varies widely in the extent of degradation and the degree of contamination. In general, a single type of test is insufficient to evaluate the condition of the oil sample.
Evaluation of the condition should preferably be based upon the composite evaluation of significant characteristics determined in suitably qualified and properly equipped laboratories. However, some users find it advantageous to carry out field screening tests.
9.3.2 Field tests
In some circumstances there is a need to perform tests closer to the point of sampling rather in the laboratory. These are typically chosen to meet the following requirements:
• obtain a prompt estimation of oil condition;
• establish the classification of service-aged oils (see 9.4);
• eliminate any changes to the oil sample's properties due to transportation to a laboratory and/or storage of oil samples.
Field tests may also be performed where there are on-site and on-line test instruments with an accuracy comparable to laboratory test instruments.
Some field tests are less accurate than laboratory tests. Field tests are usually limited to visual inspection (colour and appearance), breakdown voltage, water content and, with less accuracy, acidity. These tests may sometimes be used for the assessment of service-aged oils in accordance with 9.4, though, more often, field tests are carried out to identify oil samples requiring laboratory evaluation.
Experience has shown that breakdown voltage and water content tests carried out on site may produce reliable results and may be used for acceptance tests.
9.3.3 Laboratory tests
A complete examination scheme includes all the tests listed in Table 1. However, these tests may be sub-divided into three groups and tests applicable to one or more groups may be required according to the specific requirements (see Tables 1 and 4).
9.4 Classification of the condition of oils in service
It is virtually impossible to set hard and fast rules for the evaluation of oil in service or recommend test limits for all possible applications of insulating oil in service. The classification and any consequent corrective action should only be taken after due consideration of the results of all tests. The trend of such results over a period of time is considered essential information when arriving at a final decision.
According to local or current industrial experience, oils in service may be classified as "good", "fair" or "poor" based on the evaluation of significant properties and their ability to be restored to the characteristics desired. Table 5 provides guidance to assist in this classification process.
• Good
Oil in normal condition; continue normal sampling.
• Fair
Oil deterioration detectable; more frequent sampling recommended (see Table 5).
• Poor
Oil deterioration abnormal; schedule effective actions.
9.5 Corrective action
In general, two types of contamination/deterioration of the oil can be considered: physical and chemical. Each one requires a different remedial action as described in Table 5 below.
The following recommendations should also be noted:
a) Where a test result is outside the limits recommended in Table 5, it should be compared with previous results and, if appropriate, a fresh sample obtained for confirmation before any other action is taken.
b) If rapid deterioration or acceleration in the rate of deterioration is observed, more frequent tests (see Table 4) should be instituted promptly and appropriate remedial action should be taken. It may be desirable to consult the manufacturer of the equipment.
It is emphasized that no action should be taken on the basis of one result and one property. Repeat samples are recommended where the result appears abnormal compared to the trend of the results previously obtained.
