IEC 60156 Insulating Liquids - Determination of Breakdown Voltage
IEC 60156 Insulating Liquids - Determination of the Breakdown Voltage at Power Frequency - Test Method
4 Test assembly
4.1 Test cell
The volume of the cell shall be between 350 ml and 600 ml.
The cell shall be made of material that is electrically insulating, transparent and chemically inert, resistant to the insulating liquid and the cleaning agents which may be used.
The cell shall be provided with a cover and shall be designed to permit easy removal of the electrodes for cleaning and maintenance.
Examples of suitable cell designs are given in Figure 1 and Figure 2.
4.2 Electrodes
The electrodes shall be made either of brass, bronze or austenitic stainless steel. They shall be polished and, in shape, either spherical (12,5 mm to 13,0 mm diameter) as shown in Figure 1 or partially spherical of the shape and dimensions given in Figure 2. The axis of the electrode system shall be horizontal, and at least 40 mm below the surface of the test liquid in the cell. No part of the electrode shall be closer than 12 mm to the cell wall or stirrer. The gap between the electrodes shall be 2,50 mm ± 0,05 mm.
The electrodes shall be examined frequently for pitting or other damage, and shall be maintained or replaced as soon as such damage is observed.
4.3 Stirring (optional)
The test may be conducted with or without stirring. Differences between tests with or without stirring have not been found statistically significant.
A stirrer, however, may be convenient especially with apparatus capable of automatic operation.
Stirring may be achieved by means of a two-bladed impeller of effective diameter 20 mm to 25 mm, axial depth 5 mm to 10 mm, rotating at a speed of 250 r.p.m to 300 r.p.m. The impeller shall not entrain air bubbles and preferably rotate in such a direction that the resulting liquid flow is directed downward. It shall be constructed so that it is easily cleaned.
Stirring by means of a magnetic bar (20 mm to 25 mm in length and 5 mm to 10 mm in diameter) is an acceptable alternative when there is no risk of removing magnetic particles.
The dimensions of the stirring device shall conform to the clearance requirements in 4.2.
5 Preparation of electrodes
New electrodes, pitted electrodes, electrodes which have not been properly stored for a considerable time shall be cleaned by the following procedure:
- clean all surfaces with a suitable volatile solvent and allow the solvent to evaporate;
- polish with fine abrasive powder (for example, jeweller's rouge) or abrasive paper or cloth (for example, crocus cloth);
- after polishing, clean with petroleum spirit (reagent quality: boiling range 60 °C - 80 °C) followed by acetone (reagent quality);
- assemble the electrodes in the cell, fill with a clean, unused insulating liquid of the type to be tested next, and raise the electrode voltage to breakdown 24 times.
6 Test assembly preparation
It is recommended that a separate test cell assembly be reserved for each insulating liquid type.
Test assemblies shall be stored in a dry place, covered and filled with dry insulating liquid of the type in regular use in the cell.
On change of the type of liquid under test, remove all residues of the previous liquid with an appropriate solvent, rinse the assembly with clean, dry liquid of the same type as that to be tested, drain and refill.
7 Sampling
7.1 Sample containers
Sample size should be approximately three times the capacity of the test cell.
Appropriate sample containers shall comply with IEC 475. An amber glass bottle is the preferred container. Clear glass bottles may be used but they shall be shielded from direct light until ready to be tested. Plastic containers which are not attacked by the liquid to be tested may be used, but these shall not be used more than once. For sealing, screw caps with polyolefine or polytetrafluoroethylene insert are preferred.
Containers and caps shall be cleaned by washing with a suitable solvent to remove residues of an earlier sample. Containers shall next be rinsed with acetone, traces of which shall be removed by blowing with warm air.
After cleaning, containers shall be immediately capped and kept sealed until used.
7.2 Sampling technique
Sampling of new and used insulating liquids shall be carried out in full compliance with procedures detailed in IEC 475.
When sampling, containers should be almost filled with sample, leaving about 3 % of the container volume as free air space.
Breakdown voltage is extremely sensitive to the slightest contamination of the sample by water and particulate matters. Special reference is made to precautions necessary to avoid contamination of the sample and the need for trained personnel and experienced supervision.
Unless otherwise required, the sample is taken where the liquid is likely to be most contaminated, usually at the lowest point of the container holding it.
8 Condition of the sample
The test is carried out, unless otherwise specified, on the sample as received without drying or degassing.
At the time of test, the temperatures of the test liquid and ambient air shall not differ by more than 5 °C and for referee tests the liquid temperature shall be 20 °C ± 5 °C.
