6.4 Surrounding Medium—The document calling for this test method needs to specify the surrounding medium and the test temperature. Since flashover must be avoided and the effects of partial discharges prior to breakdown mimimized, even for short time tests, it is often preferable and sometimes necessary to make the tests in insulating liquid （see 6.4.1）. Breakdown values obtained in insulating liquid are often not comparable with those obtained in air. The nature of the insulating liquid and the degree of previous use are factors influencing the test values. In some cases, testing in air will require excessively large specimens or cause heavy surface discharges and burning before breakdown. Some electrode systems for testing in air make use of pressure gaskets around the electrodes to prevent flashover. The material of the gaskets or seals around the electrodes has the potential to influence the breakdown values.
6.4.1 When tests are made in insulating oil, an oil bath of adequate size shall be provided. （Warning—The use of glass containers is not recommended for tests at voltages above about 10 kV, because the energy released at breakdown has the potential to be sufficient to shatter the container. Metal baths must be grounded.）
It is recommended that mineral oil meeting the requirements of Specification D3487, Type I or II, be used. It shall have a dielectric breakdown voltage as determined by Test Method D877 of at least 26 kV. Other dielectric fluids are suitable for use as surrounding mediums if specified. These include, but are not limited to, silicone fluids and other liquids intended for use in transformers, circuit breakers, capacitors, or cables.
18.104.22.168 The quality of the insulating oil has the potential to have an appreciable effect upon the test results. In addition to the dielectric breakdown voltage, mentioned above, particulate contaminants are especially important when very thin specimens （25 μm （1 mil） or less） are being tested. Depending upon the nature of the oil and the properties of the material being tested, other properties, including dissolved gas content, water content, and dissipation factor of the oil also have the potential to affect the results. Frequent replacement of the oil, or the use of filters and other reconditioning equipment is important to minimize the effect of variations of the quality of the oil on the test results.
22.214.171.124 Breakdown values obtained using liquids having different electrical properties are often not comparable. （SeeX1.4.7.） If tests are to be made at other than room temperature, the bath must be provided with a means for heating or cooling the liquid, and with a means to ensure uniform temperature. Small baths can in some cases be placed in an oven （see 6.4.2） in order to provide temperature control. If forced circulation of the fluid is provided, care must be taken to prevent bubbles from being whipped into the fluid. The temperature shall be maintained within 65°C of the specified test temperature at the electrodes, unless otherwise specified. In many cases it is specified that specimens to be tested in insulating oil are to be previously impregnated with the oil and not removed from the oil before testing （see Practice D2413）. For such materials, the bath must be of such design that it will not be necessary to expose the specimens to air before testing.
6.4.2 If tests in air are to be made at other than ambient temperature or humidity, an oven or controlled humidity chamber must be provided for the tests. Ovens meeting the requirements of Specification D5423 and provided with means for introducing the test voltage will be suitable for use when only temperature is to be controlled.
6.4.3 Tests in gasses other than air will generally require the use of chambers that can be evacuated and filled with the test gas, usually under some controlled pressure. The design of
D149 – 094 such chambers will be determined by the nature of the test program to be undertaken.