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. 
6.4.1.1 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. 
6.4.1.2 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 
ASTMD149美標標準固體絕緣材料電壓擊穿的實驗方法（十） 
6.5 Test Chamber—The test chamber or area in which the tests are to be made shall be of sufficient size to hold the test equipment, and shall be provided with interlocks to prevent accidental contact with any electrically energized parts. A number of different physical arrangements of voltage source, measuring equipment, baths or ovens, and electrodes are possible, but it is essential that （1） all gates or doors providing access to spaces in which there are electrically energized parts be interlocked to shut off the voltage source when opened; （ 2）clearances are sufficiently large that the field in the area of the electrodes and specimen are not distorted and that flashovers and partial discharges （corona） do not occur except between the test electrodes; and （3） insertion and replacement of specimens between tests be as simple 

and convenient as possible. Visual observation of the electrodes and test specimen during the test is frequently desirable. 7. Hazards 
7.1 Warning—It is possible that lethal voltages will be present during this test. It is essential that the test apparatus, and all associated equipment electrically connected to it, be properly designed and installed for safe operation. Solidly ground all electrically conductive parts that any person might come into contact with during the test. Provide means for use at the completion of any test to ground any parts which fall into any of the following cases: （a） were at high voltage during the test; （b） have the potential to acquire an induced charge during the test; or （c） have the potential to retain a charge even after disconnection of the voltage source. Thoroughly instruct all operators in the proper way to conduct tests safely. When making high-voltage 
tests,particularly in compressed gas or in oil, it is possible that the energy released at breakdown will be sufficient to result in fire, explosion, or rupture of the test chamber. Design test equipment, test chambers, and test specimens so as to minimize the possibility of such occurrences and to eliminate the possibility of personal injury. 
7.2 Warning—Ozone is a physiologically hazardous gas at elevated 
concentrations. The exposure limits are set by governmental agencies and are usually based upon recommendations made by the American Conference of Governmental ndustrial Hygienists.8 Ozone is likely to be present whenever voltages exist which are sufficient to cause partial, or complete, discharges in air or other atmospheres that contain oxygen. Ozone has a distinctive odor which is initially discernible at low concentrations but sustained inhalation of ozone can cause temporary loss of sensitivity to the scent of ozone. Because of this it is important to measure the concentration of ozone in the atmosphere, using commercially available monitoring devices, whenever the odor of ozone is persistently present or when ozone generating conditions continue. Use appropriate means, such as exhaust vents, to reduce ozone concentrations to acceptable levels in working areas. 8. Sampling 

8.1 The detailed sampling procedure for the material being tested needs to be defined in the specification for that material. 
8.2 Sampling procedures for quality control purposes shall provide for gathering of sufficient samples to estimate both the average quality and the variability of the lot being examined; and for proper protection of the samples from the time they are taken until the preparation of the test specimens in the laboratory or other test area is begun. 
8.3 For the purposes of most tests it is desirable to take samples from areas that are not immediay adjacent to obvious defects or discontinuities in the material. Avoid the outer few layers of roll material, the top sheets of a package of sheets, or material immediay next to an edge of a sheet or 
roll, unless the presence or proximity of defects or discontinuities is of interest in the investigation of the material. 
8.4 The sample shall be large enough to permit making as many individual tests as required for the particular material （see 12.4）. 
9. Test Specimens 
9.1 Preparation and Handling: 
9.1.1 Prepare specimens from samples collected in accordance with Section 8. 9.1.2 When flat-faced electrodes are to be used, the surfaces of the specimens which will be in contact with the electrodes shall be smooth parallel planes, insofar as possible without actual surface machining. 
9.1.3 The specimens shall be of sufficient size to prevent flashover under the conditions of test. For thin materials it will often be convenient to use specimens large enough to permit making more than one test on a single piece. 
9.1.4 For thicker materials （usually more than 2 mm thick） it is possible that the breakdown strength will be high enough that flashover or intense surface partial discharges （corona） will occur prior to breakdown. Techniques that are suitable for use to prevent flashover, or to reduce partial discharge （corona） include: 

9.1.4.1 Immerse the specimen in insulating oil during the test. See X1.4.7 for the surrounding medium factors influencing breakdown. This is often 
necessary for specimens that have not been dried and impregnated with oil, as well as for those which have been prepared in accordance with Practice D2413,for example. （See 6.4.） 
9.1.4.2 Machine a recess or drill a flat-bottom hole in one or both surfaces of the specimen to reduce the test thickness. If dissimilar electrodes are used （such as Type 6 of Table 1） and only one surface is to be machined, the larger of the two electrodes shall be in contact with the machined surface. Care must be taken in machining specimens not to contaminate or mechanically damage them. 
9.1.4.3 Apply seals or shrouds around the electrodes, in contact with the specimen to reduce the tendency to flashover. 
9.1.5 Materials that are not in flat sheet form shall be tested using specimens （and electrodes） appropriate to the material and the geometry of the sample. It is essential that for these materials both the specimen and the electrodes be defined in the specification for the material. 
8 Available from American Conference of Governmental Industrial Hygienists, Inc. （ACGIH）, 1330 Kemper Meadow Dr., Cincinnati, OH 45240, http:// www.acgih.org. D149 – 09 5 
9.1.6 Whatever the form of the material, if tests of other than 
surface-to-surface puncture strength are to be made, define the specimens and the electrodes in the specification for the material. 
9.2 In nearly all cases the actual thickness of the test specimen is important. Unless otherwise specified, measure the thickness after the test in the immediate vicinity of the area of breakdown. Measurements shall be made at room temperature （25 6 5°C）, using the appropriate procedure of Test Methods.