FED-STD-228A
Method 6221.1
18 APRIL 2014
DIELECTRIC STRENGTH, CLOTH AND TAPE
1. SCOPE
1.1 This method is intended for use in determining the average voltage per unit of thickness that an
insulating cloth or tape will withstand without breakdown. It is particularly applicable to varnished cloth or
tape.
2. SPECIMEN
2.1 The specimen should be a piece of the varnished cloth approximately 2 feet in length taken from a
single tape (layer) of the inspection unit.
3. APPARATUS
3.1 The apparatus shall be as follows:
3.1.1 A transformer with an adjustable output voltage supplied by a source having a frequency of 25 to
100 cycles (preferably 60 cycles) and a wave form approximating a sine curve as closely as possible.
3.1.2 A set of brass or copper electrodes ¼ inch in diameter. The electrodes shall be rounded to a
radius of 1/32 inch and finished with a flat polished contact surface. They shall be self-aligning and
mounted exactly opposite each other so as to prevent any surface burning of the tape or flashovers. The
electrodes shall be clean and polished.
3.1.3 Clamps and blocks of insulating materials for holding the specimen.
4. PROCEDURE
4.1 Approximately 2 feet of each tape layer shall be removed from the inspection unit. Unless otherwise
specified in the detail specification, 10 percent of the tapes but not less than five tapes shall be drawn at
random for test.
4.2 The specimen shall be conditioned for 48 hours at a temperature of 23° ± 1°C (73.5° ± 2°F) and a
relative humidity of 50 ± 4 percent.
4.3 The edges of the specimen shall be clamped between blocks of insulating materials under a pressure
of approximately 100 pounds per square inch to prevent flashovers occurring before puncture. Starting at
zero, the voltage shall be increased uniformly to breakdown at a rate of 500 volts per second, except that
if breakdown occurs at this rate in less than 40 seconds, the voltage rate shall be decreased so that the
breakdown will require at least 40 seconds. If the material fails at less than 5 kilovolts, the minimum time
shall be reduced from 40 to 20 seconds. The breakdown voltage shall be measured to the nearest 50
volts and the value recorded as V. Five tests equally spaced along the length at the center of the
specimen shall be made.
4.4 The thickness of the specimen shall be measured as described in method 1051 except that one
thickness measurement shall be made near each puncture in such a way as to represent as closely as
possible the thickness at the point of puncture and the value recorded as T.
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