FED-STD-228A
4.2 Minimum thickness.
4.2.1 The inspection unit shall be examined for thin places in the insulation. The specimen shall be
taken from the inspection unit so as to include the point where the insulation is considered the thinnest
and shall be free from mechanical damage. Any saturants or extraneous materials shall be removed from
the insulation by means of the cloth and solvent. The insulation shall be removed from the specimen on
the side opposite the point considered thinnest. A measurement including the thickness of the insulation
at the point considered thinnest and the diameter of the conductor or separator (if one is used) shall be
made by applying the caliper in the manner described in 4.1.1 for determining the diameter over the
insulation and the value recorded as d1.
4.2.2 The insulation at the point considered thinnest shall then be removed from the conductor and the
diameter over the conductor or separator (if one is used) measured as described in 4.1.2 and the value
recorded as d2.
4.2.3 The measurement over the insulation and the diameter over the conductor or separator shall be
determined as nearly as practicable at the same point.
4.2.4 If no thinnest point is located in the insulation of the inspection unit, the procedure shall be as
described in 4.1.
5. RESULTS
5.1 Calculations.
5.1.1 Average thickness. The thickness of the insulation at any point should be calculated as follows:
Thickness, inch=D1-D2
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Where:
D1 = the diameter over the insulation at the point measured, inches
D2 = the diameter over the conductor or separator at the point measured, inches
5.1.2 Minimum thickness. The minimum thickness of the insulation should be calculated as follows:
Thickness, inch = d2-d1
where:
d1=the thickness over the insulation and conductor or separator at the thinnest point, inches
d2=the diameter over the conductor or separator, inches
5.2 Average thickness.
5.2.1 Unless otherwise specified in the detail specification, three specimens from each inspection unit
should be tested.
5.2.2 The average thickness of the insulation of the inspection unit should be the average of the results
obtained from the specimens tested.
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