FED-STD-228A
4.2 Unless otherwise specified in the detail specification or specification sheets, the insulation resistance
shall be measured 1 minute after the circuit is closed.
4.3 The test voltage shall be as specified in the detail specification or specification sheet.
4.4 The insulation resistance test may be made before the braiding or finishing operation during
manufacture, but not before any vulcanizing process. Single conductor cable shall be tested between the
conductor and water in which it is immersed. Lead covered single conductor cable shall be tested prior to
application of the lead sheath. Individual conductors of a multiple conductor cable shall be tested before
assembly as described for single conductor cable. After assembly, the multiple-conductor cable shall be
tested between a single conductor against all other conductors arranged in two or more groups for
testing; these tests may be conducted without immersion or with immersion using the water as a ground.
After covering, a lead-covered cable shall be tested between each conductor and the lead sheath, or
between each conductor and its contiguous conductors and to the sheath, without immersion.
4.5 The specimen shall be immersed in water for a period of at least 1 hours before application of the
potential. Unless otherwise specified in the detail specification or specification sheet, tap water will be
used. The water in which the specimen is tested while immersed, as well as the lead covered and
multiple conductor cable, shall be at a temperature between 10° and 27°C for at least 30 minutes before
the test. The ends of the specimen shall be kept well above the surface of the water and the covering
removed from the surface of the insulation for several inches at each end in order to reduce surface
leakage. The ends of the specimen shall be dipped into melted paraffin or other satisfactory material to
minimize the effect of moisture in reducing insulation resistance between the covering and conductor.
The water in the bath shall be stirred well to maintain a uniform temperature throughout the bath during
the test. The specimen shall be assembled in the test circuit as described in 4.6.
4.6 The test instruments shall be mounted so that they will be reasonably free from vibration and will not
be affected adversely by local conditions. A typical test circuit, as shown on figure 6031, shall consist of a
grounded galvanometer with Ayrton shunt in series with a suitable switch, a known resistance, and a
battery with the negative terminal connected to one end of the conductor of the specimen. The circuit
shall be completed through the water electrode or metallic sheath which shall be grounded.
4.7 The galvanometer constant K shall be determined with the battery lead C connected to ground at A.
The galvanometer shunt constant is defined as the value of resistance required in series with the
galvanometer to produce a galvanometer scale deflection of one scale division, either with no shunt or for
shunt constant equal to unity and the value of voltage equal to that required in 4.3. (In paragraphs 4.8
and 5 the constants S and s for shunt tap settings are assumed to be the ratio of the tapped resistance to
the total resistance of the shunt (Ayrton type) normally connected across the galvanometer terminal.)
4.8 To allow for any leakage current to ground from the battery, instruments, and all connections, the
galvanometer deflection and corresponding shunt constant s shall be recorded with the connection
between the battery lead C and either the specimen or ground open. The galvanometer deflection and
shunt constant S, for the insulation resistance of the specimen, shall then be recorded with the battery
lead C connected to one end of the conductor of the specimen at point B. The galvanometer deflection
shall be recorded after an electrification period of 1 minute. In the case of a single coil where the
deflection is small and is decreasing, the galvanometer scale reading may be taken at the end of 15
seconds.
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