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Testing High Voltages With Spark Gaps

The A.I.E.E. recommend these rules for testing high voltages with a spark gap. Measurements with Spark Gaps. If proper precautions are observed, spark gaps may be used to advantage in checking the calibration of voltmeters when set up for the purposes of high-voltage tests of the insulation of machinery.

Ranges of Voltages. For the calibrating purposes set forth the sphere gap shall be used for voltages above 50 kv., and is to be preferred down to 30 kv. The needle spark gap may, however, be used for voltages from 10 to 50 kv.

The Needle Spark Gap. The needle spark gap shall consist of new sewing needles, supported axially at the ends of linear conductors which are at least twice the length of the gap. There must be a clear space around the gap for a radius of at least twice the gap length. The sparking distances in air between No. 00 sewing needle points for various root-mean-square sinusoidal voltages are as follows:

The above values refer to a relative humidity of 80 per cent. Variations from this humidity may involve appreciable variations in the sparking distance.

The Sphere Spark Gap. The standard sphere spark gap shall consist of two suitably mounted metal spheres. When used as specified below, the accuracy obtainable should be approximately 2 per cent.

No extraneous body, or external part of the circuit, shall be nearer the gap than twice the diameter of the spheres. By the "gap" is meant the shortest path between the two spheres.

The shanks should not be greater in diameter than 1/5 the sphere diameter. Metal collars, etc., through which the shanks extend, should be as small as practicable and should not, during any measurement, come closer to the sphere than the maximum gap length used in that measurement.

The sphere diameter should not vary more than 0.1 per cent and the curvature, measured by a spherometer, should not vary more than 1 per cent from that of a true sphere of the required diameter.

In using the spherometer to measure the curvature, the distance between the points of contact of the spherometer feet should be within the following limits:

In using Sphere Gaps constructed as above, it is assumed that the apparatus will be set up for use in a space comparatively free from external dielectric fields. Care should be taken that conducting bodies forming part of the circuit, or at circuit potential, are not so located with reference to the gap that their dielectric fields are superposed on the gap; e.g., the protecting resistance should not be arranged so as to present large masses or surfaces near the gap, even at a distance of two sphere diameters.

In case the sphere is grounded, the spark point of the grounded sphere should be approximately five diameters above the floor or ground.

When the variation from sea level is not great, the relative air density may be used as the correction factor ; when the variation is great, or greater accuracy is desired, the correction factor corresponding to the relative air density should be taken from table below.

Values of relative air density and corresponding values of the correction factor are tabulated below. It will be seen that for values above .9, the correction factor does not differ greatly from the relative air density.

The Spark-Over Voltage, for a given gap, decreases with decreasing barometric pressure and increasing temperature. This correction may be considerable at high altitudes.

The spacing at which it is necessary to set a gap to spark over at some required voltage, is found as follows:

Divide the required voltage by the correction factor given in Table 4. A new voltage is thus obtained.

The spacing on the standard curves obtained from Table 3, corresponding to this new voltage, is the required spacing. The voltage at which a given gap sparks over is found by taking the voltage corresponding to the spacing from the standard values of Table 3, and multiplying by the correction factor.

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