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
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
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
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
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
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
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