The paper presents a measurement system of peak value of high voltage (H.V.) using 150？mm diameter sphere gap, disposed in vertical position. Experimental breakdown results have been compared with standard values at atmospheric reference conditions STC (20°C, 101.3？kPa, or 760？mmHg). The main experimental and theoretical characteristics are presented. 1. Introduction Choosing the most suitable method for measuring high voltage depends on the measured voltage value, on the available measuring devices, and on whether the need is to measure the peak or the effective value. It is known that spark gap spheres measure the peak AC voltage with a precision of ±3%. The breakdown voltage depends on the distance between the spheres, on the spheres diameter, and on the type of voltage: direct, alternating, or impulse. The breakdown voltage is a nonlinear function of the gap distance which is due to the increasing field inhomogeneity [1]. Sphere gaps can be arranged vertically, with the lower sphere grounded, or horizontally. In both cases, one must take into account the parameters and which define the field distribution between the two spheres that are closest to each other. For 150？mm sphere diameter, the minimum value of is and the maximum value is (where is the sphere diameter) while the minimum value for parameter minimum is (where is the spacing) [2, 3]. Sphere gap can be considered as an approved calibration device, with a limited accuracy, but with high reliability and simplicity [4]. The breakdown field distribution can be controlled by the geometry of the electrode and by the air density. In some cases, the spark gap needs to be irradiated with ultraviolet light or X-rays sources in order to obtain consistent values for smaller sphere gaps, with the gap spacing less than 1？cm [3, 4]. The breakdown voltage of a sphere gap is affected by some factors: atmospheric conditions, nearby earthed objects, voltage ramp-up time, and irradiation which is necessary when the diameter of the sphere gap is less than 10？cm and measured voltage below 50？kV [3–6]. The breakdown probability depends on the air density and on ion density in the air. Therefore, in the successive breakdown experiments it is necessary to have identical ion densities in the gas gap, before every new voltage application [7, 8]. The factors that can modify the breakdown voltage have been analyzed in the scientific literature [4, 9, 10]. The breakdown voltage increases with humidity. The increase is about 2-3 percent for normal humidity values between 8 and 15？g/m3 [4]. For sphere gaps spacing less
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