A study of the field utilization factor and the maximum electric field at sparkover of the standard sphere gaps

The field utilization factor η (the mean electric field/the maximum electric field) of the standard sphere gaps was calculated by the charge simulation method, taking into account the ground plane and shanks. A simple equation: η = 1 − ( g /3 r ), where g is the gap length and r is the radius of a sphere, can be used generally as a representative value of η, although it changes by the length and the diameter of shanks in the range permitted by the IEC standard. The maximum electric field at sparkover of a standard sphere gap describes a U‐shaped curve against ( g/r ), up to the diameter of 1 m. Peek's equation characterizes that the right‐hand side of the curve is almost constant. However, the effect of the ground and the shanks makes η smaller and the part of the curve is rather expressed as monotonously increasing. The curves of the maximum electric field of diameters 1.5 and 2 m are different from U‐form. The curve for a 1.5‐m sphere has its maximum point at g = 60 cm, and for a 2‐m sphere, at 50 cm. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 139(4): 26–32, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.1165