The main requirements for an electric unit at the stages of its design, development, production and usage are described in technical specifications (TS) and standards (GOST). The electric unit should work in accordance with a specific purpose and have significant reliability, durability and safety. The reliability and durability of electric unit significantly depends on restoring strength speed value, that is growth of breakage voltage in arc pass for eliminating repeated arc strike. This article describes several methods of test identification of restoring strength, which were carried out at special testing laboratory units. They are described in relation to conditions of measuring the residual arc column of AC current after it reaches zero point and can be used in designing arc blowout units of low voltage.