Covers minimum performance requirements for ventilated and electrically heated precision ovens for thermal endurance evaluation of electrical insulating materials and other appropriate applications. It covers ovens designed to operate over all or part of the temperature range from 20 K above room temperature up to 300°C. Two possible methods of achieving the required performance are described: a) where the required performance is achieved by precise control of temperature in a simple single chamber oven, i.e., upgraded versions of ovens conforming to IEC 60216-4-1, and, otherwise b)where the required performance is achieved by utilizing a second chamber (iso-box), mounted within the chamber of a single-chamber oven, the purpose of which is to reduce the magnitude of any temperature changes to an acceptable level whilst maintaining the required levels of air changes and circulation.