For the past fifteen years the author has been closely involved with the development of a range of medium and high performance puffer interrupters based on partial duo-blast concept for ratings up to 550 KV. This paper will present examples to illustrate the importance of computer-aided field analysis in the evolution of EHV switchgear. The studies include critical comparisons of 4 - and 2 - break circuit breaker dielectric performance which enabled the insulation design of one-break circuit breaker to be optimised and more accurately predicted prior to high voltage type testing.