A new method for the synthesis and analysis of automation systems is described, in the context of a project for an EHV substation with digital automation systems. There are many reasons why an automation system fails to operate perfectly. Following an analysis of the situation, the present paper describes the general characteristics to be sought in an automation system, in terms of its functional diagrams. We then suggest a method of synthesis and analysis whereby these general characteristics can be obtained. The method is easy to apply, and can be computer-backed in the case of large and complex systems.