INTRODUCTIONIt is only in the last ten years that transistorised equipment has been widely used in power systems. This delay was as a result of reluctance to use modern electronic technology where previously electro-mechanical instruments had been used satisfactorily. It is just a little surprising, therefore, that digital equipment has become accepted during the last three years or so. For example, earlier experience with digital frequency meters had been disappointing. These suffered from electrical interference when installed in the particularly noisy environment of an electrical substation. The interference problems were eventually solved using isolation transformers, RF filters, and optical isolators in the production of digital equipment which has performed well in a growing number of substation installations. The solution of the interference problem was a breakthrough. It permitted manufacturers to offer, with confidence, microprocessor-based equipment which had significant advantages in terms of cost, size and performance over conventional electro-mechanical and electronic alternatives.The main applications of microprocessors in substations are: