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Electro-mechanical (EM) relays have been used in safeguarding electrical systems since the early days of electrical system protection. Static analog relays have been available for over 20 years and can provide multiple protective functions and tasks, They can come in single-phase or multi-phase units. Newer static digital relays utilize microprocessor technology which provides additional functionality such as communications, self diagnosis, metering, event recording, waveform capture and much more. However, advancement in technology brings the question: "Do you want all your relay eggs in one basket" ? Single-phase, single-function electromechanical devices may not provide all the features, but are considered by some to provide more security and dependability. This paper discusses the advantages and disadvantages of using the latest microprocessor technology in applying protective relays (Figure 1) in typical Petroleum and Chemical Industry applications. Electro-mechanical , static analog, and static digital relays are compared regarding features, maintainability, performance, and application. The goal of this paper is to aid industrial electrical engineers to select the best application combination of relay technology and equipment for operations and maintenance personnel. (Relaying for transmission and distribution applications and outdoor primary substations are not included in the scope of this Paper-)

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