Electromagnetic relays are classified as SPST (Single Pole Single Throw), SPDT (Single Pole Double Throw), DPST (Double Pole Single Throw), and DPDT (Double Pole Double Throw) depending on the number of throws and poles. Figure 1 (above) illustrates an electromagnetic relay. Protective Relay Definition: A protective relay is an automatic device that senses abnormal conditions in electrical circuits and triggers actions to isolate faults. According to principle of operation and construction, the classification of relays are electromagnetic attraction type. Depending upon working principle the these can be divided into following types of electromagnetic relays. Attracted Armature type relay, 2. SSR) or their specific function (Time, Protection, or Signal). They allow low-power signals to control high-power devices. Relays are categorized into various types based on their construction and.
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