This handbook covers the code of practice in protection circuitry including standard lead and device numbers, mode of connections at terminal strips, colour codes in multicore cables, dos and donts in execution. Also principles of various protective relays and schemes including special protection. The most common types of relay symbols used in circuit diagrams include: Figure: Different relay symbol types including SPST, SPDT, DPST, DPDT, Latching, and Solid State Relays (SSR). The Single Pole Single Throw (SPST) symbol is the simplest version. It features a coil and a basic ON/OFF switch. presentation of protection and control relaying. In electric power systems and industrial automation, ANSI Device Numbers can be used to identify equipment and devices in a system such as relays, circuit breakers, or instruments. The device numbers are enumerated in ANSI / IEEE Standard C37. These numbers are based on a system that is adopted by a standard for automatic switchgear by Institute of Electrical. There are two methods for indicating protection relay functions in common use.
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