Hard circuits refer to logic control and protection functionalities implemented through physical wiring, including relays, manual buttons, and other electromechanical components. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. Programmable relays have further simplified the zoning process, accommodating various inputs. In modern industrial systems, the concepts of “hard circuits” and “soft circuits” (or “hard wiring” and “soft wiring”) are commonly used to describe different methods of implementing logic control and protection functionalities. Because of their small size and excellent dependability these relays are used extensively. Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function. 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.
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