Time constant of a first order unit step input system is defined as the time at which unit step response reaches
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For a rising exponential response: At \(t = 1\tau\), response reaches \(63.2%\). For a decaying exponential response: At \(t = 1\tau\), response drops down to \(36.8%\) of its initial values. Keeping this distinction clear avoids confusion during evaluations!