${\displaystyle \rho \left(t\right)={\displaystyle \underset{-\infty }{\overset{t}{\int }}\text{ε}\left(\tau \right)}\text{d}\tau \text{=}\{\begin{array}{l}0\text{for}t<0\\ t\text{for}t>0\end{array}}$, where ε(t) is the unit-step function

SEE: Figure 4b), Figure 4c) and CEI 60050-103:2009, 103-03-03.