\documentclass{standalone}

\usepackage{tikz,pgfplots}
\usepackage{siunitx}
\usepackage[siunitx,european]{circuitikz}
\usepackage{mathrsfs}
\usetikzlibrary{arrows}
\usepackage{amsbsy}
\usetikzlibrary{fadings}
\usetikzlibrary{patterns}
\usetikzlibrary{automata,positioning}
\usepackage{mwe,subcaption,tikz}
\tikzset{boximg/.style={remember picture,red,thick,draw,inner sep=0pt,outer sep=0pt}}
\pgfplotsset{compat=1.11}
\begin{document}
\definecolor{mygreen}{rgb}{0,0.78,0}
\begin{tikzpicture}[scale=0.6]

\draw[->, thick] (0,-0.1) -- (0,4) node[above]{$I$};
\draw[->, thick] (-0.1,0) -- (8,0) node[right]{$t$};
\draw[color=blue, thick] (0.02,0.02) -- (0.02,3) -- (1.98,4) -- (1.98,0.02) -- (5.05,0.02) -- (5.02,3) -- (6.98,4) -- (6.98,0.02) -- (7.5,0.02) node[right, above]{$i_P$};
\draw[color=mygreen, thick] (0,0.02) -- (2.02,0.02) -- (2.02,4) -- (4.98,3) -- (4.98,0.02) -- (7.02,0.02) -- (7.02,4) -- (7.5,3.844) node[right, below]{$\frac{i_S}{N}$};
\draw[<->] (1.25,3) -- (1.25,4) node[above]{$\Delta I_m$};
\draw[dashed] (1.6,3) -- (-0.25,3)
(1.1,4) -- (2.25,4);

\end{tikzpicture}
\end{document}