step response of a circuit having an inductor loop
This circuit is not adequately modeled with ideal components. In this model, for the current source to produce a step output, its voltage must approach infinity, because it will be trying to produce an infinite \$\frac{\mathrm{d}I}{\mathrm{d}t}\$ through an inductive load. 

If you include a parasitic shunt conductance or interwinding capacitance in parallel with each inductor, your model will be more realistic and the results won't have infinite voltages.

If you don't include parasitic elements, most SPICE simulators will automatically add something like 1 Gohm resistors from each node to ground, and your simulation result will depend on these "hidden" parasitic elements.