No energized line is ever truly "not loaded". There will be line charging currents flowing. Line length, voltage, line geometry all enter into the picture. The lower the voltage and the shorter the line the lower the charging current.
The spark in disconnect switches is usually caused for the capacitive trapped charge in the system. To minimize this effect, many disconnect switches are specified with arching horn.
The enclose picture copied from in the net is a good example of the problem described above.
