Following energized winding of a D.C. machine should not be opened as it would produce high inductive voltage which may be dangerous to personnel and may cause its own insulation failure.

Explanation:
In a DC machine, the shunt field winding is connected in parallel with the armature winding. When the machine is energized, the shunt field winding carries a significant amount of current. If the shunt field winding is suddenly opened while the machine is running, the current flowing through it will abruptly drop to zero. This rapid change in current will induce a high voltage across the winding due to the principle of electromagnetic induction.
This induced voltage can be extremely dangerous to personnel working on the machine and can also cause insulation breakdown within the winding itself. Therefore, it is crucial to de-energize the DC machine before opening the shunt field winding to avoid such hazards.