What is the principle of capacitance isolation?
When the capacitor is connected to the DC circuit, there will be a brief charging process. When the positive and negative plates are filled with charge (when the voltage across the capacitor is equal to the supply voltage), there is no current and then flow, so the capacitor is isolated DC current
The structure of the capacitor is two plates, separated by a layer of insulator, so under normal circumstances the capacitor is no current through (unless the middle of the insulation was breakdown).
When the capacitor is connected to the AC circuit, there is also a charging process in which the charge is stopped when the capacitor voltage is equal to the supply voltage, and then the power supply voltage drops and the capacitor begins to discharge.
When the supply voltage rises in the reverse direction, the capacitor is charged in the reverse direction, and then the discharge is reversed. So the charge and discharge and the AC current is really the same, so the capacitor can pass the AC current.
