Thevenin equivalent and RC circuit
For t &lt; 0 ms, the circuit can be re-drawn as shown.







simulate this circuit &ndash; Schematic created using CircuitLab

at t = 0 ms the \$30\Omega\$ resistance comes in parallel with the old resistance. Now the circuit becomes.







simulate this circuit

You can directly write \$R_{th}\$ and \$U_{th}\$ from this circuit.

The only thing happened here is that you made the capacitor's charging fast. But it won't have any effect on voltage across C if it has reached V1 volts before t = 0 ms.