When an L-R-C series circuit is connected across a 120-V ac line, the voltage rating of the capacitor may be exceeded even if it is rated at 200 or 400 V. How can this be?

An inductor is a passive two-terminal device that stores energy in a magnetic field when current passes through it. When an inductor is attached to an AC supply, the resistance produced by it is called inductive reactance$\left({\mathrm{X}}_{\mathrm{L}}\right)$.

A capacitor is a two-terminal device that stores energy in a electric field when current passes through it. When a capacitor is attached to an AC supply, the resistance produced by it is called capacitive reactance$\left({\mathrm{X}}_{\mathrm{C}}\right)$.

Resistance is measure of opposition to the flow of current in a closed electrical circuit. It is measured in Ohm (Ω).

The capacitor is charged during the positive half cycle of the supply voltage and discharged during the negative half cycle, and it can exceed the supply voltage's magnitude. As a result, the capacitor's rated voltage referred to the highest permissible DC voltage across the capacitor.

Therefore, In an RLC circuit with 120 V AC source, the voltage rating of the capacitor may exceed 200 or 400 V.