Inductor act as open circuit because inductor is current storage device.Initially, when the switch in a series RL circuit is closed, the inductor acts as an open circuit. Current in an open circuit is zero, hence the inductor current is zero after that with the passage of time it store current and then act as a short circuit . it only act as a open circuit during initial condition when t=0.

When the switch in a series RL circuit is closed, the behavior of the inductor can be explained as follows:

1. Inductor in DC:
- Initially, when the switch is closed, a direct current (DC) starts flowing through the circuit.
- Since DC current is steady and constant, the rate of change of current is zero.
- According to Faraday's law of electromagnetic induction, the induced voltage across an inductor is proportional to the rate of change of current.
- Therefore, when the current is not changing (as in the case of DC), the induced voltage across the inductor is zero.
- As a result, the inductor behaves as an open circuit, meaning it offers high resistance to the flow of current.

2. Inductor in AC:
- In an alternating current (AC) circuit, the current continuously changes direction and magnitude over time.
- When the AC signal is first applied, it takes some time for the current to build up from zero to its peak value.
- During this transient period, the inductor resists the change in current flow by generating a back-emf (electromotive force) that opposes the applied voltage.
- The inductor behaves as if it is an open circuit, limiting the current flow.
- However, once the current reaches its steady-state value, the inductor allows the AC current to pass through with minimal opposition.
- At steady state, the inductor behaves as a low impedance element, similar to a short circuit for AC signals.

In summary, when the switch in a series RL circuit is closed, the inductor initially acts as an open circuit, offering high resistance to the flow of direct current. However, once the current reaches its steady-state value in an AC circuit, the inductor behaves as a low impedance element, allowing the AC current to pass through with minimal opposition.