The overcurrent relays for the line protection and loads connected at the buses are shown in the figure.

The relays are $IDMT$ in nature having the characteristic

$t_{op}=\dfrac{0.14\times \text{Time Multiplier Setting}}{(\text{Plug Setting Multiplier})^{0.02}-1}$

The maximum and minimum fault currents at bus $B$ are $2000$ $A$ and $500$ $A$ respectively. Assuming the time multiplier setting and plug setting for relay $R_B$ to be $0.1$ and $5A$ respectively, the operating time of $R_B$ (in seconds) is ______________

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