TElSimpleFTPSClient.Active property

Specifies whether the session has been established and the data can be sent and received.

    This read-only property returns True if the session was established and the client can send data. If the client can't send data now, False is returned. The client should check the value of the property before calling Send method.
    However, if the connection is lost on the socket level, there is no way to find out that the connection has been closed, and Active property can still be True. The only way to check the actual connection state is to send something and get an error if the connection has been closed. This is by design of TCP/IP.
    Your code should always be ready to handle unexpected disconnections and other network errors and reconnect if needed.