When is an interlocking appliance considered "blocked"?

An interlocking appliance is considered "blocked" when an approved device is applied to prevent operation. This means that specific safety measures are actively in place to ensure that the interlocking system cannot be operated or inadvertently activated. This is crucial for maintaining control and ensuring safety within the rail system, as blocking an interlocking appliance effectively prevents trains from passing through certain areas until the necessary conditions are met to allow safe operation.

The other choices do not sufficiently define what constitutes a "blocked" status. While securing the appliance by a qualified employee or having a movable frog may play roles in operational procedures, they do not inherently mean that the appliance is blocked from function. Likewise, if the appliance is functioning normally, it is clearly not blocked, as it is still operational. Thus, the key aspect of “blocking” relates specifically to the implementation of an approved device to halt operations for safety.