What signifies that an interlocking appliance is blocked?

The correct answer is that an employee has applied an approved device to prevent operation. This signifies that an interlocking appliance is blocked because it indicates that a specific safety measure has been put in place to ensure that the appliance cannot be operated until the device is removed. This practice is essential in maintaining safety protocols, as it helps to prevent accidental movements of the appliance while maintenance or other critical operations are underway.

In contrast, while a visual warning signal or mechanical locking in a fixed position may indicate that an appliance is not currently in operation, they do not explicitly signify that an interlocking appliance is blocked through the application of a safety measure by a trained employee. Additionally, undergoing routine maintenance would not inherently mean the appliance is blocked; it may simply be temporarily out of service for inspections or repairs without a formal blocking mechanism in place. Thus, the application of an approved device is a direct and clear indication that the appliance is securely blocked.