What must be visible in isolation points for work over 750 volts?

For work involving voltages over 750 volts, it is crucial to have a visible open point at isolation points. This ensures that anyone working in the vicinity is aware that the system has been intentionally interrupted, making it safe to proceed with their tasks. The visibility of an open point indicates that the circuit is de-energized and can help prevent accidental energization during work.

Other options, while potentially helpful in a broader context, do not provide the same immediate assurance that a system is safe to work on. A safety warning sign may indicate the presence of high voltage, but it does not confirm that the circuit has been opened. A maintenance log and a detailed work plan may be important components of a safety protocol, but they do not serve the direct function of visually communicating the status of the electrical system. Only a visible open point confirms to workers that they are operating in a safe environment.