What indicator must be visible on all AC power primary sources?

The visibility of a 'power on' indicator on all AC power primary sources is essential for ensuring that the system is operational and receiving adequate electrical supply. This indicator serves a critical function by allowing technicians and users to easily verify that power is flowing into the device or system. It provides an immediate visual cue that can help in troubleshooting and maintenance tasks, indicating whether the power source is functioning properly.

While other indicators, such as a power level gauge, battery status light, or surge protection indicator, can offer additional information about the state of the system or circuit, they do not universally guarantee the presence of AC power. The 'power on' indicator specifically confirms that the device is energized, which is a fundamental requirement for any system relying on AC power to function effectively.