Which two features are mandatory for the mine telephonic communication system?

The correct answer emphasizes the necessity of a backup power supply and suitability for use in a designated emergency risk zone (ERZ0) for mine telephonic communication systems.

A backup power supply is critical in mining operations as it ensures that communication systems remain operational even during power outages, which can happen due to various situations like equipment failure or severe weather. This is crucial for maintaining safety and effective communication in emergency situations where timely responses can be life-saving.

Being suitable for use in an ERZ0 highlights the safety requirements mandated in areas where there are potential hazards due to mining activities. ERZ0 classifications indicate areas where additional precautions must be taken to protect workers and equipment from hazards such as explosive atmospheres. Therefore, the communication system must be designed to operate safely in such environments, evidencing compliance with safety regulations and standards.

In summary, the combination of a backup power supply and suitability for ERZ0 clearly meets legal and practical requirements that ensure operational safety and reliability in mining environments. The features in this correct answer align with the overarching goal of risk mitigation and worker safety.