What is the primary measure to ensure safety when testing boiler safety valves?

The primary measure to ensure safety when testing boiler safety valves is to maintain a minimum pressure. Safety valves are designed to open at a predetermined pressure to release excess pressure from the boiler system, preventing dangerous situations such as overpressure conditions that could lead to catastrophic failures. By maintaining a minimum pressure, operators can ensure that there is sufficient pressure for the safety valves to operate correctly and to verify that they will relieve pressure as intended without causing damage to the system or creating unsafe conditions.

The testing of safety valves is critical to ensure their reliability, as they play a vital role in system safety. If the pressure is too low, the safety valve may not function correctly, failing to relieve pressure when needed. Thus, keeping a minimum pressure ensures that the system is operating within a safe range while still allowing for effective testing of the safety equipment.

This principle emphasizes the importance of maintaining the appropriate pressure conditions within the boiler system during any testing procedures, making option C the correct choice.