Where instrument air systems are provided with a standby header, the header shall have how many cylinders attached?

The requirement for a standby header in instrument air systems to have a sufficient amount of cylinders for one hour of normal operation is critical for ensuring that the system can maintain functionality during potential outages or maintenance activities. This provision aligns with safety and operational standards that aim to prevent disruptions in service, especially in environments where instrument air is essential for various tools and processes.

Having enough cylinders for one hour of normal operation ensures that there is adequate reserve capacity to handle peak demands or unexpected downtime without compromising the performance of the system. This safeguard is particularly important in medical gas installations where consistent pressure and flow of gas are vital for patient safety and equipment efficacy.

Other options such as an average day's supply or a specific number of cylinders do not guarantee timely and sufficient backup when immediate operational continuity is essential. Therefore, the emphasis on sustaining at least an hour’s worth of operating capacity is a well-founded strategy in maintaining reliable instrument air supply systems.