When must sound rapid strokes on the bell for 5 seconds at intervals not less than one minute in a zone of restricted visibility?

Sound rapid strokes on the bell for 5 seconds at intervals not less than one minute must be made by vessels in a zone of restricted visibility when they are at anchor. This signal is intended to warn other vessels of their presence in low visibility conditions, ensuring safety and preventing collisions. The practice is part of the International Regulations for Preventing Collisions at Sea (COLREGs).