Final answer:
The statement is true; ballcocks should be installed to have the critical level above the overflow pipe opening, preventing back-siphonage and ensuring safe plumbing systems.
Step-by-step explanation:
True, ballcocks in water closet tanks should be installed such that the critical level of the water inlet is above the full opening of the overflow pipe. This is to prevent potential contamination of the potable water supply due to back-siphonage. Back-siphonage can occur if the critical level is at the same height as or below the overflow pipe, as negative pressure in the water supply line could draw contaminated water back into the potable water system. Ensuring that the critical level is adequately above the overflow pipe is therefore a critical aspect of the safe design and installation of plumbing systems.
To prevent the tank from overfilling, the critical level of the water inlet should always be positioned higher than the full opening of the overflow pipe. This ensures that when the water reaches the critical level, it will not overflow and cause damage.
For example, if the ballcock is installed in a way that the critical level is below the full opening of the overflow pipe, water will continue to flow beyond the desired level and overflow through the overflow pipe, wasting water and potentially causing water damage.