How deep is the trap system designed to maintain its seal?

The trap system is designed to maintain its seal at a depth of 38 mm (1-1/2 in) primarily because this specific measurement is effective in preventing the passage of gases while still allowing liquid to flow. The depth of 38 mm strikes a balance between providing an adequate seal and not requiring excessive space, which could be problematic in terms of system design and space limitations. A trap with this depth effectively captures and retains enough liquid to form a barrier against sewer gases, while also accommodating normal fluctuations in liquid levels due to usage or system dynamics.

In contrast, other depths either do not provide an adequate seal or could lead to operational issues in a system. Shallower depths might not hold enough liquid for an effective seal, while significantly deeper traps could complicate maintenance or lead to blockages within the trap system. Therefore, the choice of 38 mm is optimal for ensuring proper functioning and reliability in maintaining a gas-tight seal in the trap system.