Regarding a house sewer intercepting device (master trap) and its fresh-air inlet arrangement, identify the incorrect statement from typical public health engineering practice.

Introduction / Context:
The master trap (intercepting trap) isolates a building’s drainage system from the public sewer, minimizing entry of foul gases while allowing ventilation via a fresh-air inlet (F.A.I.) pipe. Understanding the components and correct features is important for sanitary design and maintenance.

Given Data / Assumptions:

• Master trap installed at the junction of house drain and street sewer.
• F.A.I. pipe arrangement with protective flap and adequate height.
• Comparison of water-seal depth with ordinary fixture traps.

Concept / Approach:
The intercepting trap includes a deeper water seal than ordinary traps (often around 7.5 cm or more), ensuring a reliable barrier against sewer gases. A cleaning eye enables rodding. The fresh-air inlet allows atmospheric air into the system; a flap that opens inwards only prevents outward flow of foul air while permitting inward ventilation. The F.A.I. termination is kept safely above ground (commonly 1.8 m or more; many texts cite approximately 2–3 m) to avoid nuisance and to admit clean air.

Step-by-Step Solution:
Check position: Between house drain and street sewer — correct.Check cleaning eye: Provided for maintenance — correct.Check F.A.I. height: A height of the order of a couple of meters (up to ~3 m) above ground is acceptable — statement is reasonable.Check flap action: Inward-opening flap admits fresh air and blocks exit of foul gases — correct.Water seal depth: The statement “less than ordinary traps” is wrong — the master trap has a deeper seal than ordinary traps.

Verification / Alternative check:
Standard sanitary engineering references specify deeper seals for intercepting traps to maintain gas barriers even under pressure fluctuations.

Why Other Options Are Wrong:

• Options A–D describe accepted arrangements and functions for intercepting traps and their F.A.I. assemblies.

Common Pitfalls:
Assuming all traps have the same seal depth; confusing the flow direction of the F.A.I. flap; placing the F.A.I. outlet too low causing odour issues.

Final Answer:
The water seal depth is less than that of ordinary traps.