Regarding a house sewer intercepting device (master trap) and its fresh-air inlet arrangement, identify the incorrect statement from typical public health engineering practice.
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AIt is provided between the lower end of the house drain and the street sewer (to intercept sewer gases).
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BA cleaning eye is provided at the top of the trap for maintenance access.
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CThe fresh-air inlet (F.A.I.) pipe fixed vertically along the wall is generally kept about 3 m above ground level.
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DA mica flap valve that opens inwards only is fitted at the top of the fresh-air inlet pipe.
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EThe water seal depth is less than that of ordinary traps.
Answer
Correct Answer: The water seal depth is less than that of ordinary traps.
Explanation
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.