EAM.6:5.2 - Shared failure and different recovery intervals
A separate constructed water system must continuously deliver 400 m³/h. Two units each supply a qualified 250 m³/h but share one feeder. An independent third source supplies 100 m³/h. Usable stored reserve is 300 m³, with delivery qualified for the deficits considered here. These are teaching inputs, not additional CityWater facts.
A feeder loss removes both 250 contributions. One returns after one hour and the other after two. During the first hour, only 100 m³/h remains, so preserving service requires (400 − 100) × 1 = 300 m³. The reserve is exhausted at one hour. During the next hour, 250 + 100 = 350 m³/h leaves another 50 m³/h deficit. The arrangement fails the continuous-service requirement despite normal capability of 600 m³/h.
Now consider a qualified additional 250 m³/h supply that can operate independently of this feeder loss and is usable after half an hour. Its support and connection are supplied as case premises.
| Interval after loss | Available contribution, m³/h | Required reserve rate, m³/h | Required reserve volume, m³ |
|---|---|---|---|
| 0–0.5 h | 100 | 300 | 150 |
| 0.5–1 h | 350 | 50 | 25 |
| 1–2 h | 600 | 0 | 0 |
Before the first feeder-dependent unit returns, required reserve is 150 + 25 = 175 m³. The initial 300 therefore covers the stated intervals, leaving 125 m³; no replenishment is assumed. From one hour, the available contributions meet the 400 requirement. The added supply is a feasible response to this constructed scenario, to be compared with other supported remedies and their burden.
If the added supply actually depends on the failed feeder, remove its contribution and the first conclusion returns: service fails after one hour. If it is independent but becomes usable only after 1.5 hours, the first hour exhausts storage and the next half-hour has a 50 m³/h deficit. Independence alone therefore does not establish timely service. Earlier availability, a further usable 25 m³ of reserve, a different restoration arrangement or another supported alternative could change that latter result.
Use actual dependency and restoration evidence before relying on such a calculation. The case establishes neither the probability of feeder loss nor the hydraulic feasibility of an unexamined network. EAM.7/.8 compare the supported remedy; EAM.11 must retain its half-hour availability and restoration conditions in the time arrangement.