Fault-Call Dispatch with a Critical Hospital Load
At 18:40 the 33/11 kV control room logs five calls within 12 minutes. (a) The 11 kV feeder breaker at the substation trips and does not reclose; 1,800 consumers are out. (b) A jumper on an 11 kV spur has snapped and the conductor is lying across a lane; 240 consumers are out. (c) An LT fuse has blown at a temple; six houses are dark. (d) A distribution transformer is leaking oil but is still energised; 90 consumers are supplied. (e) A dedicated 11 kV hospital feeder is showing low voltage; one critical consumer is affected. Two crews are available. Crew 1 has an 11 kV line permit and a bucket truck; Crew 2 has only LT tools and a ladder. Travel times from the control room are: substation 15 min, spur 35 min, temple 10 min, leaking transformer 25 min, hospital 20 min. Estimated repair times are: substation 40 min, spur 60 min, temple 15 min, transformer 30 min, hospital 25 min. The hospital has a 30-minute backup generator with 20 minutes of fuel left.
1. Rank the five calls and assign the first job of each crew. Justify using safety, criticality and consumer-minutes.
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Consumer-minute estimates: substation 1,800×40=72,000; spur 240×60=14,400; leaking transformer 90×30=2,700; temple 6×15=90; hospital 1×25=25. Pure consumer-minutes would put the substation first, but two calls override the arithmetic. The snapped 11 kV conductor across a lane is a public-safety hazard; it must be guarded and treated as live until tested. The hospital has only 20 minutes of generator fuel, so the control room must arrange fuel or a portable generator immediately-before any crew arrives-because the hospital is 20 minutes away and repair takes 25 minutes.
Crew 1 should go to the substation first: it has the 11 kV permit and bucket truck, restoration affects 1,800 consumers, and the trip may be caused by the spur fault. Crew 2 should go to the temple (10 min travel, 15 min repair), then to the leaking transformer to cordon and temporary-repair it. A line helper or police should be sent to guard the snapped conductor until Crew 1 can isolate the spur. Do not send Crew 1 to the hospital first; the fuel arrangement is the time-critical action there.
2. At 19:55 the substation breaker is restored, but the spur is still dead and the hospital generator has run out. Recalculate the position and decide the next two jobs.
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The substation outage has lasted 18:40–19:55 = 75 minutes, so 1,800×75 = 135,000 consumer-minutes have already been lost; the 40-minute repair estimate was optimistic. The hospital lost its generator at 19:00, so it has had no supply for 55 minutes-this is now the highest criticality call, regardless of its low consumer count.
Next jobs: Crew 2 should take a portable generator or fuel to the hospital immediately, while Crew 1 isolates and repairs the spur. The spur's 240 consumers have also been out for 75 minutes; repairing it saves 240×60 = 14,400 additional consumer-minutes, compared with 90×30 = 2,700 for the leaking transformer and 6×15 = 90 for the temple. If the hospital can be stabilized by Crew 2, Crew 1 goes to the spur, then the transformer. The lesson is that the original ranking by consumer-minutes alone would have left a critical customer without supply.