Summer Lab Shutdowns on a Shared Circuit
A 16-PC computer lab has one 16 A, 230 V wall circuit. Each PC has a 450 W PSU and a 22-inch monitor; measured draw is 180 W per PC and 25 W per monitor under load. Eight PCs and their monitors are connected to UPS-1 and the other eight to UPS-2; each UPS is rated 1.5 kVA at 0.8 power factor. A 1.5-ton air conditioner and a 300 W laser printer are also on the same wall circuit. Between 11:00 and 15:00, PCs restart randomly, both UPS units beep, and the breaker trips when the printer and a vacuum cleaner run together. Room temperature reaches 39 °C and dust is visible on intake filters. The lab assistant proposes replacing the 16 A breaker with a 32 A breaker and cleaning the PCs with compressed air while they are switched on.
1. Calculate the load on the wall circuit and on each UPS. Identify at least two separate causes of the shutdowns and explain why replacing the breaker is unsafe.
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Wall circuit capacity is 16 A × 230 V = 3680 W, and a continuous load should stay near 80% of that, about 2944 W. The PCs and monitors draw 16 × (180 + 25) = 3280 W. Adding the 300 W printer gives 3580 W, already close to the breaker rating before the air conditioner. A 1.5-ton AC typically adds roughly 1500–2000 W, so the circuit is clearly overloaded. Each UPS can deliver 1.5 kVA × 0.8 = 1200 W, but each carries 8 × 205 = 1640 W, so both UPS units are also overloaded.
There are therefore two independent faults: UPS overload and wall-circuit overload, worsened by AC start-up surge and by the vacuum cleaner. Replacing the 16 A breaker with a 32 A breaker is unsafe because the wiring, sockets and switchgear are still rated for 16 A. The breaker would no longer protect the cable; overheating, melted insulation or fire could occur before the breaker trips. The correct response is to reduce and redistribute load, not to remove protection.
2. Give a safe immediate action sequence for the next hot afternoon and a revised preventive-maintenance plan covering cleaning, UPS checks and documentation.
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Immediate sequence: notify the lab in-charge; stop nonessential loads such as the printer and vacuum cleaner; shut down PCs in an orderly way if the UPS alarms continue; do not bypass or uprate the breaker. Move the AC to a separate circuit only if a qualified electrician confirms it. Reduce the number of PCs per UPS to four so each UPS carries about 820 W, well below 1200 W. Open the room early, use fans only on circuits that are not overloaded, and record UPS load displays and room temperature before students arrive.
Preventive plan: schedule monthly filter cleaning and quarterly internal cleaning with the PC powered off and unplugged, using ESD precautions and compressed air outdoors with eye and respiratory protection. Never clean a running PC. Test each UPS quarterly with a controlled load and replace batteries that fail runtime tests. Log room temperature, UPS load, breaker trips, cleaning dates and battery replacement dates. Add a rule that high-power appliances are not connected to the lab circuit, and review the electrical load before adding any new device.
3. The lab has no budget for a new circuit before exams. Propose a temporary configuration that keeps as many PCs usable as possible for a two-hour exam, stating assumptions and residual risk.
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With no new circuit, the only defensible plan is to reduce simultaneous load and separate the largest appliance. Assumption: the AC can be moved to a different existing circuit that is verified by an electrician; if it cannot, the AC must remain off during the exam. With the AC on a separate circuit, run 12–14 PCs, printer off, vacuum off, and split the PCs evenly between the two UPS units so neither exceeds 1200 W: 6 PCs per UPS is about 1230 W including monitors, so 5 per UPS is safer, or 6 only for a short exam with monitored load. If the AC stays on the lab circuit, limit the lab to 8 PCs with the AC running only if the measured total stays below about 2944 W; otherwise use a second room on another circuit. Seat students in two shifts if needed. Residual risk: nuisance breaker trips, heat stress and UPS battery depletion. Keep a named person at the breaker, a phone, and a plan to save work every 15 minutes.