Barrack Electrical Load and the Fuse Rating
An old barrack is wired to a 230 V supply protected by a 15 A fuse. The connected load is: 10 LED bulbs of 9 W each, 4 ceiling fans of 70 W each, one television of 100 W, two mobile chargers of 10 W each, one water heater of 1,500 W, and one clothes iron of 1,000 W. The heater and iron are never used at the same time as each other, but each may be used with the lights, fans, TV and chargers. The warden now wants to add a 1 HP (746 W) water pump for the barracks. Assume a power factor of 1 and negligible start-up surge. The electrician says a 15 A fuse will hold because the pump is 'small'. The store has 20 A and 25 A fuses, and the wiring is rated for 18 A.
1. Compute the normal load current without the heater and iron, then with the heater, then with the iron. Is the 15 A fuse adequate for these normal conditions?
Read the worked answer
Base load = (10 × 9) + (4 × 70) + 100 + (2 × 10) = 90 + 280 + 100 + 20 = 490 W.
Without heater or iron: I = 490 ÷ 230 = 2.13 A. With heater: (490 + 1,500) = 1,990 W; I = 1,990 ÷ 230 = 8.65 A. With iron: (490 + 1,000) = 1,490 W; I = 1,490 ÷ 230 = 6.48 A.
All three values are well below 15 A, so the fuse is adequate for normal use. The heater and iron are not simultaneous, so adding them together would be an invalid test unless the interlock fails.
2. The warden wants the pump, heater and all base loads running at once, with the iron off. Compute the current and decide whether the 15 A fuse is safe. What is the least-risk fix if the heater and iron are later used together with the pump?
Read the worked answer
Pump + heater + base = 490 + 1,500 + 746 = 2,736 W. I = 2,736 ÷ 230 = 11.90 A. This is below 15 A, so the fuse should hold if start-up surge is negligible. The electrician's claim is correct only for this combination.
If the heater, iron and pump run together: 490 + 1,500 + 1,000 + 746 = 3,736 W; I = 3,736 ÷ 230 = 16.24 A. The 15 A fuse would blow. The wiring is rated 18 A, so this current is still below the cable limit, but the fuse is doing its job.
Least-risk fix: keep the heater and iron on a mutually exclusive switch or interlock, put the pump on a separate circuit, or enforce a load schedule. Do not fit a 20 A or 25 A fuse just to stop blowing: those ratings are above or too close to the 18 A wiring limit and reduce protection.
3. A clerk replaces the 15 A fuse with a 25 A fuse to stop nuisance blowing. Explain the hazard and the principle the clerk has misunderstood.
Read the worked answer
The wiring is rated 18 A. A 25 A fuse will carry up to 25 A before opening, so a fault or an added load between 18 A and 25 A can overheat the cable without blowing the fuse. The fuse protects the wiring, not the appliance and not the convenience of the user.
The clerk has treated the fuse as a nuisance trip rather than as the weakest designed link. The correct response is to reduce the load, improve the circuit, or install cable and a protective device with a higher coordinated rating-not to uprate only the fuse.