One Fuse, Seven Amps and an Earth Pin on the Neutral
The Sharma family in Jalandhar has one 5 A lighting circuit fed through a 5 A fuse at the distribution board. Connected to it are three LED bulbs of 9 W, two ceiling fans of 70 W, a television of 80 W, and a new 1.5 kW room heater plugged into the same circuit through a 5 A socket and a multi-plug adaptor. The supply is 230 V. On the first cold night the fuse wire melted. The electrician's advice is to replace the 5 A fuse wire with 15 A wire, so that it will not trouble the family again. The heater's flexible cable has three cores and the socket's earth pin is connected, but on opening the box the earth terminal was found joined to the neutral wire. The inverter in the same room is rated 600 VA and hums audibly; its transformer is warm even when nothing is running on it. During a power cut the family wonders whether the heater can be run from it.
1. Calculate the circuit current with and without the heater, and explain the fuse failure.
Read the worked answer
Without the heater: 3 × 9 W = 27 W of lighting, 2 × 70 W = 140 W of fans, 80 W television, total 247 W. At 230 V, I = 247/230 ≈ 1.07 A.
The heater alone draws 1500/230 ≈ 6.52 A. With everything on, the total is 1747 W, giving 1747/230 ≈ 7.6 A.
A 5 A fuse is designed to melt when the current stays well above 5 A; at about 1.5 times its rating the wire heats and opens within a short time. So the fuse behaved exactly as intended - it opened before the 5 A socket, the adaptor and the 1.0–1.5 mm² lighting cable could overheat. The fault is not the fuse but the load placed on a circuit that was never designed for a 1.5 kW appliance. Any reading of the event that treats the fuse as a nuisance gets the safety logic backwards: the fuse is the deliberate weak link that protects the wiring.
2. Explain why the electrician's 15 A fuse advice is dangerous, and state the correct fix.
Read the worked answer
A fuse protects the smallest conductor in the circuit, not the appliance. The lighting cable is thin - around 1.0 to 1.5 mm² - and the socket and adaptor are 5 A rated. If the fuse is raised to 15 A, a 7.6 A overload no longer blows anything: the current flows happily through wiring, the multi-plug and the socket contacts, which heat up at joints and inside the conduit. The fuse would only act at currents the cable can never survive, so the failure mode changes from a blown fuse to scorched insulation or a fire inside the wall.
The correct fix is to keep the 5 A fuse on the lighting circuit and give the heater its own circuit: 2.5 mm² copper cable, a 15/16 A socket and a 16 A MCB at the board, with no multi-plug adaptor. An MCB also trips quickly on overload and can be reset, but its rating must match the cable it protects, not the annoyance it causes.
3. Judge two things: the earth terminal joined to the neutral, and the idea of running the heater from the 600 VA inverter. What does the humming and warmth tell you?
Read the worked answer
Joining earth to neutral is dangerous. The neutral carries current in normal operation, and it can rise above earth potential or be broken upstream at the meter or at a loose joint. If the neutral opens, the metal body of the heater is connected to the live side through the heating element and sits at full 230 V; anyone touching the body and a tap or floor completes the circuit. If the socket polarity is reversed, the body is live immediately. The practice can also mask faults from an RCCB. The earth pin must go to a separate earth electrode of low resistance, and that resistance should be measured.
The 1.5 kW heater cannot run on a 600 VA inverter. The load is 1500 W against a 600 VA rating - about 2.5 times over - and the start-up surge is worse, so the inverter will trip on overload or its output devices will fail. A heater is a poor inverter load even at matched rating, because it drains the battery fast. The audible hum is normal: the transformer core laminations change length slightly twice per cycle, giving a 100 Hz buzz, and the warmth with no load is ordinary iron loss plus magnetising current. A suddenly louder, hotter transformer points to loose laminations or a failing winding, not to a load that is not connected.