The fuse blows as soon as the cigarette lighter is pushed in. The problem is likely a:

• A. bad heating element, reducing the resistance inside the lighter.
• B. short to ground in the lighter socket.
• C. fuse with too high a rating.
• D. improper wiring of the lighter.

Answer: (A)

When a fuse protects a circuit, it opens the path if the current becomes higher than what the fuse is designed to handle. A cigarette lighter is just a load that draws current to heat a element. The amount of current it draws depends on how much resistance the heating element presents: I = V/R. If the heating element develops a fault that lowers its resistance, significantly more current will flow than the circuit is designed to carry. That surge causes the fuse to heat up and blow immediately as soon as the lighter is plugged in.

Other faults could also cause strong current, like a short to ground, but the symptom described points to the lighter itself having a lower-than-normal resistance. If the heating element becomes highly resistant or open, the lighter wouldn’t draw current and the fuse wouldn’t blow.