The taillights of a vehicle are dimly illuminated whenever the brake lights are on. Technician A says this could be caused by a blown fuse. Technician B says an open ground circuit could cause this problem.

Who is correct?

Answer: Technician B

Explanation: A taillights, and several kind of ilumation devices, works by converting a current flow trought a material in Light. This mean that the Current Flow (Amperes) which goes through a lights is proportional to the light it gives (Lumens).

This means, that if the tailights has a dimly light the current that goes through is less than the one it is suppose to be.

A fuse is a protection device for over-current, if the current at any given time goes beyond a limit (designed on the fuse) the fuse will melt and cutting all the current to the circuit. A blown fuse will cut all the current from the circuit and the tailights will be completely off.

However a open ground circuit is a differente kind of failure. In this cases, there is a pact from where the current on the circuit "escapes" from it. This could be by several reason, unprotected wires the most usual. While the current escapes from his intended course, not all of the energy goes away from the load. This explain why the tailight still has enough energy through it to light dimly.