In the last post Electrical Light Switch Installation Could Have Been Deadly – Part 1 the lamp glowed even after the light switch was turned off. The question was why did the light glow whenever it rained. Here’s the answer:
First a little about neutral wires. The neutral wire circuit is normally white and tied to the building ground (or separately installed grounding rods) near the incoming electrical service box. The ground rod is often connected to the neutral bus bar in the circuit panel (depending on the local authority having jurisdiction or, AHJ). So under ideal conditions, the neutral conductor is at or very near ground voltage – basically at 0 volts, though it could be higher. So when the hot or line circuit is energized through an electrical device – the flood light fixture in this case – any path to ground after the light would cause it to turn on.
Since the light fixture was always energized, ANY path to ground on the neutral side of the bulb being grounded would cause the flow of electricity through the bulb. In my friend’s case, there was a neutral wire exposed in a metal lamp post nearby on the same circuit. The lamp post was buried in the ground about a foot deep. When the weather was wet, electricity flowed through the neutral to the grounded metal lamp post. Since the resistance to ground was high when conditions were dry, the light would stay off, but a hazardous voltage condition existed on the lamp post where it was energized to 120 volts because of the open neutral at the switch. When it rained, the resistance to earth ground would drop and electricity would flow through the flood light fixture through the neutral wire to the metal lamp post to ground. This was because the lamp and it’s neutral wire were always energized even when the switch was off. The electricity simply found another path to ground.
Hope this makes sense to you, but remember – if you don’t understand electricity and local electrical code requirements, call a licensed electrician for any electrical work. It could save your life and the lives of others.