Light fixture still live when power turned off

Question

I'm sorry if this is an ignorant question, but it is one that I cannot seem to figure out. When I was changing a light bulb in my ceiling pot light fixture, the power switch was turned off, and I touched the base of the lighbulb (the metal part) as I was screwing it in. I got zapped. Just a short buzz, really.

I was always under the impression that no power would be supplied to a light fixture when the power switch is off. I use a multimeter, and surely enough, the base of the lightbulb is live even when the switch was off. I tried 2 other light fixtures in the house, one was similarly wired, whereas another was as I had originally expected, where there was no live portion of the socket when the wall switch was turned off.

Does this mean that the light fixtures in question are improperly wired up? Or am I not understanding the way the light switch works?

Thanks in advance for your help, and again sorry for the noob question.

manassehkatz-Moving 2 Codidact · Accepted Answer

Typically there are 3 connections involved for lights, receptacles and small appliances (some larger appliances like ovens, air conditioners, heaters, clothes dryers are different).

Hot
Neutral
Ground

Normally, hot is switched. Neutral and ground are not switched. Normally, that results in a safe environment where this is nothing "live" when hot is switched off.

This safety is based on neutral being the same as (or very close to) ground and the exposed metal of appliances, light fixtures, etc. being connected to ground. There are a number of things that need to be done correctly for this to be the case, including (but not necessarily limited to):

Neutral and ground connected together (bonded) in only one place (typically the main breaker panel).
Ground (and by virtue of the neutral/ground bonding, neutral as well) being connected to the actual ground via ground rods and/or connection to rebar and/or connection to metal cold water pipes.
Fixtures wired correctly so that the wires that should be hot are hot and the wires that should be neutral are neutral.
Ground wires used correctly so that there is really only one connection between neutral and ground.
Ground and neutral wires used correctly so that neutral is never piggy-backed onto ground or vice versa.

Anything that can go wrong, will. That's why when working on even something as simple as replacement of a fixture that is controlled by a switch, we don't rely on the switch to provide full protection but instead turn off the breaker or pull the fuse to make sure that the circuit is off and will stay off.

All that being said, with a properly wired house, changing a light bulb should be a very safe activity.

It sounds like you have a number of possible problems, which could include swapped wires (hot vs. neutral), neutral/ground not connected properly (too many places or none at all) or other problems.

In a case this extreme, I suggest getting a professional electrician to check things out.

One quick test you may want to do is to check various fixtures & receptacles with that multimeter. The values should be similar throughout the house - e.g., in the US that would be in the 110 - 120V range for most things. If you get wildly varying numbers then you likely have a very serious problem such as a "lost neutral" that would require immediate professional assistance.

Harper - Reinstate Monica · Answer

Ignoring the equipment safety ground wire, a lamp has 2 wires going to it: hot and neutral.   One those two wires is interrupted by the switch.

What makes a neutral a neutral?  It is intentionally wired so as to be close to the safety earthing in voltage.  Under normal operating conditions, it is not harmful.

Light circuits are also supposed to be wired so the switch severs the hot wire.

However, some switches (powered switches such as dimmers, lighted switches, motion sensors and smart switches) leak power through the switch at all times, expecting a ~200 ohm lamp to be at the other end.   These make the tip hot at all times.

Light bulbs are supposed to be wired so the tip is hot, and the shell is neutral.

So the third thing above, combined with either of the first two, will give you that surprise.

Also, many switches are wired in a switch loop.  That means that mechanically, power comes into the lamp first.  Then it runs down to the switch on a common /2 cable, going down in one wire as always-hot and returning in the other as switched-hot.  But one wire in the /2 cable is the normal color of neutral.   You're supposed to re-mark it with paint or tape to say it's being used for a hot, but most installers don't.  This can confuse people later.

Anyway, you have a double failure, in 2 or more of those places.

Steve Klein · Answer

I know because I was about to wire my lights yesterday to cause this, why someone might choose to set up the light to be hot when the switch is off.
I wired one light fixture into the switch, then realized I still had to wire the 2nd fixture into the same switch. So I had at least two options, but only one came to mind at first:

Upon realizing this would make the 2nd fixture always hot, I realized I should instead try this erroneous way:

This^ results in LED bulbs flickering a lot. It may look wrong but it's because one of the lights had only one 12/2 wire running out to it. Turned out running light fixtures in series doesn't work so well. Voltage drop or something. Changed to parallel and it was fine:

Light fixture still live when power turned off

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