I have an old circuit which has absolutely nothing but 1 light fixture with the pull string attached to it (so you pull light goes on, pull again, goes off). With the neutral disconnected from the bus bar. The light continues to work. How is this possible?

 

The neutral you disconnected at the bus bar might not be for that circuit. Do you have a multi meter or electric tester to check the wires at the fixture. You could check to see if the neutral or ground wire is completing the circuit.

 

If you are 100% sure that the neutral you disconnected is for the same circuit your light is on, it is possible something is wired wrong.
There may be a borrowed neutral from different circuit (which is wrong)

It may be part of multi wire branch circuit (MWBC). Is it on 2 pole breaker? Never turn this circuit on with neutral disconnected because it will introduce over 120V on some circuit and can fry what ever is on the circuit.

You may have neutral shorted to ground somewhere.

 

Well, an update. I confirmed today with my DMM on continuity (and resistance) mode that the neutral does belong to the same circuit. Furthermore, because I changed every single breaker in my panel I know for a fact that neutral is in that circuit. And yes, with neutral disconnected, the light is on.

Basically this entire circuit can be thought of as a spare circuit. It was connected to something probably and they terminated it and wired up that light fixture. The light fixture is 2 feet away from the breaker box. So imagine there is a circuit breaker -> 3 feet of NMB -> light fixture. It's pretty simple. There is absolutely nothing else to it.

The light fixture has one of those pull strings so it has a switch built in. Can that explain why? I mean I'm just quite puzzled how this is possible and how there can be no explanation. I opened up the light fixture and I see black, white, and ground cables connect to the light fixture properly.

 

Possible the ground wire was connected to the neutral/shell of the lamp fixture. Not where you want it, obviously.

Inspect the back of the lamp fixture.

 

I did. In fact, I rewired everything to make sure Maybe the fixture is defective?

 

Does the bulb burn brighter or dimmer with the neutral disconnected?

It's possible you have a Multi-Wire Branch Circuit where two hots from different phases share a neutral.
If this is the case, when you disconnect the neutral at the panel, you create a 240 volt circuit (Hot leg to Hot leg).

If your breaker has two "trip handles" (above-and-below or side-by-side), do not disconnect the neutral before disconnecting both hots.

 

No sir its now MWBC. All other breakers were off anyways.

 

Post a picture. It is getting a return path from somewhere.

 

Is it an LED bulb or a standard incandescent? LED bulbs can light up from very small currents even when their hot line is off. This can happen sometimes when there is an electronic dimmer in the circuit and the LED sees a path through the electronics. I think you have a separate thread about what may be a surge protector on your main panel. That could possibly be a path for a very small current that might light an LED.

 

Just a guess but perhaps the light fixture is grounding out to the box it is mounted to and completing the circuit through ground. If not mounted to a box then grounding through the fixture to the ground wire in the NM.

skeeter