NCV fails to still detect voltage once the circuit is loaded?

Question

The light above the cooktop in our kitchen started flickering occasionally, and eventually just was steadily very dim. I hated the fixture, so thought I'd replace it, and also just hoping the connection in the fixture was bad. But, beyond troubleshooting the flickering light, I have a few questions about the switch (S1) that powers the light…

1. In the associated metal switchbox (S1), there is only a 2-wire cable, a yellow and a black (I guess could be really faded blue???), and no ground in the metal device box. It does NOT appear as though the metal box is grounded. It's an old house (pre-1960) but has had several diy remodels, so a mixture of old/newer wiring I think… and I assume the yellow/black cable is probably original, since it's not the white/black/ground I'm used to seeing in the other areas of the house I've looked at. With everything separated, afaict, my volt pen was indicating the YELLOW was the live wire. What does yellow wire mean typically in older wiring? I always thought black was supposed to be the hot wire?
2. Here’s where I probably sound like a rookie… or maybe I don’t understand how my little pen-style voltage detector works… but after taking off the wallplate, with the light switch OFF, my pen lights up as soon as I get close to the box at all… I don’t even have to get close to the terminals on the switch. However, when I flip the switch ON, all the kitchen lights go on and the pen doesn’t light up AT ALL… even with direct contact to the terminals (see pics). I was not expecting this? Every other time I’ve used the pen, I kind of just thought it would light up near live wires, period. What stupid thing do I not understand?

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Answer 1

Yellow is hot and black is switched-hot at this location

That circuit, at the very least, and perhaps the rest of your house as well, is wired using metal conduit. This means two things:

1. Properly installed metal conduit is a highly effective grounding path, so your box likely is grounded even though there's no ground wire there
2. Electricians working in conduit have the rainbow at their disposal instead of being limited to the fixed color schemes of cabling (save for white and grey, which are reserved for neutrals, and green, which is saved for grounding wires). As a result, you will see multiple hot and/or switched-hot wires distinguished by color in conduit jobs, where they'd all otherwise be the same color (or retagged) in places using cable wiring methods.

As to your volt detector...

As to why your NCV does not detect voltage with the switch ON, it's because a NCV is designed to detect a static potential, basically, as a "quick check" for live parts. With current flowing, the NCV doesn't see anything because the path it completes to do that detection (a very high impedance path) is effectively shorted out by the load (obviously a low impedance path).

Answer 2

In old house wiring from that era, the wall switch interrupts the hot wire. So those 2 wires in the box are both hot, interrupted by the switch. No neutral or ground in there. Based on your measurements, it looks like the metal box is not grounded. Try connecting one end of your probe to a known grounded outlet.

Some houses were wired with Hot going directly to the light fixture, then to the switch to interrupt it. That's a really bad design because the hot wire is always live in the light fixture, whether or not the wall switch is open/closed. Be careful!