Yes, techroomt is correct. Which is why I mentioned using only one lamp above. I thought I covered this earlier, but still had my mind wound up on leakage current.

I once had a car that, at one point, had trouble cranking over. The engine ran fine when started, the battery was was fine and all its cable connections were checked and looked good. It was quite a puzzler at the time. Must be a bad/weak starter, right (cheesy starters are so notorious)?

Nope. It turned out that the negative/black cable going to the motor/chassis ground was about 5-6 feet long (somewhat unusual), and had become corroded internally. With too much cable resistance, the battery could not supply the needed current to the starter.

But under load, the battery voltage would no longer appear at the starter due to the resistance in the cable. As i said before,

If the lamp is good, and has 12V across it, it must light.

Maybe you are talking about the case where the lamp is missing or defective?

 

If resistance increases wouldn't you expect lower voltage as read by a digital multimeter?

 

Now I'm seeing circles.

Perhaps incorrectly, I took techroomt's statement to mean from the supply side. In my car example, the battery would measure fine even when trying to power the starter, but the starter wouldn't work (or crank very good). And if the all the connections are good, what remains as bad? Took awhile to think it was the cable itself.

So you could measure 12V at the supply with a good lamp installed, but the lamp doesn't light.

Saying that you are measuring 12V across the lamp itself, and a known good lamp doesn't light, is nonsense (as I earlier implied).

Again, if they had unplugged all the "bad" lamps except one, they may have notice that one would light to some extent. One or two strands of wire would probably be able to supply sufficient current to one lamp. The voltage drop measured across the lamp would indicate a voltage droop along the line.

 

Saying that you are measuring 12V across the lamp itself, and a known good lamp doesn't light, is nonsense (as I earlier implied).

Perhaps the word "lamp" is used when "socket" would be better. Measuring across a bulb base can be challenging. Sockets do indeed go bad.

 

Ok guys, thanks for the comments, but I think I'm driving you crazy. Let's leave it at "the contractor gave bad info - impossible to diagnose!"

Thanks anyway and I apologize for the improbable conundrum.

 

Thanks anyway and I apologize for the improbable conundrum.

Come on Steve! Even though I wasnt contributing, yet I learned quite a bit merely by going through the discussion. I'd rather say, Thank you for initiating this discussion!
Thats the beauty of forums, specially well moderated ones