Some years ago my then-girlfriend smelled something burning in her house. We finally tracked it down to a bad electrical connection up in a ceiling fixture. A bit scary, though. Poor connections have a higher resistance, and when you ask for lots of current through them, they can heat up, and in some cases cause a fire.
So how can you find these bad connections? Just measuring the voltage at outlets won’t tell you anything. You need a special tester that first measures the voltage at an outlet, then places a brief but heavy load (typically 12, 15, or 20 amps) and measures the new voltage. The amount by which the voltage drops tells you if you have a problem. Anything over 5-6% drop is not good. By testing outlets sequentially on a single circuit, you can generally figure out where any big drop is.
A lot of house inspectors and even electricians don’t seem to know about testing for voltage drop under load, but once you start doing so, you won’t want to go back. And yes, I’ve found one or two instances where an outlet that otherwise looked fine had an unacceptable voltage drop.
— Dale Grover
Ideal Suretest 61-164
Available from Telephone Stuff
(Some cheaper models are available; see manufacturer’s site below.)
Manufactured by Ideal Industries