Existing circuit working fine until suddenly the lights on that circuit turn off. Circuit tester showing hot and ground reversed in an outlet on that same circuit. Then a while later the lights are back on and testing shows all is proper. Breaker is not tripped. Loose wire somewhere? What’s wrong...

 

Those plug in testers can be fooled by an open neutral. That is most likely what is happening there.

The typical problem is wiring connected to the push-in terminals on receptacles gets loose and causes intermittent problems. You need to identify everything on the affected circuit. Then determine what is going off when the problem occurs. The problem can be at the last working location in the circuit or the first dead location.

 

This.

It can be a loose backstab in a receptacle upstream, or even a loose wire inside a wirenut that LOOKS like it's tight. Could be a loose terminal screw in the panel. Especially check anything that may have been added between that room and the panel since construction. Often I'll find it in a ceiling fan that someone installed using the handiest live cable they found in the attic.

 

Imagine some one has rewired a lamp and reverse the polarity of the lamps wiring, every time you turn the lamp on the ground wire will be hot and not hot if the light is out. How old is this home? if 1950 or earlier built the circuits used only 2 wires not 3, there were no grounds only a hot leg and a neutral, often times bootleg grounds were made when the 3 hole outlets replaced 2 hole outlets and a bootleg ground can be very deadly and a plug in analyzer will never detect thsi condition known as RPBG or Reversed Polarity Bootleg Ground. In this condition the ground and neutral terminals at the outlet are jumpered making the plug in circuit analyzer think a ground wire exists when in fact its just neutral jumped to the ground terminal to fool an inspector whom is too lazy to open the outlet and see two wires not 3 and the neutral terminal jumped to the ground terminal, they can operate this way for years however imagine a plumber is called out and must go under the home and plugs in his trouble light with a metal shield for the light bulb, he crawls on the dirt usually damp dirt and accidentally touches the trouble lights metal shield and is electrocuted dead under the house! In older homes I found you can never trust the ground wire or terminal to reference with a voltmeter to read other wires, I go outdoors and hammer a coper spike into the earth outdoors and with a long wire attached to the copper sike know my ground is in fact a real ground, then the readings you get will make sense and identify the real problem, imagine you read neutral to ground and think is ground cause no voltage was read but in reality both ground and neutral are hot and with the same hot leg so putting a voltmeters probe on the same side of power(the hot leg) the meter read nothing and you think cool and never know both ground and neutral are hot, Google RPBG or reversed polarity bootleg ground, not many techs know this even contractors I have seen had no idea what this was .

 

Loose wire indeed. Holy smokes we could have had a serious problem. This from the receptacle in the corner nobody pays attention to. It melted the insulation on the wire and melted the casing on the back of the receptacle.

 

Was it connected using the push-in connections ?
There may be more there that need checking out.
It's advisable to move all push-in connectins to the screw terminals

 

I don't suppose the wiring could be aluminum?

 

Copper Wiring. Solid Core.

Wire was terminated onto screw type connector in back of receptacle. It just wasn't fully curled as in the picture and worked its way loose. It was barely touching and was likely arcing at times. Hence the heat...

 

BTW is this something UL would want to see? They did certify the product. I am not sure if this degree of heat damage meets what criteria they would have?

 

UL has tested the device to be safe based on expected connection parameters.
A loose screw would remove the fault from the manufacturer and transfer it to the installer.