Lost power in my guest room? What happened?

Factual Questions
1

Not sure if this is more appropriate for FQ or IMHO, so I’ll start here and see where it leads.

Last week I finally removed the window unit air conditioners from both of my guest rooms. After carrying them out to the garage, I noticed our Internet was down. The modem/router is in one of those rooms so naturally I assumed I blew a circuit and went out to the garage to reset it. But there were no breakers tripped. I checked all the GFI I could find ( none in that room, but I know with renovations, especially DIY renovations, wiring gets…creative). No luck.

Anyway, called my daughters boyfriend (electricians apprentice), and he came over with a voltmeter ( I think that’s the name of it), plugged it in to the outlet that the AC had been plugged into. Top and bottom read 30 volts (should have been 120). So do all of the other outlets in that room. So we put in a new outlet (after identifying and turning off the breaker). When we flip the breaker back on, nothing has changed.

We decide to check out the other room. The overhead light/ceiling fan is still on in this room, so I never thought to check the outlets. When we do ( starting with the one the AC was plugged into, we find 30 v in the top outlet but 120v on the bottom. So we replace that outlet as well. When I flip the breaker off this time, the light and ceiling fan go off too. We change the outlet, restore power, and problem solved ( in both rooms).

So it seems to me pretty clear that there was a problem in that outlet (in Guestroom #2), and that it affected all outlets on the circuit that were “downstream” ( surely not the right word). The light/ fan was presumably “upstream” from the fault and was thus not affected. Both rooms appear to be on the same circuit breaker.

But here are my questions: (may or may not factual answers vs conjecture, thus I would not object to moving to IMHO if moderators see fit)

  1. if there was a fault of some kind, why didn’t the circuit breaker trip?

  2. what kind of problem would cause PARTIAL voltage to be measured, especially in the same outlet?

  3. Both AC units were working when we last used them about a month or so ago (as was the internet) so whatever fault occurred was clearly caused by unplugging them, unless it was a staggering coincidence.

  4. Did not notice any arcing when I unplugged the AC, not did I see any scorching or signs of arcing in the outlet. All wires appeared to be secure and tight.

Not sure if that’s enough info to diagnose, if you have questions let me know, I may or may not have other relevant info. As the problem is fixed, I’m not terribly concerned about it, just curious.

2

I had a similar problem with a ceiling fan wall switch producing very low voltage. Prior to the problem, I saw that the switch was installed upside down, so I removed it and rotated 180 degrees. That’s when the problem began. The issue was a loose wire behind the switch. It seems that low voltage indicates something is not making a full connection between your main panel feed and the problem circuit. Another area to investigate in the circuit breaker itself. Did you reset the breaker? You didn’t explain how the problem was resolved.

3

The breaker never tripped, but I did turn it off and back on while we replaced the outlet. Replacing the outlet fixed the problem.

4

To correct an apparent misunderstanding by the OP:

Circuit breakers do not trip for any and all circuit faults. Ordinary breakers trip only in response to a significant overcurrent condition. Which you’re not having.

There are more modern “arc fault” breakers (AFCBs) which attempt to also detect and trip for lousy intermittent connections hidden somewhere in the walls or in attached devices. But these are expensive and prone to false alarms which result in nuisance trips with no real fault.

It’s possible, though not certain, that if you’d had that circuit on an arc fault breaker it’d have been tripping routinely as the defective connection deteriorated. OTOH, AFCBs also tend to work poorly on any circuit powering an electric motors. Such as fans and HVAC units.

5

As I understand though, a loose connection can generate heat at the poor junction, since essentially it’s a resistance. It can be an eventual source of fire, depending on the load and what’s around it. At the least you’d possibly end up with a scorched outlet.

(This was a risk in the good old days when builders first started using aluminum wiring - it deforms easier than copper, so the risk is it would deform in a connection to make the connection screw less firm.)

6

A high-resistance (and thus faulty) connection anywhere in the circuit will result in a weird voltage reading on a modern DMM. We call these “ghost voltages,” and it is due to the DMM’s very high input impedance. To eliminate ghost voltages, you must use a DMM that has a LoZ option. In your case, a DMM w/ LoZ would have displayed a voltage reading close to 0 V instead of displaying 30 V. A reading of 0 V makes more sense than 30 V (or 20 V, or 60 V) when troubleshooting power circuits.

7

Yea, a poor connection can result in a glowing contact or arcing, both of which are bad. And a standard circuit breaker couldn’t care less.

8

Thank you all for the responses. I didn’t realize that there were some faults that a circuit breaker could not detect.

9

This is raising a red flag. Do you still have the outlet that you replaced? If so, check on the sides of the outlet to see if the metal tabs that connect the screws for the top outlet to the screws for the bottom outlet are intact:

If they are both intact then ignore my post. If the silver one is intact but the gold one is broken then half the outlet was likely controlled by a wall switch and if you didn’t break the tab on the new outlet you have bypassed the wall switch.

If both tabs are broken that indicates that the top and bottom outlets were likely on different circuits and if you replaced the outlet without breaking both the tabs (as well as making sure you replaced the wires exactly they were on the original outlet) you may have connected two circuits together - potentially a very dangerous situation.

10

Overall it sounds like a bad neutral. If that 30 volts was measured with a digital voltmeter it is probably just reading some floating voltage. Those meters are very sensitive. An old analog meter might give a better clue. I’m surprised your electrician didn’t use one of those inexpensive outlet testers. They display any problems.

https://www.homedepot.com/p/Power-Gear-3-Wire-Receptacle-Tester-50542/206212329?source=shoppingads&locale=en-US

11

As a very concrete example - I had AT&T at the house a few weeks ago installing a new fiber connection. When they drilled through the exterior wall, they nailed an electric line. A little smoke, smell of burnt stuff, etc. The one downstream outlet stopped working. The circuit did not flip, though.

(I turned it off myself, and had an electrician out the next morning to properly cap the wires until he could get a proper patch kit to reconnect things.)