Help from the engineers

[donheff]

I woke up this morning to a total power outage, which is unusual in DC because the lines are underground. Turns out some underground collapse messed up some cables. The power has been restored but only to half the house. This happened once before and was explained away as "a phase thing." The guys outside confirm that and explain that as they test lines we may see thing go on and off in sections of our houses. Sure enough, the furnace, which came back on when the first part of the house was restored, has now gone off.

Can anyone explain in layman's terms how the heck we can get full power on some circuits but none on others? And why would it jump around?

 

[frayne]

What you're describing involves the way power is distributed to your house, which is typically on a split-phase system. Here’s a simplified explanation:

1. Split-Phase Power Basics:
   Most homes in the U.S. receive electricity through a split-phase system. This means the power coming from the grid is divided into two "legs" (or phases) of 120 volts each, with a neutral line in between. These two legs can combine to provide 240 volts for larger appliances like furnaces, dryers, or ovens, while most outlets and lights run on just one of the 120-volt legs.
2. The "Phase Thing":
   If one of these legs is damaged or not fully connected (like from the underground collapse you mentioned), only the circuits tied to the working leg will have power. This is why parts of your house are lit up while others remain dark.
3. Why It Jumps Around:
   When the utility workers test and restore lines, they’re reconnecting parts of the system and may temporarily switch which circuits are getting power. For example:
   • Initially, they restored one leg, and your furnace, connected to that leg, came on.
   • Later, when they adjusted the connections to test the other leg, the power supply shifted, causing some devices (like your furnace) to lose power while others gained it.
4. Why Large Appliances Are Affected:
   Larger appliances often require both legs to provide 240 volts. If one leg is out, these appliances won't work, even if one leg has power.

Until the utility fully restores and stabilizes both legs, you’ll likely see this inconsistent behavior. It’s frustrating but normal during complex repair work.

 

[donheff]

Thanks Frayne. That confirms my basic guesses. I suspect that I missed something since it certainly seemed that the furnace had been on while one leg was off but then switched off while the rest of the on leg was still on and the rest of the of leg was still off. In other words, it seemed like the only thing that switched off was the furnace and not a whole leg. That doesn't make sense to me so maybe I missed something. At any rate, everything is on now. Unfortunately, there are still a bunch of people and trucks working the problem and they are saying they think it was a small gas explosion underground. So, we may have more messing around to come later in the day.

 

[frayne]

You're welcome! Full disclosure: I'm not an electrical engineer by training, but I did stay at a Holiday Inn Express one time or two. I also queried your question to the all-knowing (but fallible) ChatGPT AI bot.

 

[marinauser]

We had this happen several years ago. The two power lines feed into our house underground under the house and up into the circuit breaker box. We woke up one day with no power on two of our four floors. Then the AC for the top two floors kicked on and suddenly we had power to all 4 floors (townhouse). AC kicked off and we lost power again two 2 floors. After digging and finding where cables fed under house and testing wires, we could tell that there was an issue underground. Through much working and using a come along pulling on the underground cables, they were recovered and found to have shorted together losing one of the two 110 feeds. Apparently somehow when the AC kicked on it was back-feeding power and powering the other two floors. This was not a good situation and we had to shut down that AC unit until all of the repairs were completed over the course of a couple of days.

 

[38Chevy454]

Split-Phase Power Basics:
Most homes in the U.S. receive electricity through a split-phase system. This means the power coming from the grid is divided into two "legs" (or phases) of 120 volts each, with a neutral line in between. These two legs can combine to provide 240 volts for larger appliances like furnaces, dryers, or ovens, while most outlets and lights run on just one of the 120-volt legs.

Just as Frayne stated. To expand a bit on above part of his reply:
The two phases are on sine waves. The frequency is 60 Hz, and on each leg the voltage goes from 0 to 120 volts, in a sine wave pattern, 60 times per second. By combining the max 120 volts on each, you get 240 volts. But as stated, most items in your house work off just 120 volts. Only high power items, like HVAC, electric water heater, clothes dryer and oven use 240 volts. Power is measured in watts, and volts x amps = watts; so having 240 volts enables to need less amps for the same power vs only having 120 volts.

 

[Yoheadden]

Could your electronics, either individual items or components to say your furnace, be at risk when this occurs?

We have underground lines in our neighborhood and one time we noticed fluxations (if thats a word) in power to our microwave (low power) and refrigerator (bright frig light). It turned out, there was a problem with one of the underground lines. It was a good thing we were home because we shut down the main breaker, but not before it did affect a circuit board on our furnace.

 

[JoeWras]

Risk to devices? Heck yeah.

Why? There's a fault in the system. Either through the mechanism of the fault, or through the repair process, there is a chance for a surge or extended period of under voltage.

And 240 v devices really get confused. The brains may work, but parts of the appliance don't. One would hope there are safeguards, but who knows? Can't be healthy.

The few times this happened to us, I threw the main breaker of the panel until the situation calmed down just like Yoheadden did.

I should mention that the power company ended up replacing all the underground lines in the neighborhood about 20 years ago. They had some sort of issue. The problem hasn't reccurred since.

 

[gutterman]

Typically your breaker panel has two rows of breakers. Left side is one phase and the right side is the other. So if one phase is out, every breaker on that side will be out. At least that is how my houses have been.

I would turn the breaker off for your HVAC system until the problem is sorted out.

 

[FLSUnFIRE]

Typically your breaker panel has two rows of breakers. Left side is one phase and the right side is the other. So if one phase is out, every breaker on that side will be out. At least that is how my houses have been.

I would turn the breaker off for your HVAC system until the problem is sorted out.

I'm pretty sure they alternate and not by side so every other breaker would be out.... 240a breakers utilize power from two phases and draw power from each phase on the same side of the box.
-Also not an engineer.

 

[CaptTom]

Yes, they alternate. The pins the breakers connect to are interleaved, like this:

 

[folivier]

The two different phases are in direct opposition to each other. IOW when one phase of 120Vac reaches 0 the other phase is at 120 volts. At least as I understand it.
That's why you can't combine 2 plugs to get 240Vac. You'll double the amps available but still be at 120Vac. Unless you happen to use the 2 plugs that are on different phases.

 

[Pluperfect]

I also queried your question to the all-knowing (but fallible) ChatGPT AI bot.

Since it's not clear from your answer, I'll ask: Did you use any personal expertise to review the answer? You said you're not an "electrical engineer by training," but that could mean any number of things, including that you're an electrician, or that you have no training/education/experience with power distribution at all.

 

[frayne]

I have a background in chemical plant operations management and have been trained to open and close 4160V breakers. I've also done some home electrical work, including 220V and generator wiring, so I’m not a complete novice when it comes to electricity. (Note: Always use a voltmeter.) I use ChatGPT primarily as a grammar checker but also as a knowledge base for a wide range of questions. It’s a remarkable tool, though the information it provides should always be verified—much like using GPS, where not all data is entirely accurate.

 

[msieweke]

FYI - Your furnace may operate on 120V, since it uses fairly low power for the fan. Air conditioners usually require 240V for the compressor.

 

[Alchemist253]

One phase shorted on the feed to my house. My utility brought in a converter that converted some of the “A” phase to the “B” phase so that my 220 appliances would work until the feeder was repaired. That situation lasted for a few weeks.