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Are there certain items in a house (anything from a computer to a furnace) that can be damaged by shutting off its circuit at the electrical panel?

If so, must one avoid turning off that circuit at all times, or should one simply take certain precautions (for example, unplugging the device) before shutting off that breaker?

A Chicago Tribune article touches on the issue, but does not go into detail.

I ask because I've seen people (who didn't have a wire tracer handy) shut off entire legs of an unlabeled panel at once, in order to quickly identify the desired breaker. Using this method, one can isolate the correct breaker in about five tries. But, I worry that it's bad for certain hardwired appliances or plugged-in devices, to often have their power cut at the breaker.

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    Are you referring to physical hardware damage only, or the possibility of lost data as well? (say, if you turn off the circuit your PC is plugged into and you have unsaved work on said PC) Also, are you covering devices that might harm someone who trips the breaker and then tries to take them apart? – ThreePhaseEel Jul 19 '17 at 1:04
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    I think that turning them back on is more likely to cause damage. – RBarryYoung Jul 19 '17 at 14:39
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    Logically, any such easily damaged device would be damaged by power outages, and would require a UPS... – DJohnM Jul 20 '17 at 4:47
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    potentially? Murphy says anything that's plugged in. But I only ask for one thing when I'm going to play the breaker game (because it's the one thing everyone knows that can be indisputably traced back to it being the fault of an otherwise competent electrician, and consists of 99% of the potential problems I've encountered): "Are there any desktop computers?" – Mazura Jul 21 '17 at 1:06
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    If a device can be damaged by changing a breaker's state, what do you think will happen when there's a blackout, or when power is restored after a blackout? – Carl Witthoft Jul 21 '17 at 14:38

13 Answers 13

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First, the article writer is arm-waving. A lot. A device which is destroyed when you sever power is a defective device. What he's trying to say is that breakers themselves are not made to be used as switches, and particularly, they're not made to interrupt high-current-drawing loads (like your water heater when cycled on). Except when they are.

As circuit breakers evolved, they often came out with special variants of the breaker that can do some parlor trick a normal breaker cannot (and are UL-listed for that trick). The breaker might be rated to also be used in a 208V/3phase panel. It might be rated for interrupting inductive loads. Often these breakers are expensive only because of the sunk engineering costs and that they are oddball -- the manufacturers know if they made every breaker that way, it would add almost nothing to breaker cost. So after a time, they often do exactly that. And often UL "levels the playing field" by changing breaker specs so all breakers must be that.

UL (Underwriters Laboratories) tests products for safety, and lists approved products. The need to use listed electrical products comes up a lot.

Here, the relevant trick is called an "SWD" breaker. This is made for a very common habit in factories and shops of using the breaker as a light switch. There is nothing wrong with this or the AHJ would not allow it. Being commercial they tend to control fluorescent or HID (sodium, metal halide, mercury) lighting. The SWD breaker is made to be used daily as a switch, tested for many more on-off cycles. The HID breaker is built extra tough to cope with the poor power factor of many HID lights. Many manufacturers simply make all their breakers SWD rated. Whether UL is behind this I don't know.

So likely your breakers are SWD, and rated for interrupting a big inductive load like an A/C unit all day everyday. Your old breakers, maybe not so much, that's what the article is trying to warn about. But even then, this is a bit of hysteria; UL tests even non-SWD breakers for thousands of cycles.

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    What's "UL" that you refer to? – Pimgd Jul 19 '17 at 9:55
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    @Pimgd Underwriters Laboratories. – Tester101 Jul 19 '17 at 10:39
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    "Hat trick" usually means achieving a feat 3 times within a certain span. Not sure how that applies here. – Timbo Jul 20 '17 at 0:39
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    Yes, I was going to ask - what are the three things for the hat-trick here? – psmears Jul 20 '17 at 9:03
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    I think perhaps Harper meant "parlor trick"; in this context, a gimmicky or niche feature that differentiates one line of a manufacturer's breakers from all the others, which they use to justify an absurd (50%+) hike in price even though it'd only cost them 1% or less to add said feature to every breaker they made. – Doktor J Jul 20 '17 at 14:19
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Most devices can at least put up with power interruptions

Save for the obvious things (like data loss), most devices won't be physically damaged by having their power interrupted for a length of time (i.e. several minutes). HVAC/R stuff that needs to protect against short-cycling or cool a hot part is the closest you'll come to this problem in most cases; there is also the occasional pump that's not self-priming, but I'd consider any pump of that style to be obsolete for any duty we'd see on this site.

But some things can bite you after the breaker's been tripped

There are a few devices, however, that have special switched-bleeder arrangements that only kick in on a normal shutdown. These devices (some duct-type electrostatic air cleaners seem to be this way) can store charge if they are shut down by way of a snap (light) switch or breaker, and thus bite you with that stored charge when you go to handle them

The breaker itself? Fine

Most 15A and 20A breakers are rated for what UL calls Switching Duty, or SWD for short. Also, there are quite a few places in the NEC where a circuit breaker, even one not listed specifically for switching duty, is allowable as a disconnecting means to turn a circuit or device off for servicing, with the understanding that this will happen infrequently -- SWD ratings are only needed when the breaker is being used as a switch on a regular/frequent basis.

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    Every switch in my circuit breaker panel has "SWD" written on it, with the exception of all double-pole circuit breakers and the GFCI circuit breaker. Thanks for explaining that. – Fil Jul 19 '17 at 2:55
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    +1 for mentioning SWD. These are normally used for lighting in buildings like warehouses where banks of switches are not desired. If your breakers are marked SWD then the breaker is rated for it. If a device can't handle the power being turned on and off it should not be sold in the marketplace. I would call it junk. It is not in any way similar to a car being started or left running. There are no moving parts to wear every time it is turned on. The device should be able to withstand thousands of cycles from off to on without failing, or it is junk. But then there is a lot of junk out there. – ArchonOSX Jul 19 '17 at 9:52
  • @ArchonOSX: I've got high-end devices that really shouldn't have their power interrupted. They make battery backups for a reason. – Joshua Jul 20 '17 at 22:25
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    @Joshua Yes the reason is convenience or data loss, not because they will be damaged by power cycling. – ArchonOSX Jul 20 '17 at 22:37
  • some things like projectors don't like being swtiched off from the circuit. they need to cool the bulb down slowly and short cycling can cause the bulbs to wear quicker or even shatter. – Matthew Whited Jul 21 '17 at 14:54
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That article does not appear to be very well-informed. I would guess that any car which uses more fuel starting than standing is either very old or shut off for a very short time. I'm dubious of the claims about insulated water heaters as well.

What I don't doubt is that electricians have advised against flipping the breakers, mainly because they're primarily a safety device. It is common to use them as switches in barns, pools, workshops, etc. but 1) Shutting the subpanel to your pool off if one breaks is much less aggravating than turning the main to your home off. 2) When used as switches the breaker turns on a well-defined set of things rather than random stuff that may be plugged in. You won't generally start a fire turning on the barn lights because the drapes blew into a bedroom space heater you forgot about.

As far as specific items go, in theory you could damage a mechanical hard drive with a sudden power loss if it happened to be in operation at the time. In practice they're designed to shut down gracefully if power is cut from the PC switch so they're fairly resilient against power loss. You'd need the perfect storm of a drive old enough to not park heads on power failures, power cut at the breaker, then the PC jostled enough to impact the disc.

That being said, many electronic devices aren't huge fans of power fluctuations so there may be more of a concern that you're varying the distribution of power more so than using the breakers. i.e. The fridge starting up causing a transient power drop on other circuits.

The second set of candidates would be anything with a duty cycle. I'm thinking specifically of an AC that's labelled, "Wait three minutes before restarting." I would take the manufacturer at their word and unplug it prior to flipping breakers willy-nilly.

The last thing I can think of is water pumps, which will generally burn out if run dry. It's more likely with pools but I have seen a well pump that had to be primed after being shut off.

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    A hard drive old enough to not park the heads on power failure will have been manufactured prior to about the mid 1980s. – Mark Jul 19 '17 at 2:37
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    What about projectors? They tend to like continuing to cool the bulb a few minutes after shutdown to prevent damage. I doubt it would cause catastrophic failure to turn it directly off at the mains/breakers but I suspect it may well harm the life expectancy of the bulb. – Muzer Jul 19 '17 at 8:19
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    @Mark true. The real problem with cutting power is with files in some indeterminate state. Any pending writes will not happen which can lead to corrupt files – Baldrickk Jul 19 '17 at 9:26
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    @Mark actually, even modern HDDs can lose up to one sector due to corruption in the middle of writing a sector (partially old data, partially new data, checksum invalid, you can't read either of the versions), as HDDs prefer head parking over writing the current sector fully. Also, HDDs reorder writes using a buffer for performance, so you may have block A depending on block B. The operating system writes block B first and block A then, but the HDD may write block A first if it's in a more convenient location. So, what you depend on isn't there, leading to file system corruption. – juhist Jul 19 '17 at 14:20
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    @Mark I suppose the drive met its end some other way, then, but it definitely tends to grind to a stop now if I plug it in, wait for it to spin up, and then unplug it. Maybe all I can do is say that if you have a cheap and failing hard drive, it may not survive an unexpected power cycle. – Darren Jul 19 '17 at 20:10
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Some projectors have fragile lenses which can crack if they cool down too quickly. These projectors would have a warning in the instruction manual, telling you not to unplug them for several minutes after you turn them off.

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    I expect that the issue here is with the bulb moreso than the lens. – Timbo Jul 20 '17 at 0:46
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    FWIW, it's a defective product if it needs this cooling but does not have internal power storage (e.g. a small rechargeable battery) to operate the fan even after power is removed. – R.. Jul 20 '17 at 3:34
  • I've got a laminator with such a warning. All the time it is operating it makes a slight noise, I think there's a small fan inside keeping the heat from the high temperature portion away from the electronics. It's cheap enough they aren't going to go the battery route. – Loren Pechtel Jul 20 '17 at 4:40
  • You're not supposed to kill the power because the fan needs to still run for a while... to cool it, or it could melt its own insides. It has nothing to do with cooling too quickly, but it might cool unevenly, which would be bad. – Mazura Jul 21 '17 at 0:54
  • I don't think the issue is cooling too quickly; I think the issue is that the lamp assembly gets very hot and other nearby parts of the projector need to be kept cool by a fan. If the lamp is hot, cutting power won't instantly cool it. Instead, its heat will transfer to nearby parts which are no longer being cooled by the fan. – supercat Jul 21 '17 at 15:37
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Some devices - video projectors and induction stoves are examples - run their fan for some time after being switched off, to save on heat related wear and tear. Switching these off via cutting their power might not exactly be damaging them, but accelerating wear.

Computers generally do not "like" being forcibly switched off, but this is more of a problem of data becoming inconsistent.

If you pathologically and constantly switched the power on and off, some things would certainly wear out: Some power supply designs suffer accelerated wear due to inrush currents, they will charge their filtering capacitors from scratch each time which is a heavy load on the rectifier. Some devices that keep state across power cycles would eventually wear out the (limited write cycle) EEPROM devices used to save that state into when powering down.

CRT based monitors or televisions (maybe also some microwave ovens?) are likely to keep the tube filaments powered at all times, constant power cycling could shorten the lifespan of the tubes here.

  • "maybe also some microwave ovens" Every microwave oven I've looked at in detail uses a power interlock, i.e. a device that physically disconnects power from the potentially dangerous parts if the door is opened. Therefore, they must be designed to tolerate power disconnection, because it happens every time you open the door.. – Jules Jul 20 '17 at 17:17
  • Is that always true of the filament? Tube filaments do not run off high voltage normally (actually, making them for high voltage operation is more difficult and expensive!) and aren't dangerous to power by themselves. If there is only one transformer with a filament winding - yes, the interlock will likely stop it. More complicated designs - not so sure... – rackandboneman Jul 20 '17 at 19:31
  • Anything with a cooling fan, especially computers. +1 – Mazura Jul 21 '17 at 0:57
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Simple answer: yes, but a little common sense and care and not overloading your breakers and you'll generally be ok.

You're likely going to get answers on both sides of this. So let's get the obvious part of the question out of the way first: Question: ...Are there things in the home that can be damaged by shutting them off... at an AC source upstream somewhere... Answer: Yes. For sake of answering what appears to be the core of the question, turning item(s) off by cutting power can break something - including the circuit breaker itself.

With this said, its a common practice, and it rarely breaks things, but it can. The real issues are probably three fold -

1.) transients (motors, compressors, and various other appliances or items with large capacitors or loads can essentially send a jolt of power back out to other devices on the same circuit and even beyond)

2.) current/power draw (even something as simple as a light bulb, going from no power to initial power on draws extra power for a moment when powered on, thus we have all seen an incandescent bulb burn out when you turn it on... regardless if by a wall switch or a breaker...) thus turning things on especially numerous things that were on before the breaker was turned off can cause both #1 & #2 above.

3.) Microsoft always tells you to shutdown cleanly before powering off your computer, a video projector, or an A/C conditioner, some dryers for example, all require cooldown time after their main cycle has just ran. From hard disks spinning down cleanly and memory in electronics getting cleared or cleaned up, to fans finishing their cooldown cycle, shutting these things off "cold turkey" so to speak, actually subjects them to high heat or abnormal conditions which significantly shorten their life or could pose a fire hazard.

These are extremes. So best precautions: Dont bake or run the a/c furnace, dryer, heck, even listen for the fridge to be on an idle cycle. Turn off lights, computers, tv's etc. Turn off items that are not in use, and are not needed for the testing. Use a few cheap items like incandescent table lamps on a hard surface. This way, the most likely thing you are going to blow is a light bulb. Further, when you turn the breaker back on, its less likely to cause a surge.

  • Yes - I would be careful about hard-interrupting the power to inductive loads with potentially large stored energy, (back-EMF). Obviously, large electric motors spring to mind:( – Martin James Jul 20 '17 at 19:48
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Medical devices can have all sorts of problems and conseqeuences if shut off unexpectedly.

Your bedside alarm clock may forget its alarm settings, and revert to having the alarm set for 00:00 Can be distressing to have your Get Up alarm go off early.

UPSs can be great and they can also be time bombs. I recall one that worked fine on utility power, but when the moment came for the UPS to act, it started smoking as the inverter failed. That was unpleasant.

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    +1 for the UPS. Even if it was recently tested and you expect it to work perfectly, you should probably give some thought to what's plugged in to it and what that will do when the battery runs out. – Chris H Jul 21 '17 at 8:13
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The only (but small) risk of destroying electrical circuit elements by abruptly interrupting the power supply occurs if a big conventional 50/60Hz transformer is connected in parallel to the device. Which most probably is no more the case: 50/60 Hz transformers were big and heavy, required loads of iron and costly copper, hummed constantly and were either fit for 110 or for 220 Volts, not for both voltages at once. The problem is that with such a transformer about half of the net sinus current from the net produces a magnetic field in the transformer's core and half of the time this field is converted back into a current that flows into the net. If the connection to the net fails abruptly and none of the other circuits needs this current the transformer is willing to produce quite high overvoltages that might destroy other circuit elements.

Fortunately in nearly all devices one uses every day the big transformer has been replaced by a small switch-mode power supply that no more does do this.

Also the ballast inductor of neon tubes might produce an overvoltage when the circuit is interrupted abruptly.

The third category of things that might break are poorly-designed intelligent devices during a firmware update: Many devices have two memories for their firmware: One with the current and one with the old version. If a firmware update is interrupted the memory with the old version still works. But some devices first erase the old firmware and then hope the power will last until they finished writing the new one. But - firmware updates normally last only minutes and occur at maximum perhaps once per year. So this risk is small...

...and the fan from your oven or the bulb from your video projector might stop if the circuit is interrupted which might mean that the surroundings of the bulb or the oven will get hotter than expected. Normally video projectors when turned off try to cool down the bulb as fast as they can: This abrupt temperature change will reduce the lifetime of the bulb. But running the fan at full speed reduces the probability that the bulb's heat will damage the lens/some electronic parts if the power is interrupted abruptly. Also it reduces the probability that the projector is transported while its bulb is still hot (and therefore more sensitive to vibration than in the cold state).

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So here is the thing. No device has ever failed (except the breaker it's self) by being turned off at the breaker. The only real problems come from two sides.

  1. Flipping the breaker is the same as "unplugging" the device. Look around your home and tell me one device that will break because you unplug it. It's also the same as the power going out because you didn't pay your bill, or there was a storm. None of the devices in your house are going to fail just because they loose power.

  2. Turning on the breaker is the same as "plugging" in a device. Look around your house and tell me one device that will break because your power came back after a storm. Again not likely, though there are some devices that should take precautions with (like the A/C needs time between starts usually)

Real Risks

The real risks with using breakers as switches is that in most cases around a house, you don't know what a breaker will turn on. It's not that the device is going to explode or anything. It's that it might not be in a safe operating state. A blender may all of the sudden come on with the lid off. The blender is fine and working well, your kitchen is now a utter mess.

Some devices are meant to be used in a certain way, and cycling the power may "break" that way. For example a router that get's it IP address from a modem, may be designed to start after the modem. But by using the breaker they both start at the same time. The router is working fine, just as intended, but because the modem wasn't ready when it started the router can't find the internet and can't do some auto configuration.

At any rate, using the breaker as a switch is not going to harm any devices that your would find around a home. I'm sure if we look hard enough we can find some odd devices in some settings that may be effected, but those devices are poorly designed or do a very specific job. Electricity is something we take for granted, but when designing electric devices it's always important (and they do) make sure that they can handle a loss of power at any time. Because many things can cause a loss of power not just throwing a breaker.

  • If a circuit has a large inductive load on it and also has a device that doesn't like voltage transients, tripping the breaker may cause the inductive load to generate a voltage spike which could damage the other device. The same could occur if both devices were plugged into the same power strip and it was unplugged from the wall, but would not occur if they were unplugged separately. – supercat Jul 19 '17 at 23:16
  • There are absolutely devices that will break from unplugging them, I listed one DEFINITE example in my answer - a projector. Projectors keep a fan running after you turn them off to cool them off and prevent fire or other shortening of bulb/projector life. So if you were using a home theater projector and just "unplug it" your chances of device failure go up DRASTICALLY. Not to mention, since there are hundreds, thousands, or even millions of different electrical devices that plug into a wall, and they range in quality - its best not to make such an assertion. – noybman Jul 19 '17 at 23:58
  • @noybman: That's a defective-by-design device. It it needs the fan for safety or for preventing premature failure of the product, it needs to have an internal battery to power the fan. – R.. Jul 20 '17 at 3:37
  • @R, indeed it is 'defective by design'. It is not the only one. Just making the point that there are indeed things in the home that can break if AC power is removed. This just one example of how of the 3 I initially listed. – noybman Jul 20 '17 at 11:31
  • "No device has ever failed ... by being turned off at the breaker." I've repaired computers for being turned off this way. This is a dangerous answer. And they're not "defective by design." They're designed to be powered down in a controlled manner to protect the circuitry. There is no way to perfectly protect anything (I'm an electrical engineer, what are you?) – JBH Jul 21 '17 at 0:49
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Several good answers here: One thing I don't see mentioned is certain space heaters. If your heater continues to blow air for a bit after the desired temperature is reached, then it is doing so to protect its components from heat sink. Basically the air continues to run so that the heat components can reach something near ambient temperature.

Turning the device off like that every now and again probably won't be a problem, but if you do it continually, you might find that the internal components fail prematurely.

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I'm not an electrician, but I do believe that you can experience power spikes, depending on your circuit breaker. Regardless of how new and well designed your circuit breaker is, it's not designed to be used like a light switch for the house, so I wouldn't do this. Additionally, if you power down the whole house, then power it all back on, all of your devices are going to start up at the same time. It's not a big deal every once in a while, probably, but if you do this every time you pop off to your mum's for the weekend, I think you could encounter a corner case that the designers did not anticipate.

Additionally, as another poster pointed out, many of these devices need to wind down before being powered on as they're not designed to be power cycled in the middle of useage. Your refrigerator or AC both can suffer from this. Additionally, I wouldn't unplug these on a regular basis without consulting the manual, and who really reads the manual for their refrigerator? Additionally, every single device in your home drawing startup power at the same time could possibly damage or cause unusual wear on your devices.

Many of your home devices, especially your water heater, do have power saving modes though, and should be able to be turned off or down when you're on vacation. I would recommend using those instead.

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The easy answer is "yes." The more complicated the electronic device, the less happy it is to be switched off by a breaker. Your dryer will never be harmed by switching it with a breaker (or a switch, for that matter). Your computer, on the other hand, HATES having the power interrupted. It has circuitry to try and prevent damage, but frankly, you should avoid it at all costs. I've had to replace client hard drives because it meant nothing to them even to use the computer's 4-second power-off switch because it was too inconvenient to shut down the machine via software. And any medical equipment in the home (such as you might find caring for the elderly) should always have a power backup to avoid snapping off the power.

However, the best answer is, "don't ever do it."

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One I don't see mentioned here is the refrigerator or freezer. Now the interesting thing about refrigerant compressors is the minimum cycle time, specifically turning them off does not cause any problems, but if you don't wait long enough to restart it you can burn out the motor. Conventional wisdom is 1/2 hour is plenty of time, and it is not an issue if the compressor was not running at power off.

Also note that there is industrial and scientific equipment that cannot handle power failure, specifically air bearings must have air flow while moving to prevent damage and often have battery backed air supplies to handle failures.

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