Power surge; breakers say "meh"
January 2, 2010 4:31 PM   Subscribe

Power surged in the house; circuit breakers did not break...

Last year we had a power surge in our house. It caused lots of damage to our electrical appliances and started a dvd/tv combo player on fire. But not one single circuit breaker went off and I can't figure out why.

Here's how it went down:

We live in southern California, USA. Our house gets fed two "hots" (each @ 120 volts) and a neutral from the mains. Last January, for whatever reason, squirrels finally gnawed through the neutral line, completely severing it. This led to a power spike in the house which caused a ton of damage - but failed to trip one single breaker. Before we could turn all the lights off they were really bright so I know that was a lot of current pushing though. But why didn't any breakers flip?
posted by shino-boy to Home & Garden (12 answers total) 2 users marked this as a favorite
Because they didn't max out. Maybe you had more current than usual running through your breaker box, but your usual load is 25% of spec for a given circuit, then even if it doubled you'd still be at 50% and the breaker would be happy.

Breakers are to defend the wires in the wall, not the things that are plugged in. The idea is that if the current flow is sufficient so that the wires in the wall could become hot enough to start a fire, the breaker (or fuse) blows first.

That's all they're intended to do. They are not surge protectors.
posted by Chocolate Pickle at 4:36 PM on January 2, 2010

Best answer: I've got 6 60W lights on right now for the same circuit. That's 6 * 60W / 120V = 3A. And most circuits in your house are 20A. That means I could overload those lights by 667% before I'd come close to blowing the circuit.

My window air conditioner is ~1000W. But when the compressor fires up, it can draw as much as 3x that amount. So 3000W / 120V = 25A. More than the circuit that it's on, but as Chocolate Pickle points out, breakers are designed to allow momentary surges of power slightly over their rating (otherwise anything with a compressor would be blowing your circuits all the time).

Point is that circuit breakers aren't there to protect your appliances from surges. They're meant to trip in the case of a sustained overload beyond the wire's rated capacity (15A wire is commonly used in your walls). If you overload the wires for too long then they heat up, and I think you can guess where that leads.

You can get whole-house surge protectors. My uncle, who lives in a rural area with flakey power, has one.
posted by sbutler at 4:55 PM on January 2, 2010

Best answer: Your breakers are there to protect against too much amperage being drawn through the wires. That is the amount of current being used by your electrical appliances and lights. When you turn on more items, the voltage doesn't go up and down, but the current being drawn does. Within limits, it doesn't matter how much voltage is being applied to the line. Your wiring is good to about 600 volts. Your voltage jumping to 240 didn't affect the wires. The amount of current being shoved through those wires is something else. If you only had a few lights and/or appliances on, you might have been drawing a little more current but not enough to trip the breakers.

Your electronic stuff is affected by voltage. All of those chips and silicone goodies are very voltage sensitive. They can overheat and fry themselves (thus the fires), but that can happen with less than the normal amount of amperage draw. You might consider protecting your electronics from surges by installing an uninterpretable power supply (UPS) between the wall and the device. We have UPSs for each of our computers. They not only protect against surges, they are great in blackouts. No data lost! You don't need them for lighting circuits of most motor-driven appliances, but they are great for electronics.
posted by Old Geezer at 5:05 PM on January 2, 2010

The breakers are there to protect the house wiring, nothing more. Your appliances are required to fend for themselves. They should have been designed to safely handle the particular fault condition you describe (and many others). Companies must submit their products to testing labs like Underwriters Laboratories to verify their safety.

The catch: safety standards only require products not catch fire and not electrocute the user. It's perfectly fine if the device never works again. The DVD player definitely wasn't supposed to catch on fire, but all the other failures are more-or-less according to plan.

And before you go out and buy a bunch of them, you should know that the average surge protector will not protect against the kind of fault you describe. The voltage is too low. The surge protection devices inside power strips are designed to dissipate very quick, very high voltage pulses. Anything less than 300-500V is not going to trigger them. Nothing less than a good UPS will protect against the kind of problem you had.

Note: there's a good chance your homeowners insurance will cover the damaged equipment.
posted by ryanrs at 5:33 PM on January 2, 2010

...there's a good chance your homeowners insurance will cover the damaged equipment.

After a year, very unlikely.
posted by Chocolate Pickle at 5:48 PM on January 2, 2010

I was thinking last year meant last week, but now I see it was last January. Yeah, nobody is going to care twelve months after the incident.

Shino-boy, next time one of your electrical devices catches fire, please file an incident report with the CPSC. It's the meat-space equivalent of flagging, and works much the same way. One incident report on a product does nothing, but several will trigger an investigation of some kind.
posted by ryanrs at 6:22 PM on January 2, 2010

NB: "Amperage" (arguably archaic) is also known as "current."
posted by ZenMasterThis at 8:01 PM on January 2, 2010

Response by poster: OK, I think I get it now.

We did contact our homeowner's insurance company, the total dollar amount (about $1100.00) was below our deductible. But they did pay to replace all the food we lost in the fridge.
posted by shino-boy at 8:25 PM on January 2, 2010

When you lose your neutral, what happens is that the two phases become connected in series at your service panel. The two 120V phases add up to 240V. But the 240V will typically divide unevenly on the two halves of your panel depending on the balance of loads on the two phases. You might see something like 140V on right half your panel circuits and 100V on the left half of your panel circuits. So in some rooms the light bulbs would be brighter and in some rooms the light bulbs would be dimmer.

If you flipped on a couple more lights in one room, the voltage and brightness of all bulbs on that half of the panel would decrease slightly and the other half of the house would get slightly brighter. In fact turning off the brightest lights, the seemingly common sense thing to do, would have the perverse effect of making things worse, increasing the brightness of the remaining lights and raising the voltage on that half of the house, potentially causing more damage.

Depending on what loads were turned on at the time of the fault, you may have had 140 to 150 volts in some circuits. This voltage would be enough to make some bulbs brighter and enough to blow up some electronics, but it your case it was not enough to increase the current in any circuit sufficiently to trip a breaker. Current might have increased by 15% or 20% on the over-voltage circuits, but unless those circuits were already running near 100% capacity, the breakers would not trip.
posted by JackFlash at 1:10 AM on January 3, 2010 [1 favorite]

I wonder if the fault was actually a hot-to-neutral short outside the service panel. Did squirrels really "completely sever" a half-inch aluminum conductor?
posted by ryanrs at 2:23 AM on January 4, 2010

Response by poster: Yes, those suckers really did chew through it. When the power company technician came to the house to diagnose the problem, he didn't even climb on the roof. He pulled out his binoculars and examined the mains connections while standing on my front lawn. He told me that this was a common problem in our area and that he had seen it many times before.
posted by shino-boy at 10:36 AM on January 4, 2010

The aluminum fetish appears to be real:

Expert: Dana Krempels, Ph.D. - 11/10/2009

Actually, squirrels gnawing on aluminum is quite widespread. I am a Assistant Steward for a land preserve that is part of the Essex Land Trust in Connecticut. We use aluminum trail markers that are painted in different colors. For some reason, the squirrels gnaw on the markers. I have noticed this for over a year. While, I have not watched them do it directly, it hard to believe that any other animal would be capable of doing it on the side of tree. The markers are nailed onto trees about 6-7' off the ground. They seem to eat the paint and the aluminum mostly in the Fall. From reading the comments to your answer, this behavior appears to be widespread in the eastern part of the US. What are your thoughts on this matter?

Dear Frank,

Squirrels do like to chew, but it's as much a mystery to me as to you why they'd want to gnaw on aluminum. Aluminum isn't an essential mineral nutrient. And I know from experience that when you bite down on it, you get an unpleasant "shock" that's painful.

I wonder if something in the paint attracts them. But it is a mystery to me, too.


Maybe they don't go for copper because it's more toxic, but they need to chew something that will wear away their teeth to keep them from getting so long they're useless, or worse than useless.
posted by jamjam at 6:01 PM on February 14, 2010

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