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-   -   why wouldn't circuit breaker prevent fire? (http://www.diychatroom.com/f18/why-wouldnt-circuit-breaker-prevent-fire-14071/)

fauer 12-04-2007 09:05 AM

why wouldn't circuit breaker prevent fire?
 
local article:
Door opener sparks garage fire

A problem with an electric door opener sparked a fire that caused about $40,000 in damage last night to a detached garage that was used primarily as a recreational room by a family at their Town of Polk home, according to the Washington County Sheriff's Department.

Sheriff's deputies, the Slinger Fire Department and Lifestar Rescue initially responded to the fire at 4762 Highland Park Drive just before 8 p.m. Monday. Slinger also requested assistance from the St. Lawrence, Jackson and Allenton Fire Departments.

No one was injured in the fire.

According to the Washington County Sheriff's Department, the family had been gathering Christmas decorations from the garage and putting them in the yard just prior to the fire.

The garage, which measured about 25-by-35-feet and its contents were a total loss. A car parked near the garage also suffered heat damage.

NateHanson 12-04-2007 09:35 AM

Well, that article doesn't say much about the cause of the fire, so it's hard to speculate on what would have PREVENTED it, but just for some basic info - Circuit Breakers are just overcurrent devices. When a 20A circuit draws more than 20A, the breaker opens the circuit. But if loose connection in a box causes heating of that wire nut, or if a opener motor jams and the motor doesn't have overload protection inside it (or it's not working right) then 15 amps of electricity could easily cause a fire, and not trip the 20A breaker.

KUIPORNG 12-04-2007 09:53 AM

this question is similar to ask "why people will still get kill/hurt if they put their finger into the socket if there is circuit breaker"....

the answer I believe is, the circuit breaker set off after the fact, that is, a large current when through the wire and towards some conducting materials, generating a lot of heat enough to cause a fire or kill someone in some circumstances such as a burnable materials close by...

ponch37300 12-04-2007 10:02 AM

I believe this is why they invented arc fault circuit breakers.

NateHanson 12-04-2007 10:33 AM

It's not that the circuit breaker would trip a moment after the fire started, it's probably that the fire started by something other than a dead fault to ground. If you touch hot to ground or hot to neutral, you'll get a massive arc and the breaker will trip. No fire will start, in all likelyhood. But if you get a slow heating from a loose wire terminal attached to the opener or something, that terminal will get hot as the electricity has to constantly jump a small gap from the wire to the screw. The motor will still only draw what it normally draws (let's say 5 amps), so the breaker won't trip, but the terminal is getting hotter and hotter, until finally the plastic housing on the motor catches fire.

The circuit breaker is important, certainly, but it's for over current protection. Electrical fires are probably very often caused by something besides overcurrent situations.

Circuit breakers also will not keep you from slipping on the wet bathroom floor. They're useful, but there are some things they just do not protect against. :)

J. V. 12-04-2007 11:52 AM

Lose connections are high resistance connections and they may never reach the current to trip the breaker. I don't like to call breakers overcurrent devises as they prefer shorting of current carrying conductors. Of course 22 amps on a 20 amp breaker will trip. But how long will it take. Maybe long enough for the sparks to ignite something?

Andy in ATL 12-04-2007 03:05 PM

Quote:

Originally Posted by J. V. (Post 78359)
Lose connections are high resistance connections and they may never reach the current to trip the breaker. I don't like to call breakers overcurrent devises as they prefer shorting of current carrying conductors. Of course 22 amps on a 20 amp breaker will trip. But how long will it take. Maybe long enough for the sparks to ignite something?

JV, I agree with most of what you say. I've watched new breakers hold 24A for over 6 hours. I don't think this over-amperage is what is causing the fire. Loose connections are the cause...in my opinion. 12AWG THHN at 24A is warm to the touch. It is certainly not ignition hot.

Andy

Tony L 03-07-2008 09:24 PM

Quote:

Originally Posted by fauer (Post 78315)
local article:
Door opener sparks garage fire

A problem with an electric door opener sparked a fire that caused about $40,000 in damage last night to a detached garage that was used primarily as a recreational room by a family at their Town of Polk home, according to the Washington County Sheriff's Department.

Sheriff's deputies, the Slinger Fire Department and Lifestar Rescue initially responded to the fire at 4762 Highland Park Drive just before 8 p.m. Monday. Slinger also requested assistance from the St. Lawrence, Jackson and Allenton Fire Departments.

No one was injured in the fire.


According to the Washington County Sheriff's Department, the family had been gathering Christmas decorations from the garage and putting them in the yard just prior to the fire.

The garage, which measured about 25-by-35-feet and its contents were a total loss. A car parked near the garage also suffered heat damage.

Need much more information to hep you out.What burned up?
Tony

nap 03-08-2008 01:06 PM

Quote:

Originally Posted by Andy in ATL (Post 78388)
JV, I agree with most of what you say. I've watched new breakers hold 24A for over 6 hours. I don't think this over-amperage is what is causing the fire. Loose connections are the cause...in my opinion. 12AWG THHN at 24A is warm to the touch. It is certainly not ignition hot.

Andy

If you review the characteristics curve of a typical 20 amp breaker, you will see that at 6 times the rated current, the trip should be simply instantaneous. At 24 amps (figuring a 20 amp breaker so 120% rating) trip time should be right at 50 seconds up to 6 or 7 minutes. This is at 77F. Higher ambient temps would cause the trip time to be lower. Lower temps, longer.

Square D breaker characteristics

If you observe a breaker that does not perform as designed, it should be replaced as a defective unit as it could possible not act as designed when required and could cause a serious problem. So andy, did you replace that breaker?



One thing that most people ignore but can be of direct importance is; people do not exercise their breakers with any regularity. I did not look for it but somewhere in life, I had read that Square D ( and I am sure the other manufacturers do as well) recommends exercising your breakers at least once per year. A breaker can "stick" and not perform as intended if it is not exercised from time to time. Also, from at least one manufacture; if a breaker trips due to short circuit, it is no longer warranted to perform as designed and should be replaced (doesn't everybody already do that?).

I cannot count the number of breakers I have come across that were never exercised and when I did, many were definately "stiff" and most likely would not have performed properly when needed.

It is one of the things I do in any customers panels when I have the chance. (don't do it when all the office women are using their computers. They hate that)

Many non-electrician type folks do not understand where the heat comes from. As others have stated, a highly resistive connection is often the cause of a great amount of heat. Think about this; a specified size of wire is required to be used for a specified current rated circuit. Why? because a smaller wire will heat up due to the current flow. So, what happens with a poor connection is you are trying to flow the the needed current flow (the wire size is not going to limit what the motor or any other use requires) through a bad connecton, which is effectively a greatly reduced wire. Guess what happens? You produce heat and sometimes a great amount of it. I have removed wire nuts (well, what was left of them) that were not installed properly and the heat created melted the entire plastic portion of the wire nut leaving the metal spring and nothing else.. What was left was effectively a metal heating coil. If this is near a combustable, it will start a fire. Luckily, most of the materials used in electrical installations are very fire resistant or there would have been a fire.

220/221 03-08-2008 01:12 PM

Keep this in mind.


Your toaster doesn't trip the circuit breaker and it gets red hot.

Under the right bad conditions wire can do this too.

bullet 03-08-2008 05:45 PM

I like the toaster analogy.

goose134 03-08-2008 10:55 PM

Very nice explanation about breakers. I'll say this though, they are mechanical devices, and as such have a tendency towards inconsistency. 20 Amp breakers DO NOT all trip at 20 amps. I don't care if you take them to the gym and run them on the treadmill. Fuses will ALWAYS go at their rating (in my experience).

But breakers are here to stay, and the should be exercised fairly regularly.

InPhase277 03-09-2008 12:46 AM

Quote:

Originally Posted by goose134 (Post 105703)
Very nice explanation about breakers. I'll say this though, they are mechanical devices, and as such have a tendency towards inconsistency. 20 Amp breakers DO NOT all trip at 20 amps. I don't care if you take them to the gym and run them on the treadmill. Fuses will ALWAYS go at their rating (in my experience).

But breakers are here to stay, and the should be exercised fairly regularly.

You know, I have told several customers that same thing. A breaker can fail in several ways: it can fail to trip, it can fail to close, it can trip on a small load, it can trip on a short but not on an overload etc. But a fuse only has one failure mode: BLOW.

This usually comes up when a customer tells me they want to get rid of their "old dangerous" fuse panel. I have explained that as long as everything was wired right, a fuse panel with the proper size fuses is fine. Am I too honest? On several occasions, to save the customer some money,I have cleaned up the old fuse box, put in a new neutral bar, and subfed it from the new 200 A breaker panel next to it. This saves them money because I don't have to waste time crawling in their attic splicing old wiring in j-boxes for the new panel. The old branch circuits are fed from their fuse panel (with tamper proof type S adapters), and their stove, water heater, dryer, and new circuits get fed from the breaker panel.

Nothing wrong with (properly sized) fuses.

InPhase277

elkangorito 03-09-2008 06:36 AM

Quote:

Originally Posted by nap (Post 105553)
One thing that most people ignore but can be of direct importance is; people do not exercise their breakers with any regularity. I did not look for it but somewhere in life, I had read that Square D ( and I am sure the other manufacturers do as well) recommends exercising your breakers at least once per year. A breaker can "stick" and not perform as intended if it is not exercised from time to time. Also, from at least one manufacture; if a breaker trips due to short circuit, it is no longer warranted to perform as designed and should be replaced (doesn't everybody already do that?).

You have read correctly. I designed & built switchboards for about 10 years. In this industry, it is common knowledge that CB's must be annually operated. Again, you are correct about CB's that have tripped from a high current fault (I squared t). I cringe when I hear people talk about buying 2nd hand circuit breakers or selling used circuit breakers. These activities should be legislated against.

hoistdoctor 11-26-2008 05:57 PM

The common and cheap breakers in a residential panel are a thermal device and will change their trip point over time if exercised by overcurrent. The heating will takes its toll.


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