Do you test your breakers?

[MillWeld]

Square D markets at least two different classes of breaker - one of which is "QO" or Quick Open. - Jay

Ah... interesting. My breaker is QOT but I don't know what that means. I will check into that. My breaker certainly didn't open within seconds.

 

[JB4310]

I actually measured the amps with a clamp-around meter. The first heater brought the amps near to 15 so I added a 2nd one which brought it to 23. The total time at 23 was probably less than a minute - didn't really time it. I didn't know the trip mechanisn was heat related; I expected a near immediate trip. I didn't want to wait and wait for 23 amps to trip it so I removed the heaters. Maybe it was premature but I replaced the breaker today.

The fan IS actually on a GFI circuit and that is what alerted me to the problem in the first place; it shut down several receptacles and overhead lights. Finally traced it to the bad fan which had TWO problems - bearing and short. I'm glad it is on a GFI given Bird's warning.

Actually, in retrospect, we have had intermittant problems with that circuit which would, apparantly at random (we thought), trip the breaker. I suppose a winding was degrading and occasionally contact the frame causing a ground fault and finally got bad enough to be permanent. I'm looking for a replacement motor.
Bob

I'm no electrician, but the fan does not need to be on a GFI, it will offer no extra protection against fire as far as I can tell.

You mentioned lights went out with the gfi trip, usually lights would not be gfi protected either, you don't want to be in the dark due to a gfi trip.
If the bearing was bad enough to prevent the motor from turning, "full rotor lock" is how I've heard that described, would spike the amp draw which should certainly trip a properly sized breaker. Maybe as fast as a short, I'm not sure how the gfi treats a short like that, but I'm guessing it tripped so fast it didn't give the breaker a chance to react??

All these comments are just me thinking out load, like I said I am not any more an electrician than the average handyman.

I have an old attic fan that was originally on a shared circuit, but when I renovated the 70 year old house, the electrician recommended having the fan on a dedicated 15 amp circuit, even though it was on the shared circuit for all those years.

JB.

 

[ducks13]

Breakers take considerable more than the rated amps to trip quickly. They are thermal magnetic devices; the magnetic part of the force that opens it is created by current flowing in a coil making a magnet, the more the current the stronger the magnet and the faster a breaker will trip. There is also a thermal portion of the force the opens the breaker (by heating a bimetallic spring) after extended (greater than a second or two) time. A breaker will typically hold 6 times the rating for a couple of seconds.

Your breaker should not be tripping on a motor overload, the motor's overload protection should be. Your motor's overload protection is probably internal to it and if the bearing was bad enough to have caused a fire I feel certain the overload would have opened and prevented it.

Here is a link to a typical breaker curve.

Technical Library - Schneider Electric Schneider Electric

I didn't know that QO stood for quick open but that is marketing, all common breakers have similar trip curves.

Notice that at your load of about 1.5 times the breaker rating it would take about 100 seconds to open. Also note that it would probably take over 15 minutes to trip at 16 amps. The breaker is there to protect the wire, not the motor. Breaker on motor circuit are called "short circuit and ground fault protection"; not overload protection. The wire can handle 16 amps for 15 minutes without overheating the insulation; or 100 amps for a couple of seconds.

Breakers are tested on some commercial buildings where power reliability is important such as data centers and hospitals. They are usually just tested during the initial installation. They are primarily concerned with it tripping when it shouldn't rather than not tripping when it should. The later is virtually unheard of in my experience.

Cheers!

 

[JB4310]

Your breaker should not be tripping on a motor overload, the motor's overload protection should be. Your motor's overload protection is probably internal to it and if the bearing was bad enough to have caused a fire I feel certain the overload would have opened and prevented it.
Cheers!

How does the overload protection work? by heat sensing? and does it reset (close) when it cools. So if you turn off the switch, then went back later and turned it on again, it would go thru the same process.

JB

 

[NC4AB]

I am the Assistant Chief with the local Fire Department and very leery of attic fans in general, our fire department has had at least 2 house fires caused by over heated attic fan motors in the past several years. The first one burned the entire attic and roof off the house (the homeowner wasn't home), the second one only burned the area around the fan (homeowner was home and turned a garden hose on the fan from the back yard).

If the fan motor is impedance protected it is not "supposed" to draw enough current to overheat even if the rotor is locked and can't turn. These types really give me the willys.

The better fan motors use thermal overload protection - a temperature switch built into the motor that will open on high temperature and will stop the fan. UL allows this to be a one time device (like a fuse) or manually reset (like a breaker).

 

[ducks13]

Or automatically reset when they cool is probably the most common on small motors. Yes, if you let it cool and turn it back on it will go through the same process. Do it enough times and the overload protection will eventually fail open and it will not run again.

 

[grsthegreat]

I am an electrician and we rate our circuits for 80% of breaker value. For instance a 20 amp breaker should only carry 16 amps under continuous load (calculated load). This works great for commercial operations where motor loads are known, but its hit and miss in residential applications. I have seen many a breaker that is marked 20 amps and pulling a continuous 23 amps without tripping. Ive seen it many times over the years. It shouldn't do it but it does. Remember, #12 wire is ACTUALLY rated to conduct 30 amps of power, but by convention its rated at 20 amps. So the 23 amps wont cause a fire per say. Now as to why it didn't trip with a short to ground tells me either the ground wire wasn't connected completely thru to the panel, or that particular breaker is toast.

I have never been asked in over 30 years to test someones breakers.....not sure how to effectively do that even. If there is a question, i simply replace it. I still remember the old advertising for Zinsco brand breakers...." Our breakers are so tough....they never trip" thats cause they were cra*.

I have not installed a powered attic vent for many many years. I have never seen one that has shorted out though. I'm sure it can happen.

QO breakers are rated very highly in our industry, but i am very skeptical about the twin (mini) breakers. I don't see how they can be as effective as the full sized breakers.

A ground fault or an ark fault breaker could add further protection to this fan in the future. HOWEVER you need to make sure the ground wire is intact all the way into the panel.

 

[tommu56]

I am the Assistant Chief with the local Fire Department and very leery of attic fans in general, our fire department has had at least 2 house fires caused by over heated attic fan motors in the past several years. The first one burned the entire attic and roof off the house (the homeowner wasn't home), the second one only burned the area around the fan (homeowner was home and turned a garden hose on the fan from the back yard).

If the fan motor is impedance protected it is not "supposed" to draw enough current to overheat even if the rotor is locked and can't turn. These types really give me the willys.

The better fan motors use thermal overload protection - a temperature switch built into the motor that will open on high temperature and will stop the fan. UL allows this to be a one time device (like a fuse) or manually reset (like a breaker).

As a electrician when I install them I recommend is a high temp cut out so that is if there is a fire it doesn't fan it (Pun intendd) as a volunteer fire fighter too.

Model EFL Fusible Link by Globe Technologies

 

[Qapla]

I have never "tested" any breakers. But I have, on occasion, deliberately "shorted" a circuit for the express purpose of "throwing" the breaker.

I have done this when I needed to work on a circuit and did not know which breaker killed the circuit. When the panel is not within sight of where you want the circuit dead and also to far away to "hear" when the circuit is dead and you don't have someone to yell "not it, not it, THAT'S IT", you use whatever method works.

I'm not saying I recommend this method, I'm just saying that I have done it. I have never had a breaker fail to trip when shorted.

BTW - I do go and "turn the breaker off" after tripping it and BEFORE doing any work.

Keep in mind also, repeated tripping will continue to heat a breaker up so that it will refuse to "reset" until it has cooled back down. Also, if you continue to trip and reset a breaker, it will fail prematurely and you will have to replace it.

 

[MillWeld]

A ground fault or an ark fault breaker could add further protection to this fan in the future. HOWEVER you need to make sure the ground wire is intact all the way into the panel.

The ground wire is intact - that is why the GFI shut the circuit down. I'm glad the circuit was on a GFI for the additional protection and I'm keeping it that way.

When we renovated the electrician used all slim tandem breakers (QOT). Is there evidence that they are not as reliable as a full size breaker?