Breaker detecting heat in an outlet?

S

sparkync

Senior Member
Location
North Carolina
I have seen outlets charred before, and the breaker not tripped. Obviously the hot and ground wires did not touch, but if the breaker will detect over amperage, which creates heat, why will it not detect, a loose connection in an outlet that causes heat?
 
In a thermal magnetic breaker, the breaker will either trip from heat in the breaker (the thermal part) or from a very large current (the magnetic part). If there is a loose connection at an outlet, the heat created would have to travel through the conductors back to the breaker to cause it to trip. The heat will be dissipated before it makes it back to the breaker, so the breaker will not heat up and cannot trip in the thermal portion of its trip curve.
 
Because the loose connection doesn't cause a current draw and current is all the breaker knows.

The loose connection causes what's called a glowing arc and this is exactly what an AFCI breaker is supposed to detect but they fail miserably.

-Hal
 
hbiss said:
Because the loose connection doesn't cause a current draw and current is all the breaker knows.

The loose connection causes what's called a glowing arc and this is exactly what an AFCI breaker is supposed to detect but they fail miserably.

-Hal
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It is what we, at least me, believed they were supposed to detect. We misunderstood. They have always worked as designed.o_O

Yes, I'm just stirring the coals.
 
d0nut said:
If there is a loose connection at an outlet, the heat created would have to travel through the conductors back to the breaker to cause it to trip. The heat will be dissipated before it makes it back to the breaker,
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^^^^^
This is the key point.

The breaker detects heat inside the breaker. Heat at a distant loose connection won't be noticeable.

But Heat at the breaker will cause it to trip. Breakers with loose connections to the bus or wire termination will have excess local heating and will trip at lower than rated current.
 
sparkync said:
I have seen outlets charred before, and the breaker not tripped. Obviously the hot and ground wires did not touch, but if the breaker will detect over amperage, which creates heat, why will it not detect, a loose connection in an outlet that causes heat?
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Does the breaker trip on a heat producing load such as a toaster? It is the exact same condition with a high resistance connection. The current, while producing a lot of heat, is not great enough to trip the breaker.
 
ptonsparky said:
It is what we, at least me, believed they were supposed to detect. We misunderstood. They have always worked as designed.o_O

Yes, I'm just stirring the coals.
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Because it is not a glowing arc, it is a glowing connection. If it were arcing, 120 volts can't sustain an arc very well and extinguishes itself before it will get out of control. Which is why AFCI's are mostly pointless to begin with.

If they would just mandated lower magnetic trip settings for everything that doesn't have a starting inrush current, this likely would accomplish most of what AFCI concepts actually do and would do so with less objectionable trip events.
 
hbiss said:
...
The loose connection causes what's called a glowing arc and this is exactly what an AFCI breaker is supposed to detect but they fail miserably.
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What ptonsparky said. They were never supposed to detect this.

Consider that a high resistance connection that stays closed is indistinguishable from a load (hair dryer, space heater, towel warmer, incandescent bulb) that is intended to create heat, there will probably never, ever be a way to design a remote protective device that will detect glowing connections.
 
LarryFine said:
They started out as protection from cords and extension cords under rugs and other bedroom flammables.
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I understand that was kind of a goal in developing them. I don't think they were all that successful at meeting that goal.

As I mentioned earlier, lower magnetic trip settings are probably rather effective for this and only nuisance trips would be with some motor starting situations.

I remember Square D had a setup they had at trade shows back in the 90's where they had these little metal links they would put in series with a breaker. Their QO breakers, particularly their 15 and 20 amp single pole breakers that do have lower mag trip than many competitors have, would trip immediately when they powered the circuit. They had competitor breakers they could put in the circuit to compare, every one of the competitor breakers would let the link burn up without tripping the breaker.

AFCI's are more snake oil and money making venture than something useful for consumers even if that wasn't the original goal it is what it became.
 
kwired said:
If they would just mandated lower magnetic trip settings for everything that doesn't have a starting inrush current, this likely would accomplish most of what AFCI concepts actually do and would do so with less objectionable trip events.
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Yes, the most extensive charcoal remains I've seen to extend beyond fire bomb boxes, occurred with old breakers missing the magnetic trip function.

However, I've seen aluminum branch wire, and outlet smolder bombs on SQ-D QO, with the lowest magnetic trip in the industry.

Mechanically operated breakers may have the advantage of ignoring electronic noise, smolder, upstream events, and utility excursions, but they also ignore Amazon unlisted junk, non-FCC tested, & failing equipment, which AFCI & GFCI are vilified for shutting down.
 
kwired said:
I understand that was kind of a goal in developing them. I don't think they were all that successful at meeting that goal.

As I mentioned earlier, lower magnetic trip settings are probably rather effective for this and only nuisance trips would be with some motor starting situations.

I remember Square D had a setup they had at trade shows back in the 90's where they had these little metal links they would put in series with a breaker. Their QO breakers, particularly their 15 and 20 amp single pole breakers that do have lower mag trip than many competitors have, would trip immediately when they powered the circuit. They had competitor breakers they could put in the circuit to compare, every one of the competitor breakers would let the link burn up without tripping the breaker.

AFCI's are more snake oil and money making venture than something useful for consumers even if that wasn't the original goal it is what it became.
Click to expand...
I came across an old trailer with a 120 Vac /12 Vdc inverter. There was a small square aluminum type breaker smaller than a quarter, would this be the QO breakers you mention from the 90s?
 
jaggedben said:
(hair dryer, space heater, towel warmer, incandescent bulb) that is intended to create heat,
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Not to mention the heating elements on your stove snd oven!

Another aspect is that a loose connection that adds resistance (whether arcing or not), becomes a resistance in series with the load. That then causes a voltage drop at the load device. If the device is a motor, the voltage drop causes an increase in current, so it is indeed “helping” to get the protective device to trip. But if the load is an another resistive device, two resistances in series results in LESS current flowing in the entire circuit, so acting AGAINST the trip function of the protective device. So although there will be more heat AT the loose connection, there will be overall LESS current seen by a breaker.

When I was on a design team for motor overload relays, the general consensus was that, assuming properly sized conductors, a typical bimetallic thermal trip device is sensitive to ambient heat within about 10-12” inches of the device, meaning heating events taking place somewhere else in the conductor more than 12” away had little effect on the trip curve in that device. Most breakers use a bimetallic trip element.
 
jaggedben said:
What ptonsparky said. They were never supposed to detect this.

Consider that a high resistance connection that stays closed is indistinguishable from a load (hair dryer, space heater, towel warmer, incandescent bulb) that is intended to create heat, there will probably never, ever be a way to design a remote protective device that will detect glowing connections.
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It's a bit like gardening. A weed is just a flower growing somewhere you don't want it to. The glow you want to brown your toast is just as capable of browning your receptacle.
 
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