Circuit breakers trip on *power* overload?

[GregJ7]

TL;DR

Do circuit breakers trip on too many amps or too many volt-amps?

Old style electrical panels protected residential circuits with fuses which tripped (and were destroyed) when the internal wire got too hot and melted. What is it exactly that causes a modern residential circuit breaker to trip? They are rated by amps, but heat correlates to power (volts x amps), not amps. Should I understand their rating as a measure of VA at a particular voltage? (I am trying to understand what is going on inside breakers with either 120V and 240V running through them.)

 

[hutchphd]

Circuit breakers are rated in amps and the mechanism by which (in the US) breakers function is magnetic response to current carried. Magnets then open electrical contacts. Fuses, incidentally, also melt because of the current through them. The trip current is chosen to limit the power delivered to the rest of the circuit because the voltage is known (e.g. 120V rms in the US) but mostly to limit the current traveling through the wires in flammable walls. For instance if you have 14 gauge wiring you don't want more than 15 amps in it for any appreciable time.

 

[berkeman]

Summary:: Do circuit breakers trip on too many amps or too many volt-amps?

What is it exactly that causes a modern residential circuit breaker to trip?

https://en.wikipedia.org/wiki/Circuit_breaker

 

[GregJ7]

Circuit breakers are rated in amps and the mechanism by which (in the US) breakers function is magnetic response to current carried. Magnets then open electrical contacts. Fuses, incidentally, also melt because of the current through them. The trip current is chosen to limit the power delivered to the rest of the circuit because the voltage is known (e.g. 120V rms in the US) but mostly to limit the current traveling through the wires in flammable walls. For instance if you have 14 gauge wiring you don't want more than 15 amps in it for any appreciable time.

I find it hard to believe that a 15 amp fuse would trip with a 20 amps through it at 1 picovolt. Thanks for the info about it being magnetically tripped.

 

[Baluncore]

I find it hard to believe that a 15 amp fuse would trip with a 20 amps through it at 1 picovolt.

A metal fuse has resistance and so drops a voltage proportional to the circuit current.
That voltage is small compared with the supply voltage. It is greater than 1 mV.
For a sufficiently high current, W = I²·R, heats and then melts the wire, which opens the circuit.

 

[hutchphd]

The breaker is in the circuit as a series element. The circuit would not draw 20 amps at 1 pV because the breaker is not an ideal element (it has some reisistence) and requires some minimal power to operate, but that was not your question.
You objection is both true and irrelevant !

 

[anorlunda]

The explanation of

I find it hard to believe that a 15 amp fuse would trip with a 20 amps through it at 1 picovolt.

Forget the voltage. The fusible strip has some resistance R. The heat dissipated in a section of wire with resistance R is I²R. So given the R and the trip current I, that tells the designers how much heat should trip it. Then they select materials with a low melting point to match. (That's one approach, there are others.)

Fine tuning is done by adjusting the width of the fusible part to a narrow neck, shown schematically in the picture below. The narrower the neck, the more resistance per unit length. Sometimes in glass fuses, you can see the neck.

 

[anorlunda]

I really like the picture posted by @berkeman . The label 8 in the picture shows the arc divider/extinguisher. That is what makes it different than a switch. A circuit breaker expects that an arc will form between the contacts as they begin to open. An arc carries current so that can defeat the purpose of shutting off the current. A circuit breaker, but not a switch, has special parts designed to get rid of the arc as quickly as possible.

A normal switch does not expect, nor can it handle, significant arcing.

 

[GregJ7]

Thanks all. I see my problem was imagining the resistance of a thin-wire fuse being much lower than it actually would be.

 

[Dullard]

Not to split hairs, but I think an uninformed reader (maybe that's me?) could come away from this thread with the wrong impression about how typical domestic circuit breakers work. They do have a magnetic component for large instantaneous overloads, but they also typically have a thermal mechanism for smaller, sustained overloads. both are really required for the best combination of protection and non-nuisance operation.

 

[anorlunda]

They do have a magnetic component for large instantaneous overloads, but they also typically have a thermal mechanism for smaller, sustained overloads. both are really required for the best combination of protection and non-nuisance operation.

Correct. Here is the Wikipedia text that describes the picture @berkeman posted in #3

1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation.
2. Actuator mechanism - forces the contacts together or apart.
3. Contacts - allow current when touching and break the current when moved apart.
4. Terminals
5. Bimetallic strip - separates contacts in response to smaller, longer-term overcurrents
6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.
7. Solenoid - separates contacts rapidly in response to high overcurrents
8. Arc divider/extinguisher

 

[jrmichler]

My curiosity was aroused, so I found a 15 amp 120 volt Square D circuit breaker in my electrical stuff box, and connected it to a power supply. At 3.1 amps through the breaker, the voltage across the breaker was 0.051 volts measured at the breaker. That's a resistance of 0.016 ohms. The voltage across the breaker would be 0.25 volts at 15 amps, so the power dissipated in the breaker would be 3.7 watts at 15 amps.

That amount of heat would eventually make the breaker warm to the touch.

 

[dlgoff]

The label 8 in the picture shows the arc divider/extinguisher.

Brings back memories of the "puffer breakers" I've seen in substations.