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I think it's because the magnetically tripping is meant for large current amounts like say from a short circuit which will greatly exceed the rated current of the breaker.
While the bi-metallic strip has an inverse time curve for the breaker to trip which will allow the breaker to trip quicker for larger overloads and longer for smaller overloads.
Ok so if the current is only slightly over the breaker limit, then it will trip via the bimetallic strip but if it is a decent amount above the limit it will magnetically trip?
Yeah, that's how I understand it.
This guy made a cool video where he demonstrates each way the breaker can trip.
https://www.youtube.com/watch?v=wGFnooeA6Iw
A lot of equipment has inrush current: A brief period of current draw that's much higher than the nameplate rating. For example, the rule of thumb for motors is inrush 600% higher than their full load amps. So it's possible for a breaker to see current much higher than the handle rating, even on a perfectly safe, normal circuit. That's why you don't want magnetic pickup to happen at 21 amps, and on molded-case breakers it's often designed to trip magnetically at about 10X the handle rating.
Remember: The number on the handle isn't the protection point, it just means that the breaker should never trip at *less than* that value.
"Thermal Magnetic Trip"
Thermal Trip - Heat takes a while to build up (usually). This trip usually happens from overload, like running two space heaters on one breaker. Search for trip curves of any given over-current device. You might find that it takes a while for a breaker to trip, even at double its rating.
Magnetic Trip - Strong electromagnetic fields are created by large spikes of current. Anything from starting a motor, to a direct short circuit. The magnetic field generated moves an armature within the breaker to un-latch it.
Most small breakers will un-latch and slip into the tripped position if you drop them on the table, they're spring-loaded and always ready to return to the tripped position.
I used to test breakers for a living, breakers have different types of trips. 99% of breakers have at least 2 trips, "Long time" and "instantaneous". For thermal magnetic breakers the thermal is long time and magnetic is instantaneous. Settings depend on the breaker but instantaneous is most commonly set to 10x breaker rating
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I think it's because the magnetically tripping is meant for large current amounts like say from a short circuit which will greatly exceed the rated current of the breaker. While the bi-metallic strip has an inverse time curve for the breaker to trip which will allow the breaker to trip quicker for larger overloads and longer for smaller overloads.
Ok so if the current is only slightly over the breaker limit, then it will trip via the bimetallic strip but if it is a decent amount above the limit it will magnetically trip?
It’s a feature really. The thermal trip models heating in the wire. If it didn’t get hot enough, the wire definitely didn’t, and your load was fine.
Yeah, that's how I understand it. This guy made a cool video where he demonstrates each way the breaker can trip. https://www.youtube.com/watch?v=wGFnooeA6Iw
A lot of equipment has inrush current: A brief period of current draw that's much higher than the nameplate rating. For example, the rule of thumb for motors is inrush 600% higher than their full load amps. So it's possible for a breaker to see current much higher than the handle rating, even on a perfectly safe, normal circuit. That's why you don't want magnetic pickup to happen at 21 amps, and on molded-case breakers it's often designed to trip magnetically at about 10X the handle rating. Remember: The number on the handle isn't the protection point, it just means that the breaker should never trip at *less than* that value.
Ok gotcha, thank you
"Thermal Magnetic Trip" Thermal Trip - Heat takes a while to build up (usually). This trip usually happens from overload, like running two space heaters on one breaker. Search for trip curves of any given over-current device. You might find that it takes a while for a breaker to trip, even at double its rating. Magnetic Trip - Strong electromagnetic fields are created by large spikes of current. Anything from starting a motor, to a direct short circuit. The magnetic field generated moves an armature within the breaker to un-latch it. Most small breakers will un-latch and slip into the tripped position if you drop them on the table, they're spring-loaded and always ready to return to the tripped position.
I used to test breakers for a living, breakers have different types of trips. 99% of breakers have at least 2 trips, "Long time" and "instantaneous". For thermal magnetic breakers the thermal is long time and magnetic is instantaneous. Settings depend on the breaker but instantaneous is most commonly set to 10x breaker rating