EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: drew1900 on July 15, 2020, 12:13:56 pm
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Hi,
From what i understand, the circuit breaker will be activated when there is an excessive load of 2 or 3 times more than the rated AMP. And there are other types of circuit breakers will only activated when the excessive load is more than 5 times.
Shouldn't it be only 1 time the excessive load? For example, if I install a 10AMP circuit breaker, it only 'breaks' if there is an excessive load of '20-30AMP'? With '20-30AMP', wouldn't that already be too "late"? Say if the accidental excessive load hits 15 AMP (without activating the circuit breaker), and my wall plug is only able to handle 10AMP, wouldn't that already cause a fire or whatever safety hazard?
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Look at the data sheet. There will be a curve for load current versus time to trip.
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Circuit breakers have usually two over current triggers, one magnetic for shorts and large current spikes (fast), and a thermal one for simple overload protection (slow). The "x times full load" relates to the magnetic trigger and is meant to cope with the inrush current of loads. The B type (3-5 times full load) is typically used for households.
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Most molded case breakers have both the thermal and the magnetic trip unit. The magnetic will trip instantly when the current exceeds some multiple of nameplate. The thermal takes time and current. That's why the trip curves are actually a curve.
This should keep you busy:
https://download.schneider-electric.com/files?p_enDocType=Data+Bulletin&p_File_Name=0600DB0105.pdf&p_Doc_Ref=0600DB0105#:~:text=See%20Figure%201%20for%20an,magnetic%20circuit%20breaker%20trip%20curve.&text=response.,heat%20associated%20with%20the%20overcurrent.
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Shouldn't it be only 1 time the excessive load? For example, if I install a 10AMP circuit breaker,
Modern modular circuit breakers start to work from about 1.1 rated current (with large time). Breacker rated current (10 A, 16A, 25A, etc) is maximum load current which breaker must conduct without tripping a circuit. From 1.0 to 1.1 (from 100% to 113%) - it is uncertain zone.
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Shouldn't it be only 1 time the excessive load?
you could,but then almost every time you turned something on the breaker would go.Try measuring the resistance of a light bulb and from ohms law calculate the current ,no you haven't made a mistake,for a very brief time the lamp will draw that current and most electrical equipment is the same,circuit breakers have to account for this start up current,some things are worse than others,motors for example tend to pull a large amount of current when starting,so the breaker need to react slower than one used on a cooker.
In the uk at least,the purpose of the circuit breaker is to protect the cable,not the appliance, so you could have a 5A load wired with a cable rated for 100A protected by a 80A fuse and this would be perfectly acceptable because in the case of a fault the maximum current that can flow is well within the current rating of the cable. If you look at the graphs linked to above you can see how long it will take for a breaker to trip for a certain current, if we take a 6A breaker the minimum cable size used for a 6A circuit would be 1mm2, the cable is rated for at least 10.5A,look at the graph and at 10.5A a 6A breaker will trip in around 500 seconds,ok not instantly ,but quick enough to avoid any damage to the cable.