How Does a Circuit Breaker / Trip Switch Work? - Pt 1

[SeanB]

How Does a Circuit Breaker / Trip Switch Work? - Pt 1

https://www.youtube.com/watch?v=Cq55tbLtnlg

[itvend]

i was wondering what is the resistance of bi-mettal ? And i uploaded european circuit breaker B6

[SeanB]

Resistance is very low, well under an ohm, and it is in the hundreds of milliohm range. It only has to warm to about 100C to trip the mechanism, and has to not trip at 70C case temperature with rated current. That makes it not too onerous resistance wise, as the example you show for the EU spec relay ( probably under 10A) has both a higher number of turns on the magnetic trip core and an insulating sleeve on the thermal sensor to ensure it heats up enough from the current.

http://www.e-t-a.co.uk/fileadmin/user_upload/Ordnerstruktur/pdf-Data/Products/Elektromechanik/2_pdf_therm_magn/2_pdf_englisch/D_4230-T_ENG.pdf

Interesting that they quote a life of 20000 operations for mechanical operation, and 6000 cycles for rated current trips. However I would say the number of cycles at maximum overcurrent would be a lot less, probably only in the tens of cycles for this massive overload regime.

[SeanB]

Tested a failed one at work, and it survived quite a lot of abuse.

https://www.youtube.com/watch?v=QXbTG4Y_0XA

Trip current was at least 140A, provided by the arc welder, using the short heavy leads to give the best current delivery to the breaker. Open circuit voltage is around 70VAC, enough to give a good arc on opening, and plenty of heat in the arc. Breaker was removed from service because the magnetic trip had lost it's damping, and would trip on motor start current pulses.

You can copy this experiment easily, but do not use an inverter type welder, as it may fail from the current spikes, or the HF start may arc across the breaker internally. Wear safety goggles, as your breaker might fail explosively. Caution, the breaker case will get very hot.