At the heart of the GCB lies the interrupting chamber which has the job of conduct-ing very high nominal currents as well as to interrupt fault currents within a few cycles. Two contact systems – the main contact system and the arcing contact system – as well as the associated concentrically mounted blast pistons are coupled to an internal gearing which is connected to a high-speed drive situated outside this chamber. The gearing is designed such that, during breaking, the main contact system parts a few milliseconds before the arcing contact system, ensuring that the full current is broken by the latter. The arc created when the arcing contact system separates is extinguished the instant the alternating current next passes through zero.
In figure we can see :
curve1: movement of arcing contact system; 2: movement of main contact system; 3: current; 4: voltage across breaker
5: arcing time; 6: arc extinguished at current zero;
t1: drive tripped; t2: main contact system separates;
t3: arcing contact system separates; t4: arc extinguished at current zero.
