Circuit interrupters current chopping

In the first case, it is easy to interrupt the circuit, which poses no problem of current chopping (premature tripping) before a current zero or a prolonged arc after a current zero for the following reasons ... 

The circuit lends to interrupt at a normal current zero and causes no current chopping. A BOCB is generally designed with two breaks per pole that help in restoring the dielectric strength promptly during an arc interruption. 

Figure 19.27 Approximate representation of assumed voltage and current waveforms illustrating a current-chopping effect and its attenuation while interrupting a circuit having 0,3 p.f.

By the immediate first current zero it is assumed that the contacts have travelled sufficiently apart to achieve the required deionization and have built up adequate dielectric strength to withstand at least 0.95 V, . If the circuit does not interrupt at the immediate current zero at a which is so near to the point of chopping Vt , the interruption will take place only by the next current zero at point h and result in another 260 strikes by then. To study more accurate behaviour of an intenupter, with the number of restrikes and the formation of the actual transient voltage waveforms on current chopping, oscillograms similar to those during a short-circuit test may be obtained (Section 14.3.6). 

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