这个开关可使接触器接通电流形成回路。

The switch closes the contacts and complete the circuit.

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于是，电流通过C1、Rt形成回路预热灯丝。

Then current through C1, RT forms the return circuit-preheating filament.

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但若故障支路形成回路或割集时，则仍需要解一组非线性方程，不过它的变量个数已大大减少。

But if there have some loops or cut sets between these fault branches, solving a set of nonlinear equations is also necessary, though the number of variables is much decreased.

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