an electrosurgical system comprises a generator and an instrument including a first electrode , a second electrode , and an insulating spacer separating the first and second electrodes . the generator repeatedly measures a characteristic of the radio frequency output such as the impedance between the first and second electrodes . the generator analyses the impedance measurements , and interrupts the radio frequency signal when the rate of change of the impedance is such as to indicate the onset of a “ flare - out ”. in this way , the power is reduced before the flare - out leads to permanent damage or failure of the instrument .