referring to fig1 , a generator 10 has an output socket 10 s providing a radio frequency ( rf ) output for an instrument 12 via a connection cord 14 . activation of the generator may be performed from the instrument 12 via a connection in cord 14 or by means of a footswitch unit 16 , as shown , connected to the rear of the generator by a footswitch connection cord 18 . in the illustrated embodiment footswitch unit 16 has two footswitches 16 a and 16 b for selecting a coagulation mode and a cutting mode of the generator respectively . the generator front panel has push buttons 20 and 22 for respectively setting coagulation and cutting power levels , which are indicated in a display 24 . push buttons 26 are provided as an alternative means for selection between coagulation and cutting modes . referring to fig2 , the instrument 12 comprises a blade shown generally at 1 and including a generally flat first electrode 2 , a larger second electrode 3 and an electrical insulator 4 separating the first and second electrodes . the first electrode 2 is formed of stainless steel while the second electrode 3 is formed from copper integrally with a body portion 9 . the surface of the second electrode is plated with a biocompatible material such as stainless steel , or alternatively with a non - oxidising material such as gold , platinum or palladium . the electrical insulator 4 is formed from a ceramic material such as al 2 o 3 . a conductive lead 5 is connected to the first electrode 2 , while lead 6 is connected to the second electrode 3 . the rf output from the generator 10 is connected to the blade 1 via the leads 5 and 6 so that a radio frequency signal having a substantially constant peak voltage ( typically around 400v ) appears between the first and second electrodes . when the blade 1 is brought into contact with tissue at a target site , the rf voltage will cause arcing between one of the electrodes and the tissue surface . because the first electrode 2 is smaller in cross - sectional area , and has a lower thermal capacity and conductivity than that of the second electrode 3 , the first electrode will assume the role of the active electrode and arcing will occur from this electrode to the tissue . electrical current will flow through the tissue to the second electrode 3 , which will assume the role of the return electrode . cutting of the tissue will occur at the active electrode , and the blade may be moved through the tissue . referring to fig3 , the generator comprises a radio frequency ( rf ) power oscillator 60 having a pair of output lines 60 c for coupling via output terminals 62 to the load impedance 64 represented by the instrument 12 when in use . power is supplied to the oscillator 60 by a switched mode power supply 66 . in the preferred embodiment , the rf oscillator 60 operates at about 400 khz , with any frequency from 300 khz upwards into the hf range being feasible . the switched mode power supply typically operates at a frequency in the range of from 25 to 50 khz . coupled across the output lines 60 c is a voltage threshold detector 68 having a first output 68 a coupled to the switched mode power supply 16 and a second output 68 b coupled to an “ on ” time control circuit 70 . a micro - processor controller 72 coupled to the operator controls and display ( shown in fig1 ) is connected to a control input 66 a of the power supply 66 for adjusting the generator output power by supply voltage variation and to a threshold - set input 68 c of the voltage threshold detector 68 for setting peak rf output voltage limits . also coupled across the output lines 60 c is a current detection circuit 80 which feeds signals to the controller 72 via line 81 . in operation , the microprocessor controller 72 causes power to be applied to the switched mode power supply 66 when electrosurgical power is demanded by the surgeon operating an activation switch arrangement which may be provided on a hand - piece or footswitch ( see fig1 ). a constant output voltage threshold is set independently on the supply voltage via input 68 c according to control settings on the front panel of the generator ( see fig1 ). typically , for desiccation or coagulation the threshold is set at a desiccation threshold value between 150 volts and 200 volts . when a cutting or vaporisation output is required the threshold is set to a value in the range of from 250 or 300 volts to 600 volts . these voltage values are peak values . their being peak values means that for desiccation at least it is preferable to have an output rf wave - form of low crest factor to give maximum power before the voltage is clamped at the values given . typically a crest factor of 1 . 5 or less is achieved . when the generator is first activated , the status of the control input 60 i of the rf oscillator 60 ( which is connected to the “ on ” time control circuit 70 ) is “ on ”, such that the power switching device which forms the oscillating element of the oscillator 60 is switched on for a maximum conduction period during each oscillation cycle . the power delivered to the load 64 depends partly on the supply voltage applied to the rf oscillator 60 from the switched mode power supply 66 and partly on the load impedance 64 . the voltage threshold for a desiccation output is set to cause trigger signals to be sent to the “ on ” time control circuit 70 and to the switched mode power supply 66 when the voltage threshold is reached . the “ on ” time control circuit 70 has the effect of virtually instantaneously reducing the “ on ” time of the rf oscillator - switching device . simultaneously , the switched mode power supply is disabled so that the voltage supplied to oscillator 60 begins to fall . the operation of the generator in this way is described in detail in our european patent application no . 0754437 , the disclosure of which is hereby incorporated by way of reference . referring back to fig2 , when the instrument 12 is in use , small particles of condensed tissue and other debris can become adhered to the edge electrode 2 and , to a lesser extent , the base electrode 3 . if the instrument is used particularly aggressively , it is possible that a conductive track of such debris can build up between the electrodes 2 and 3 across the ceramic insulator 4 . such a conductive track is shown schematically at 11 in fig2 . if no action is taken to prevent it , this conductive track 11 will develop into a “ flare - out ” in which the current passing directly between the electrode 2 and the electrode 3 will cause the instrument to overheat and finally fail . the following description explains how the generator 10 detects and compensates for just such a situation . at regular intervals , in this case every 10 ms the current is measured across the load 64 by the current detector 80 and the current value is sent to the controller 72 . the controller uses the current value to determine repeatedly the impedance across the load 64 . the difference between successive impedance values , irrespective of their absolute value , is calculated , and summed for 16 consecutive readings to give a first total z 1 . the current measurements continue every 10 ms until a further 16 consecutive impedance calculations have been made , which calculations are again summed to give a second total z 2 . if z 1 and z 2 are both less than the threshold criteria for the sum q of the impedance changes , then the generator continues to supply rf signals to the instrument 12 . the process is continued with further current measurements being sent to the controller 72 every 10 ms . this normal operation is shown in fig4 , in which the voltage across the electrodes 2 , 3 is shown by trace 31 , the current flowing by trace 32 and the impedance measured by the generator by trace 33 . if a flare - out starts to develop between the electrodes 2 and 3 , the current measured across the load 64 will start to fluctuate widely , and with a high frequency of oscillation . this is shown in fig5 , with the build up to the flare - out being shown at 34 and the onset of the flare - out at 35 . in these circumstances z 1 and z 2 ( representing difference in impedance values , not absolute impedance values ) will both be above the threshold for the sum q of the impedance changes , and this causes the controller to send a signal to the power supply 66 to cause the power to be interrupted . a typical value for q is 1000 ohms , for a 16 measurement cycle . in addition to interrupting the power supply , the controller may cause a message ( such as “ clean tip ”) to be displayed by the display 24 . the controller does not allow power to be restored to the output of the generator until the surgeon has pressed a reset button to indicate that the tip has been cleaned , and will repeat the interruption process if the impedance measurements show that the flare - out conditions are still in existence when the power is recommenced . it will be appreciated that criteria other than the changeability of the impedance across the output of the generator could be employed to give an indication of the onset of a flare - out . these include , non - exhaustively , the high frequency content ( e . g . the number of high frequency components ) of the modulation of the current or voltage signal , or the d . c . thermionic current flowing between the electrodes 2 and 3 . the latter will be measured in the manner disclosed in u . s . pat . no . 6 , 547 , 786 , the contents of which are incorporated herein by reference . it will also be appreciated that , while the embodiments of the invention have been described with reference to the elimination of flare - outs , the invention could in some aspects be used to prevent overheating of electrodes without the actual existence of a flare - out . the generator , detecting criteria indicating the start of a potential overheating situation , could reduce the power or alter the radio frequency signal in other ways so as to maintain operation of the electrosurgical system operating within proper parameters . those skilled in the art of electrosurgical generators will readily be able to establish suitable detection criteria to keep the generator operating within safe and effective limits .
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 .