a fault - tolerant multi - point flame sense circuit utilizes a single electronic switch to signal the presence or absence of flame to an electronic controller . multiple flame sense electrodes may be input to this circuit . by configuring these electrodes in accordance with the present invention , cross - contamination of a single failed flame sense electrode will not affect the other flame sense electrodes &# 39 ; ability to sense a flame at their associated burner . the circuit provides inputs for a number of flame sense electrodes via input channels that are capacitively coupled to the line voltage and resistively coupled to an rc network that controls the state of an electronic switch . when a flame is present at any one of the electrodes , the resulting unbalance current flow through the rc network turns the switch off to indicate the presence of flame . this operation is not affected by a short on any other electrode in the circuit .