895,044. Controlling burners. MINNEAPOLIS-HONEYWELL REGULATOR CO. June 27, 1958 [June 28, 1957], No. 20696/58. Class 75(1) An electrical safety control apparatus for a burner (not shown) comprises a lock-out bimetal time delay switch 37 actuated by an electric heater 38 energized in the absence of a burner flame and a bimetal time delay switch 42 actuated by an electric heater 43 and adapted to open the circuit to an electric igniter 22 at a predetermined time after a flame detector 16, which may be a bimetal element, has detected a burner flame, the electric heater 38 for the lock-out switch 37 being in parallel with the electric heater 43 for the time delay switch 42 and the impedance of the heater 38 being small compared with that of the heater 43 so that the heat developed by the heater 43 is; not sufficient to operate the time delay switch 43; until the other electric heater 38 has been deenergized by the flame detector 16. In operation, when a heat anticipating type thermostat 49 closes its contacts, a first portion 27 of a control relay 26 and the heater 38 of the lock-out switch 37 are energized from a mains transformer 23 through closed contacts 34, 36 of a flame relay 32, closed contacts 46 of the time delay switch 42 and closed contacts 40 of the lock-out switch 37. Control relay 26 then closes switch 31 to energize a motor 11 supplying fuel oil and air to the burner and switch 30 which connects the high resistance heater 43 for the time delay switch 42 and the second portion 28 of the control relay 26 in parallel with the heater 38 for the lock-out switch 37. The burner is then ignited by electrodes energized by an ignition transformer 22 actuated by the closed contacts 45 of the time delay switch 42 (The ignition transformer is not controlled by the thermostat 49 and is energized except when the contacts 45 are opened by the time delay switch 42 on establishment of a flame.) The flame detector 16 then closes contacts (not shown) to energize the flame relay 32 which opens contacts 34, 36 to de-energize the low resistance heater 38 for the lock-out switch 37. The high resistance heater 43 then starts to develop more heat and after a time it actuates time delay switch 42 to open contacts 45 to de-energize the ignition transformer 11 and to open contacts 46. The flame relay 32 when energized'also closes contacts 34, 35 disposed in parallel with the contacts 46 of the time delay switch 42. On the demand for heat being satisfied thermostat 49 opens its contacts to de-energize the control relay 26 so that switch 31 opens to de-energize the burner motor 11 and switch 30 opens to de-energize the heater 43 for the time delay switch 42. The burner goes out and flame detector 16 opens its contacts to de-energize the flame relay 32 so that contacts 34, 36 again close. The starting sequence cannot however start on a further demand for heat, until heater 43 has cooled sufficiently to allow the time delay switch 42 to close contacts 45, 46. If ignition of the -burner fails at starting the flame detector does not energize the flame relay 32 and consequently resistance 38 actuates the lock-out switch 37 to open contacts 40. On cooling of the bimetal element of the lock-out switch the contacts 40 are closed by a manually operated reset button 41. If flame failure occurs during burner operation, the flame detector de-energizes the burner motor 11 and the starting sequence is automatically started when the time delay switch 42 cools and closes the contacts 45, 46; if ignition is not successful the lock-out switch opens as described above.