Patent GB-429684-A (GB)
Title: Improvements in pressure operated electric switches
IPC: H01H35/32
Filed: 1933-12-02
Abstract: 429,684. Actuation of switches; switch cases. TRIGGS, W. W., 57, Lincoln's Inn Fields, London.-(Dunham Co., C. A.; 1523, Davenport Road, Toronto, Canada) Dec. 2, 1933, No. 33961. [Class 38 (v)] A pressure - operated switch, for use in a heating system of the kind in which a constant pressure difference is maintained between the supply and return pipes, comprises casings 9, 9<1> fitted with bellows 16, 16<1> clamped in position by plates 14, 141 joined by struts 35, 36. The ends of the bellows are connected to the ends of a rod 39 having bearings in domed centre parts 37, 37<1> of the plates 14, 141. A lever 46 pivoted at 47 is connected at 50 with the rod 39 and bears at its free end an insulated contact 80 connected by a flexible lead to a conductor 72. A spring 51 acts at 56 on the lever 46 and is adjusted by a nut 58 so that when the desired pressure difference exists between supply and return pipes connected to the nipples 23, 23<1>, the lever 46 occupies the central position. If the pressure difference falls, the contact 80 is moved to engage the contact 77 to energize a relay to start the exhausting motor which runs until the pressure difference has increased sufficiently to cause the contact 80 to engage a contact 79 to deenergize the relay and stop the motor. A modified form of contact mechanism is shown in Fig. 5. The pressure operated lever 461 has a forked end 118 carrying screws 119, 120 which engage a stem 127 passing through a flexible part of an evacuated glass tube 122. The stem 127 carries a contact 126 which can be moved to engage either of the contacts 123, 124 sealed into the tube. Fig. 6 shows the circuit, arranged for direct current mains 98, 99. The main switch is shown at 100 and the main switch for the pressure control mechanism is shown at H. Switches 103, 105 are arranged so that when switch 103 is open, the switch 105 can be closed to operate the exhausting motor 97 continuously. A float operated switch 108 closes the motor circuit when an accumulation of condensate requires removal. For normal operation the switches 100, 103, H will be closed and switches 105, 108 will be open. Then, with the motor 97 stopped, the pressure difference will decrease until contact 80 moves to engage contact 77 thus completing the energizing circuit of relay 84 through wire 71 wire 113, negative main 93, positive main 91, wire 111, relay 84, wire 112, wire 72 and contact 80. The relay 84 closes the motor circuit from positive main 98, switch 100, motor 97, wire 104, switch 103, wire 95, switch H, wire 92, relay contacts 87, 86, 88 and negative main 93. At the same time a hold-on circuit for the relay is completed from positive main 91 through wire 111, coil 84, wire 112, wire 114, resistance 89, wire 115, relay contacts 87, 86, 88, negative main 93. The hold-on circuit remains energized after the contact 80 has moved away from contact 77 and the motor runs until, when the maximum pressure difference has been established, the contact 80 engages contact 79. A shunt circuit for de-energizing the relay coil 84 is then completed from positive main 91 through wire 116, resistance 90, wire 117, wire 73, contacts 79, 80, wire 82, wire 72, wire 114, resistance 89, wire 115, relay contacts 87, 86, 88 and negative main 93. The relay 84 then opens breaking both the operating circuit and the motor circuit.