Fire Alarm Telegraphy
The article “Non-Interference and Succession Boxes in Fire Alarm Telegraphy,” appearing in the June Number of The FIRE ENGINEER, was read by me with mixed feelings of pleasure and disappointment. Pleasure in that an attempt has been made to lay before those who are interested, reliable information with regard to a matter about which little is generally known or written. Disappointment in that the description of the functions of some of the apparatus mentioned, while technically all that could be desired, yet was meagre and involved and not sufficiently clear as to give the uninitiated a proper understanding of the subject.
I hesitate to supplement anything written about fire alarm boxes by one so eminently qualified to write as Mr. Faller, and I appreciate that the article was merely one of general description and not of detailed information, but in the interest of a spread of useful information I am taking the liberty of elaborating somewhat on what Mr. Faller has said about the Wiederhold box.
This type of box operates normally on closed circuit, but will function and preserve its non-interfering features whether the line is open or closed. When the box is operated the first function of the mechanism is to ground the circuit, which ground connection is broken when the first notch on the code wheel passes under the circuit breaking device. As the circuit breaking device passes out of the notch and rides on the periphery of the code wheel, the ground connection is again restored to the line. This operation is repeated for each make and break of the circuit in accordance with the predetermined number of the box, and it follows from this, that while this type of box is in motion, the circuit is alternately grounded and opened by the mechanism.
Referring to the attached diagram, it is apparent at once that when box 2 is set in motion and a ground connection is established at G2, two circuits having a ground return, G2-G, have been made, one containing battery B, relay A, and Box No. 2, and the other containing battery Bl, Relay Al, and Boxes 4 and 3. When the circuit breaking device passes into the first notch of the code wheel, it is obvious that both these circuits are opened simultaneously, causing the Relays A and Al to function. Similarly if box No. 4 is operated and no other box on the line is functioning, two circuits will be alternately made and broken, one containing battery Bl, Relay Al, and box 4, and the other containing battery B, Relay A and Roxes 2 and 3. It is evident then that when only one box on the line is operating, the effect upon the line relays is exactly the same as would be produced by a plain interfering box. If, however, boxes 2 and 4 are operating simultaneously, it is obvious that box No. 2 will actuate relay A only as it is manifestly impossible for any electrical disturbance set up in box. No. 2 to pass through the line to relay Al since the line at box 4 is either grounded or open; and similarly no disturbance in the circuit set up in box No. 4 will actuate relay A for the reason that the line is either grounded or open at box No. 2. Hence there can be no inter ference of signals when two boxes are operated on the same circuit at the same time. But it follows naturally that all boxes connected on the line in that portion of the circuit away from the relays and between the two boxes in motion are inoperative. Referring to the diagram, it should be plain that while boxes No. 2 and No. 4 arc operating, box No. 3 is shunted out or insulated from the circuit by the alternate grounding and opening of the circuit at boxes No. 2 and No. 4.
Flight per cent of all fires last year were due to electricity. Cheap insulation, amateur wiring, short circuits and arcs caused more fires than the old kerosene lamp.