abcs of fire alarm part 10

7/26/2019 ABCS of Fire Alarm Part 10

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Page IMSA Journal36

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Street Corner Fire Alarm BoxesSo far, we have dis-cussed the workingsof a protected premisere alarm system, buthave not gone into de-tail about the specicmethods of signalingto an outside authority.One of the most tradi-tional methods is via aCity Master Box that is

a street re alarm boxmodied so that it can be actuated by a firealarm control panelin addition to beingmanually operated.

Radio type re alarm boxes are now replacing theelectromechanical versions in various areas, especiallywhere the cable plant has deteriorated badly due to budget cuts, etc. We will discuss mostly the electrome-chanical type box in this installment.

The electromechanical city box is most commonlyfound in the northeast and in pockets elsewherethroughout the country. This box has a wind up clock-work mechanism mounted in a weatherproof shell,with a cottage roof. These boxes were rst installed inBoston in the nineteenth century and thousands arestill in use in the east.

We will discuss the operation of a wired municipalre alarm system here, since an understanding of thissystem will make it easier to understand the working ofthe city master box. Otherwise, the only way we coulddescribe the operation of a master box would be that itconsists of a “black box that works by magic”.

The box mechanism contains normally closed contactsthat are connected in series with a huge series “balloon”loop that could stretch for miles. A municipal switchboard maintains a current in the loop of 1/10 amperethat has remained unchanged for almost a century,except we now use the modern way of expressing itas 100 milliamperes. Supervision is provided via thecurrent owing through the loop.

Depending on the size of the municipality, other ener-

The ABC’s of Fire Alarm Systems - Part X

By Anthony J. Shalna © 2009 Principal IMSA Representative to the Automatic Fire Alarm AssociationPresident: Southeastern Signalmen of Massachusetts

Retired Approvals Manager: Gamewell-FCI by Honeywell

gized single stroke devices, such as gongs, electrome-chanical punch registers, etc. could also be connectedin series with the boxes. The punch registers perforatea strip of paper in accordance with the box code andthen time and date stamp the strip, giving a hard copyof the code and time of alarm. (The TV show “AmericanPickers” recently showed a punch register they boughtfor resale, and the “Pickers” didn’t have a clue as towhat it was!) Some towns even have solenoid valvesinstalled in the loop to blow large air horns. Largersystems connect all gongs, solenoids, etc. into a sepa-

rate alarm circuit rather than intermingle them with boxes. Switchboards in large cities now have electronicdecoders, but older systems in small towns might verywell have these gongs, etc. in use. A gong located ina re substation will automatically sound the code ofthe box in alarm.

The older municipal switchboards operate from aForm 4 power supply which is further dened in theNational Fire Code, NFPA 72. In brief, a large set of 24volt batteries is connected to a large battery charger thatis also suitable for use as a power supply. The chargerhas excess capacity; that is to say, if the municipal loops

require 5 amps of current for operation, the charger willhave a capacity 5 amps greater than that required torecharge the batteries in the event of a power failure.

Thus, the charger will actually power the system. Inthe event of power failure, the charger will die andthe system will continue to operate from the batteries.This is a true uninterruptible power supply! In oldersystems, solid state inverter power supplies are thenused to step up the 24 VDC to the voltage required tomaintain current in the loops.

To operate the box, one opens the pull guard and pulls

the hook that becomes exposed. This requires twohands to operate, in an effort to reduce the possibil-ity of “impulse” alarms, while the pull guard affordsprotection to the hook. The pull guard has a large openhandle that is pulled to pull the guard forward. Thishandle was originally designed to be an ice breaker, soyou could insert a mittened hand and pull the guard,even if the guard was coated with ice.

The act of pulling the hook causes the clockworkmechanism to unwind, rotating a code wheel with



November/December 2010 Page 37

The ABC’s of Fire Alarm Systems – Part IX . . . Continued from page 36

teeth cut in accordance with thecode assigned to the box. The teethon the wheel cause the contactsto open and close in accordancewith the code. Note that I said thatthe contacts are normally closed.Since supervisory current is ow-ing through the circuit, a break inthe municipal loop will cause theenergized single stroke gongs, etc.in the circuit to fall out and sounda single blow. This differentiates a break from an alarm, and indicatesthe need for an investigation bythe re department. An operated box will sound its code on all thegongs, registers, etc. In small towns,a solenoid valve connected in serieswith the loop sounds an air horn

that enables volunteer re ghtersto respond directly to the scene ofan alarm.

This code is usually repeated fora total of four rounds, althoughexceptions may occur. Some yearsago, the City of Chicago used a codewheel equipped with a double set ofcoded teeth. The wheel rotated foronly two rounds, but each revolu-tion sounded two sets of code! Dueto the large number of boxes then

in use, and heavy alarm activity,the four rounds were transmittedin half the usual time. The tim-ing of the box is specied by themunicipality. Large cities will usevery fast time, while a small townwith an air horn connected into theloop will use a very slow time toallow the horn to sound distinctive“honks”. In another almost humor-ous instance, one small town not farfrom here only used boxes that wereadjusted to sound two rounds, thereason being that their compressedair tanks only had enough capac-ity to sound two rounds of codeon the air horn! Thus, economicspreempted the National Fire AlarmCode.

The use of a series loop with nor-mally closed coding devices wouldappear to be fraught with problems, but actually, this setup is exceed-ingly well planned. The boxes have

a ground connection, and the main switchboard is usually equipped withgrounding capability. Therefore, in the event of an alarm being turnedin after a loop break, the box will automatically signal through the intactpart of the loop and the box ground, using the ground as a conductor.Thus the only thing that will stop a box alarm from going through would be multiple breaks at the same time, and even then only the boxes located between the breaks would be disabled. About the only time this couldhappen would be during an ice storm with multiple skidding accidentsall involving box knockdowns. The boxes connected to either leg of theintact loop would still operate.

The boxes signal on a series non-interfering basis. A box in alarm shutsoff any alarmed boxes further downstream from itself. Any other alarmed boxes will idle until they see a clear line, and then will signal their code inturn. A wound mechanism has the capability of sounding about 25 rounds before requiring re-winding, and will open the line if it is completelyrun down. The Fire Departments usually have a standing regulationrequiring the ofcer in charge of the response to re-wind the box. If analarmed box is unable to signal after idling for 20 rounds or so, the box

has a “bulldog” feature that enables it to take over the line, shut off anysignaling boxes, and send its code before the spring is run down. Thenon-interfering feature is so effective that in demonstrations, as many asve boxes have been pulled simultaneously without affecting their abilityto sound their codes!

This electromechanical form of signaling has proven so reliable and

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foolproof that during the great earth-

quake in San Francisco in the 1970’s, itwas the only operational emergencyreporting system in the city!

Radio Street BoxesRadio (or wireless) boxes are some-what different from the convention-al street boxes, since they transmitradio signals to the central ofce

when pulled. Radio boxes requirepower for operation, usually bat-teries, and a means of chargingthe batteries, often a solar panel

mounted adjacent to the box. Ba-sically, the radio box transmits aperiodical test signal and the cen-tral console issues a “box missing”signal if the box test signal hasn’t

been received. Thus, supervi-sion of the box is provided. Most boxes also feature a motion oper-ated switch that sends a signal inthe event of tampering or beingknocked down by a vehicle.

These boxes are most suited forareas where the conventional box

circuit cables have been allowed todeteriorate, where new develop-ments would require expensivecable installations, or even wherethe water table is very high andunderground cables wouldn’t bepractical.

The ABC’s of Fire AlarmSystems – Part IX . . .

Continued from page 37

abcs of fire alarm part 10

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