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The ABCs of Fire Alarm Systems - Part XI

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

MASTER BOXES

In the last installment we discussedhow a typical municipal re alarmsystem works. Electromechanicalstreet boxes which contain a clock-work mechanism having normallyclosed contacts are connected in se-ries in a series circuit, also known asa balloon loop, since this is whatthe circuit resembles. In smaller

systems, other devices may be inter-mingled with the boxes. The loopmay include re stations containingsingle stroke gongs, punch registersor solenoid valves that operate airhorns. When a box is pulled, thepull hook pushes a trip lever onthe mechanism downward, start-ing the mechanism that containsholding coils that enable it to sendits code without interference fromother boxes. The gongs also soundin accordance with the code, as dothe other devices. Thus the codesounds on the gongs in remote restations eliminating the need forheadquarters to relay the alarm toother stations. A break in the loopsounds a single blow on all devicesto indicate a trouble condition. Anyboxes placed into alarm during aloop break will signal their code viaground and the intact portion of theloop (if the main switchboard is soconditioned).

The city Master Box is basically a

street box equipped with a tripunit which is a sort of rotary sole-noid. An extension is also placedon the box trip lever, and extraterminals are provided for the con-nections to the trip unit.

There are two types of trip units:the Shunt trip and the Local Energytrip. Their appearances are similar.The trip unit is spring loaded andmechanically set by rotating the triplever. This cocks the mechanismin much the same manner as a re-volver, which is then set off by pull-ing a trigger. The trigger in thiscase is energization of the magnetictrip coil. The spring lets go and thetrip lever rotates rapidly, physicallyhitting the trip lever extension onthe box mechanism, and thus start-ing the box.

The shunt trips were the rst to beinvented and have become obso-

lescent. The coil in the shunt tripis actually placed in series with thecity circuit. This coil is shunted outof the city circuit by normally closedcontacts of the initiating device.When the initiating device goes intoalarm, the contacts open and allowcity current to flow through thetrip coil, energizing it and startingthe box.

The National Fire Code, NFPA 72,limited the initiating devices ap-

proved for a shunt trip to manualstations and waterow switches.

The use of dry, control panel con-tacts for setting off shunt trip is alsonot permitted as supervision is im-possible with this arrangement.

The shunt trip proved exceedinglypopular about 50 to 75 years ago,when it was combined with a sprin-

kler waterow switch to providebuilding protection. No controlpanel was required, and the city cir-cuit furnished the operating current,having its own battery standby.There were no evacuation signals,of course, but property protectionwas secured. A few panels weredeveloped for use with a shunttrip but they usually had eitherunsupervised, or AC operated no-tication appliance circuits at best,and were quite primitive by todays

standards.

The biggest drawbacks of the shunttrip are that a tripped shunt mecha-nism adds resistance to the cityloop, and also wiring with city cur-rent will be on private property viathe normally closed contacts of thewaterow switch. This means thatcity personnel cant enter privateproperty to correct any groundson that portion of the circuit. Thiswould usually irritate them to no

end. The usual x was to discon-nect the box from the city circuit atthe nearest pole! The shunt trip alsocontained extra Form C contactsthat had to be connected to a resetsupervisory panel, since there wasno other way of knowing that theshunt trip is unset.

To overcome these problems, theLocal Energy trip was developed.

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that short circuit the EOL resistor.The advantage of this type of box isthat it can send multiple coded sig-nals that may be used for signalingtrouble, supervisory or zoned alarmsignals, as desired. Its power is ob-

tained from the protected premisescontrol panel. The current electronicmaster box is Listed for use at a greatrange of temperature and humidityextremes.

RADIO MASTER BOXES

Radio master boxes are somewhat

different from street boxes, sincethey are located where plenty ofpower is available. These masterboxes might be located in ordinarylooking sheet metal enclosures andneed not be located outside thebuilding, since they are not wiredto a city circuit. They somewhatresemble a conventional re alarmcontrol panel except the noticationappliance circuits are replaced by aradio transmitter and antenna. This

The ABCs of Fire Alarm Systems Part IX . . . Continued from page 32

This trip unit also contains a coil,but the coil is connected to a dedi-cated, supervised city box circuit ofa re alarm control panel listed formunicipal connection. The trip coilacts as an end of line device, and

only one trip unit may be connectedto this circuit. The LE trip derives itsname from the use of local energyfrom the protected premises controlpanel. When the panel goes intoalarm, current is applied to the LocalEnergy trip unit, allowing it to rotateand set off the box mechanism. Onevery important thing to remember isthat there is no electrical connectionbetween a LE trip and the city boxmechanism or circuit. The connec-tion is purely mechanical, as the LE

trip lever spins and physically hitsthe box trip lever extension. Resetsupervision is accomplished bymeans of a set of normally closedcontacts in the trip unit in series withthe coil. These contacts are nick-named suicide contacts and openwhen the unit is tripped. They closewhen the trip is physically reset.Hence, when the unit is tripped, itwill create a trouble condition on thecity box circuit until reset. An inter-esting fact is that the LE trip unit isactivated by applying current. Theunit is set to trip at approximately.250 milliamperes, voltage not beingrelevant. Hence, it can be tripped bya 12 or 24 VDC control panel, and isnot involved in battery calculations,since the suicide contacts immedi-ately disconnect it from the circuit.At one time, the trip was also ratedfor higher voltage AC use, requiring1/3 amp. of AC current.

ELECTRONIC MASTER BOXESRecently, thanks to the latest elec-tronic technology, an electronicmaster box has been introduced.This unit connects in place of theold electromechanical unit, oftenin a similar housing. It containselectronically controlled municipalsignaling contacts and features two-wire initiating circuits with end ofline resistor that are activated bydry contacts on the control panel

master box could contain any num-ber of Class A or B initiating circuits,depending on the manufacturer,and, like the electronic master box,is capable of transmitting multiplecoded signals.

Connections to a radio master boxare similar to those in the electronicmaster box. Alarms are created byshorting the initiating circuit(s) bymeans of dry contacts in associatedre alarm control panels or directlyfrom detectors or stations. The radiomaster boxes can also send differentzone signals to the central console,giving the re department valuableadvance information regarding highrise res, special hazards, special oc-

cupancies, etc. Trouble and Supervi-sory signals can also be transmitted.A central ofce for radio signaling isthe same as for radio street boxes.Redundant receiving consoles areused for fail-safe reasons. Testsignals are transmitted periodicallyfor supervision. A missing test signalresults in a box missing signal be-ing displayed on the console.

With the recent economy problems,some re departments are preferringmultiple or zoned alarm signals.Ordinarily a large response is madeto an alarm of re, often with laddertrucks, etc. only to nd a small rethat activated a single heat detector.Meanwhile resources are limitedin the event of a simultaneous orsubsequent alarm that may requirea maximum response. Thus activa-tion of a single device such as adetector or pull station will allowthe re dept. to provide a minimum

response and not tie up scarce re-sources that might be required torespond to other alarms. Multiplezones or devices in alarm wouldindicate a large, expanding re al-lowing the re department adequatereserves to provide a maximumresponse.

Keltron Radio Box

SigComm Radio Box