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The ABC’s of Fire Alarm Systems - Part XI By Anthony J. Shalna © 2009 Principal IMSA Representative to the Automatic Fire Alarm Association President: Southeastern Signalmen of Massachusetts Retired Approvals Manager: Gamewell-FCI by Honeywell MASTER BOXES proved for a shunt trip to manual stations and waterflow switches. The use of dry, control panel contacts for setting off shunt trip is also not permitted as supervision is impossible with this arrangement. In the last installment we discussed how a typical municipal fire alarm system works. Electromechanical street boxes which contain a clockwork mechanism having normally closed contacts are connected in series in a series circuit, also known as a “balloon loop”, since this is what the circuit resembles. In smaller systems, other devices may be intermingled with the boxes. The loop may include fire stations containing single stroke gongs, punch registers or solenoid valves that operate air horns. When a box is pulled, the pull hook pushes a trip lever on the mechanism downward, starting the mechanism that contains holding coils that enable it to send its code without interference from other boxes. The gongs also sound in accordance with the code, as do the other devices. Thus the code sounds on the gongs in remote fire stations eliminating the need for headquarters to relay the alarm to other stations. A break in the loop sounds a single blow on all devices to indicate a trouble condition. Any boxes placed into alarm during a loop break will signal their code via ground and the intact portion of the loop (if the main switchboard is so conditioned). Page 32 The city Master Box is basically a street box equipped with a “trip” unit which is a sort of rotary solenoid. An extension is also placed on the box trip lever, and extra terminals are provided for the connections to the trip unit. There are two types of trip units: the Shunt trip and the Local Energy trip. Their appearances are similar. The trip unit is spring loaded and mechanically set by rotating the trip lever. This “cocks” the mechanism in much the same manner as a revolver, which is then set off by pulling a trigger. The “trigger” in this case is energization of the magnetic trip coil. The spring lets go and the trip lever rotates rapidly, physically hitting the trip lever extension on the box mechanism, and thus starting the box. The shunt trips were the first to be invented and have become obsolescent. The coil in the shunt trip is actually placed in series with the city circuit. This coil is shunted out of the city circuit by normally closed contacts of the initiating device. When the initiating device goes into alarm, the contacts open and allow city current to flow through the trip coil, energizing it and starting the box. The National Fire Code, NFPA 72, limited the initiating devices ap- The shunt trip proved exceedingly popular about 50 to 75 years ago, when it was combined with a sprinkler waterflow switch to provide building protection. No control panel was required, and the city circuit furnished the operating current, having it’s own battery standby. There were no evacuation signals, of course, but property protection was secured. A few panels were developed for use with a shunt trip but they usually had either unsupervised, or AC operated notification appliance circuits at best, and were quite primitive by today’s standards. The biggest drawbacks of the shunt trip are that a tripped shunt mechanism adds resistance to the city loop, and also wiring with city current will be on private property via the normally closed contacts of the waterflow switch. This means that city personnel can’t enter private property to correct any grounds on that portion of the circuit. This would usually irritate them to no end. The usual fix was to disconnect the box from the city circuit at the nearest pole! The shunt trip also contained extra Form “C” contacts that had to be connected to a reset supervisory panel, since there was no other way of knowing that the shunt trip is unset. To overcome these problems, the Local Energy trip was developed. Continued on page 34 IMSA Journal The ABC’s of Fire Alarm Systems – Part IX . . . This trip unit also contains a coil, but the coil is connected to a dedicated, supervised city box circuit of a fire alarm control panel listed for municipal connection. The trip coil acts as an end of line device, and only one trip unit may be connected to this circuit. The LE trip derives its name from the use of “local” energy from the protected premises control panel. When the panel goes into alarm, current is applied to the Local Energy trip unit, allowing it to rotate and set off the box mechanism. One very important thing to remember is that there is no electrical connection between a LE trip and the city box mechanism or circuit. The connection is purely mechanical, as the LE trip lever spins and physically hits the box trip lever extension. Reset supervision is accomplished by means of a set of normally closed contacts in the trip unit in series with the coil. These contacts are nicknamed “suicide contacts” and open when the unit is tripped. They close when the trip is physically reset. Hence, when the unit is tripped, it will create a trouble condition on the city box circuit until reset. An interesting fact is that the LE trip unit is activated by applying current. The unit is set to trip at approximately .250 milliamperes, voltage not being relevant. Hence, it can be tripped by a 12 or 24 VDC control panel, and is not involved in battery calculations, since the suicide contacts immediately disconnect it from the circuit. At one time, the trip was also rated for higher voltage AC use, requiring 1/3 amp. of AC current. ELECTRONIC MASTER BOXES Recently, thanks to the latest electronic technology, an electronic master box has been introduced. This unit connects in place of the old electromechanical unit, often in a similar housing. It contains electronically controlled municipal signaling contacts and features twowire initiating circuits with end of line resistor that are activated by dry contacts on the control panel Page 34 that short circuit the EOL resistor. The advantage of this type of box is that it can send multiple coded signals that may be used for signaling trouble, supervisory or zoned alarm signals, as desired. Its power is obtained from the protected premises control panel. The current electronic master box is Listed for use at a great range of temperature and humidity extremes. RADIO MASTER BOXES Keltron Radio Box SigComm Radio Box Radio master boxes are somewhat different from street boxes, since they are located where plenty of power is available. These master boxes might be located in ordinary looking sheet metal enclosures and need not be located outside the building, since they are not wired to a city circuit. They somewhat resemble a conventional fire alarm control panel except the notification appliance circuits are replaced by a radio transmitter and antenna. This Continued from page 32 master box could contain any number of Class A or B initiating circuits, depending on the manufacturer, and, like the electronic master box, is capable of transmitting multiple coded signals. Connections to a radio master box are similar to those in the electronic master box. Alarms are created by shorting the initiating circuit(s) by means of dry contacts in associated fire alarm control panels or directly from detectors or stations. The radio master boxes can also send different zone signals to the central console, giving the fire department valuable advance information regarding high rise fires, special hazards, special occupancies, etc. Trouble and Supervisory signals can also be transmitted. A central office for radio signaling is the same as for radio street boxes. Redundant receiving consoles are used for “fail-safe” reasons. Test signals are transmitted periodically for supervision. A missing test signal results in a “box missing” signal being displayed on the console. With the recent economy problems, some fire departments are preferring multiple or zoned alarm signals. Ordinarily a large response is made to an alarm of fire, often with ladder trucks, etc. only to find a small fire that activated a single heat detector. Meanwhile resources are limited in the event of a simultaneous or subsequent alarm that may require a maximum response. Thus activation of a single device such as a detector or pull station will allow the fire dept. to provide a minimum response and not tie up scarce resources that might be required to respond to other alarms. Multiple zones or devices in alarm would indicate a large, expanding fire allowing the fire department adequate reserves to provide a maximum response. IMSA Journal