The ABC's of Fire Alarm Systems - Part XI - IMSA

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).
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 0 to 7 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
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IMSA Journal

The ABC’s of Fire Alarm Systems – Part IX . . . Continued from page 32
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
.2 0 milliamperes, voltage not being
relevant. Hence, it can be tripped by
a 12 or 2 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/ 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
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
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.
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