1 - ericssonhistory.com

Radio Alarming of Fire Brigade
ANDERSSON, LM ERICSSON TELEMATERIEL AB, STOCKHOLM
UDC 621.396.9
614.842
LME 861 852
Of the systems used hitherto for alarming of part-time staff within a fire
defence organization there have been three main types, namely sirens, signals
issued to the firemen's telephones via a signal distributor, or bells installed in
the firemen's homes and connected in series to an alarm loop.
None of these systems has been considered to meet the demands of today,
and therefore LM Ericsson Telemateriel AB, in close co-operation with
Svenska Radioakliebolaget, have adapted the ERIC ALL radio paging system,
designed by the latter company, for fire alarm purposes.
Fig. I
Keyset
The result of this team project is reported in this article.
In order to gain experience of the usability of the system for the purpose,
an ERIC ALL system was installed at the new fire station at Oxelosund during
1463 and was in full operation by February 1%4. During the period of six
months from the start of the full-scale test, detailed statistics were kept of
failure of signals, false signals, battery consumption, repairs etc. An analysis
of the statistical data showed that supplementary equipment must be added
and how the receiver should be redesigned for adaptation of a normal
ERICALL system into a fire brigade alarm system.
ERICALL — Normal System Design
The trial system consisted of an ERICALL system of normal design, which
may be briefly described as follows.
A complete system comprises a keyset, encoder with relay register and tone
generator, transmitter with aerial, and up to 392 receivers.
The keyset (fig. 1) has eight numbered buttons with associated indication
lamps.
The encoder (fig. 2) contains a relay register, AF generator and power pack.
The register is operated from the keyset and the combination of three successive
tones modulates the transmitter carrier.
The transmitter (fig. 3) is crystal-controlled and works with amplitude modulation
at a frequency of 27.45 Mc/s.
The aerial is usually of ground plan type with a radiator and three ground
plan elements directed obliquely downwards.
The receiver is sized 142 X 57 X 16 mm and issues audible as well as visible
signals in response to an alarm call. The selective calling principle is used, so
that a call actuates only the called receiver.
Operation
In addition to the eight numbered buttons with indication lamps, the control
board has a green lamp which lights when the mains voltage is switched on,
a red lamp which indicates that an alarm is being issued, and a stop button
for interrupting an alarm signal.
Fig. 2
Encoder
Each button corresponds to one of eight audio frequencies within the range
250-425 c s in the encoder. The AF code which modulates the transmitter
carrier is composed of three of these frequencies.
100