222 Fire Warning System
The system was not accessed using 999 as this could so easily be muddled up with the subsequent actions on finding a fire which would involve dialling 999 on a BT line. 222 was chosen as the number to set off the warning tone as level 2 was spare, cabled out already to the IDF and could be barred to off site phones using the coin box discrimination feature of the group selectors.
When 222 is dialled, the group selector steps to level 2 and seizes the 222 relay set. The next two digits are counted on a uniselector in the relay set. Should the digits be anything but 22 the relay set returns NU tone to the caller. If the digits are 22, then the relay set calls the PA via a line circuit of the Telecoms to PA Interconnection Equipment.
The shop staff can stop the shop bell, make an announcement over the PA (temporarily over riding the warbling tone) and if required, stop the warning completely. The shop staff also have the facility, by pressing two start buttons simultaneously, to start the alarm without the need to dial 222.
If the alarm is not stopped it will switch off after about a minute. The caller can hang up once he has initiated the warning tone and this leaves his phone free for use to report the fire to a central point. If he does not hang up then his line will be released when the relay set has timed out and stopped warbling. Should the PA be in use for music or a telephone access call, then the fire warning tone will have preference.
So long as the relay set is free, it connects a battery condition to the P wire. A loop extended from the group selector will operate relay A.
BA2 starts the ringer.
When the caller dials, A1 operates relay C with the magnet current and steps the wipers at the end of each dial pulse. Relays B and C hold during the dial train as they are slow to release.
When the caller dials the last digit, relay C again operates and holds to the magnet current and the uniselector wipers step on from outlet 2.
Relay ALR operating sets off the alarms via sets of contacts that extend conditions to external bells and lamps.
Once every revolution of the TM wipers, relay PU is operated briefly. Note that should there be any failure of the TM circuit a time pulse is connected to the second PU winding and this will ensure that the tone does eventually switch off.
PU1 and PU2 step the main uniselector every time PU is operated. This step occurs about every ten seconds.
Eventually the earth on wiper 5 causes a short circuit of the B and AL relays, which release.
The relay set is now restored and ready for further use.
Note that if the caller had hung up while the relay set was operational, relays A and B would have released. This would have taken the earth off the P wire and released the caller. Relay BA would though have been held by contact ALR5 and maintained the relay set until it had completed its period of tone warbling. It would then have released and made the P wire free again by reconnecting the battery at WS6 or the homed uniselector wiper.
Note that the shop can start the relay set warbling by looping the "start alarm keys" wires. This would operate relays BP and in turn BPR, BA, AL and ALR to start the warbling sequence.
Note that the shop "stop alarm key" will place a short circuiting earth on to cause the release of relays B and AL and therefore the stopping of the warbling sequence.
The following diagrams are available in PDF form :