System E Siren Control System
Introduction
The forerunner of Cold War WB600 was System E, a system of using direct current signalling along dedicated wires from the Police Station to the Siren Point. This was much simpler than the carrier system that replaced it and would not have been prone to EMP, which would have destroyed the carrier system's Germanium transistor circuitry. This may seem strange, one can only assume that EMP was not known about when WB400/WB600 was designed.
Many thanks to Chris Middleton who provided this section.
System E
The control systems for Fire Brigade call-out sirens ( e.g. system D ) used tone signalling but System E used DC. Signalling. It may be that this was thought to be more simple and thus more robust.
The Post Office part of the system allowed for a signal from a control station to be sent to one or more terminal stations ( siren sites ). The line length of each circuit was limited to that which had a loop resistance of 1500 ohms. To cater for longer lines, one or more "intermediate stations" could be used. An intermediate station would receive the signal from a control station or even from another intermediate station and would repeat it out to further intermediate stations or to terminal stations.
The "control station" may well be a police station where Home Office apparatus ( Autowailer ) would be installed to generate the signals to the Post Office equipment.
The siren at the terminal station could be an exclusive Air Raid warning siren or could be a siren normally used by the Fire Brigade to alert its retained firemen. A shared siren would normally be switched to Fire Brigade use and would need to be switched over to Air Raid use as circumstances dictated. An exclusive siren would normally have its motor fuses removed ( to prevent false alarms ? ) and the fuses would have to be inserted to make ready the siren if necessary.
The Post Office System E only relayed an "ON" signal from the Home Office control equipment out to siren points. It was not concerned in the generation of the signal in the first place. The use of line signalling and monitoring relays allowed for some line faults to be notified to the control station together with "power failed" alarms from the remote sites.
Normal Condition
The normal line condition is shown above. The B wire has -24 volts via a relay at the remote site and an earth via a relay at the control site. The line relay at the remote site is operated and it holds the signalling relay disconnected from the A wire. The relay at the control site, which is also operated, is monitoring for line faults. It will release if there is a fault such as A/B reversal, Earth contact, power fail at the remote site etc.
Operate the Siren Condition
The control equipment is provided with a "Hand Control" switch which can be used to operate the control equipment. This switch is on the Post Office equipment. The more normal operation would be a signal from the "Local Control" apparatus which would be an Autowailer provided by the Home Office.
The Home Office Autowailer is an electric motor driven, timer device with three buttons on the front panel. Attack, Raiders Passed and Stop. It generated timed siren "ON" signals
The "ON" signal can be of any duration over one second, thus an Attack Warning of 4 sec on / 4 sec off from the autowailer would cause the remote equipment to repeat that signal to the siren contactors. Note: A contactor is a heavy duty relay connecting the 3 phase 415 volt mains supply to the siren motor. Similarly a one minute permanent "ON" signal from the autowailer would be sent as an all clear signal.
To send an "ON" signal....
The control equipment replaces its earth on the B wire by a 24 volts negative. This causes the line relay at the remote site to release thus connecting the signalling relay to the A wire. The monitoring relay at the control site also releases. The control equipment sends 24 volts positive out on the A wire and this operates the signalling relay at the remote site. As this relay operates, its contact removes the 24 volts negative that it was sending on the B wire and replaces it with an earth. This re-operates the monitoring relay at the control as an indication that the remote equipment has operated.
Remote Site Layout
Where possible, the remote equipment was fitted in a building which carried or was near to its siren. If no suitable building existed, a "Street Pillar" could be used to house the equipment.
