Pre- Wire Broadcast, Siren Control Systems

Before WB600 was introduced to control the UK Air Raid sirens in the early 1960's they were activated by system designated as Post Office 'System E'. This was just one of a number of siren control systems, but the most important due to its widespread use prior to the introduction of the carrier system.

The History of Siren Control Systems

The BT Archives contain a series of reports compiled by the GPO Engineer-in-Chief. These document the developments immediately post WW2 up to the development of a carrier system, which is described separately in this HANDEL section of the website. The most informative is the 1950-51 report.

During the last war, the P.O. was responsible for the design and development of a number of systems for the remote control of public air raid warning sirens from a central point.

In the immediate post war period, most of the equipment was scrapped but a certain amount of it was adopted, at the request of the Home Office, for the purpose of controlling bells and sirens used for calling out firemen in rural districts, thus dispensing with watchkeepers. The various systems developed for this purpose became known subsequently as Remote Control Systems A, B and C. They were all designed for operation over private circuits and were dependant upon the availability of A.C. mains supplies at the controlling points.

Readers may not be aware that prior to the introduction of firemen's personal radio pagers in the seventies, retained staff had a bell fitted in their house to call them to the fire station to man the appliance. During daylight hours when they could be away from home a siren was used to make them aware of the callout. Retained stations are mainly in rural areas whereas major towns and cities have a full time crew on standby 24/7.

The report goes on to say. More recently, there have been a number of further developments to relieve the Fire Services of the burden of private circuit rentals and to secure independence of A.C. mains supplies at the controlling point. At about the same time, there was a pressing Home Office demand for a restoration of the public warning system.

In the light of experiences during the last war, it was decided that an improved method of control was essential. various proposals were examined and the following is a summary of the main lines of the development.

System D

The 1950-51 report says, This system for controlling fire bells and sirens is intended, primarily, for villages served by automatic exchanges where the fire staion is unattended. It provides for the remote operation of firemen's call bells and the operation of the "call-out" siren during the day. Calls are originated by an ordinary telephone call from an attended fire station and a special voice-frequency signal is transmitted.

Temporary equipment, made up from war surplus components, has given a useful degree of service in 50 experimental installations. A development contract has now been placed for a new and improved design to eliminate the weaknesses which came to light as the result of experience with the temporary equipment.

System DX

First mentioned in the 1951-52 E-i-C report This new and improved design of System D is intended, primarily, for controlling firemen's call bells and sirens in small towns served by automatic exchanges. . . . A prototype system has been tried out successfully by the Fire Service, and Home Office approval has been given. An order for 200 systems has been placed.

System E

The operation is fully described in the topic below, but the 1950-51 E-i-C Report sheds light on its development. This is a new system which has been developed, and tried out successfully by the Metropolitan Police, for controlling air raid warning sirens. To avoid the production and maintenance difficulties inherent in the use of 100 c/s. signalling equipment as used in the war-time equipment, the system has been designed to use positive battery signalling. Continuous line test and answer-back facilities are provided, and safeguards against false operation by contact with other circuits, due to line faults, have been incorporated.

Before the sixties much of the line plant was carried on overhead poles using uninsulated copper wires. High winds and contact with trees could cause these wires to touch. While this may cause a telephone bell to tinkle annoyingly, it was undesirable for a similar fault to set a siren off especially if it didn't stop for hours.

The reference in the 1950-51 report to unreliability of the 100 c/s signalling for siren control during WW2 is borne out by a number of documents held in the BT Archives. These are research reports into improving certain components in the design made by the staff at the P.O. Research station at Dollis Hill. TCB/422/11222 titled "100 c/s Generator with Increased Output for Remote Control of Sirens" and TCB/422/11204 titled "Tuned Mercury Relays for the Remote Control of Air-raid Sirens" both dated January 1941. These are not particularly interesting documents, however the co-author is Mr T.H. Flowers, the gentleman credited with designing the code breaking 'Colossus' computer at Bletchley Park. The Colossus Mk1 first ran at Dollis Hill in November 1943, 35 months later. Read about the great man of WW2

The next year the 1951-52 E-i-C Reports quotes: Pending the development of further air raid warning signals, the Home Office has decided to reinstate remote control systems to provide the 'Alert' and 'Raiders Passed' signals used during World War II. For this purpose, System E has been selected and manufacture is in hand. The necessary information for installing this system has been issued.

Installation progress is is reported by the 1952-53 E-i-C Report: Most of the apparatus for System E (see last report, page 16) has now been manufactured. Installation is now proceeding throughout the United Kingdom on schemes down to Stage V of the national plan . . . . The Post Office part of the work has generally been well ahead of that for which local authorities are responsible. . . .

By the time of the 1953-54 Report, the development of the Carrier System is underway and thoughts move towards a national warning system, similar to Handel. The installation of the main program of Remote Control System E in the larger towns is now nearing completion and consideration is being given to meeting the requirements of smaller towns and villages. It is possible that a simplified, less expensive system will be adequate.

The requirements for an audio network to link System "E" and Carrier Distribution centres to the primary sources of information have been discussed with the Home Office and the operation of equipment which would be suitable for use in such a network has been demonstrated.

The primary source of the national warning is the Home Office, Warning Officer, co-located at the Air Defence Operations Centre, but this fact is not mentioned by the report.

System F

The 1950-51 report says, although this system was developed it sacrificed facilities of system 'E' and was rejected by the Home Office.

System G

The 1950-51 report describes the system: This is an alternative to System D for use in manual exchange areas only. The voice frequency signals used for system D are liable to interruption by manual exchange operators. At this time, there were many small manual exchanges still in service. It wasn't until the seventies that the last manual public exchange was replaced by an automatic system. The report goes on to say An initial provision of 50 sets of equipment has been recommended pending a Home Office review of actual demand.

System K

Described in the 1950-51 report as This is a new system designed to supersede Systems A, B and C for the control of firemen's call bells and sirens over private circuits. By using positive battery in lieu of 100c/s signalling, the system is made independent of the availability of A.C. mains. In those days, some places were still fed by D.C. mains supplies.

. . . . Opportunity has been taken to incorporate the various other facilities demanded by the Fire Services, e.g. speech over the private circuit (for reporting purposes), for dealing with calls originated by the public at the unattended station. . . Fire Stations have a telephone outside so the public can speak to the central Fire Service operator to make a Fire call. This is known as the running call telephone, as the unfortunate person would run to the Firestation to summon help. At that time very few members of the public had landline telephones and had to rely on public phone boxes.

By the time of the 1952-53 E-i-C report manufacture is proceeding on the first order for equipment for this system, which for new work, will supersede the earlier systems based on the use of apparatus originally designed for air-raid warning purposes in the 1939-45 war.

System L

This is the last system described in the 1950-51 report. Certain sirens used during the last war were provided with shutters at both ends, each of which could be operated separately, to provide additional warning signals. These shutters were operated by D.C. signals

To meet the possibility that similar facilities might again be required, a modification of System E has been developed and is known as System L. This provides for an additional signalling path, without sacrificing the other facilities given by System E. Both shutters are closed simultaneously; to allow either to be closed separately, a third signalling path would be required and experimental work has started.

The 1951-52 report says A prototype System L has successfully completed a trial by the Metropolitan Police. Experimental work to develop a system which will give facilities for the independent operation of two shutters as well as the siren motor is still in hand.

The simultaneous operation of both shutters were going to be used to give a 'Grey Warning' when fallout is expected in one hours time, but the idea was dropped and not included when HANDEL came along. Independent operation of each end will allow a two tone Dee-Dar-Dee-Dar sound, apparently used as a Broadmoor prisoner escape warning signal.

System E Siren Control System


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 electromagnet Pulse (EMP), which would have destroyed the carrier system's Germanium transistor circuitry. This may seem strange, research shows that EMP was not known about when WB400/WB600 was designed.

Many thanks to Chris Middleton for providing the information in this section.

Detailed Description of System E

The control systems for Fire Brigade call-out sirens ( e.g. system D ) used tone signalling but System E used Direct Current Signalling. It may be that this was thought to be simpler and thus more robust.

System 'E' Schematic

System E Network Diagram

The Post Office part of the system allowed for a signal from a "Central Station" to be sent to one or more "Terminal Stations" (Sirens). The line length of each circuit was limited to that which had a loop resistance of 1500 ohms. The signal could pass through one or more "Intermediate Stations". An intermediate station would receive the signal from a central station or even from another intermediate station and would repeat it out to further intermediate stations or to terminal stations. An intermediate station could have an optional Autowailer but this would only control its own sirens and those of any dependent stations.

The terms "Central Station", "Intermediate Station" and "Terminal Station" are used for Post Office engineering purposes, the customer (Home Office in England and Wales) may use other terminology such as Main Control Centre, sub-control centre to distinguish between central and/or intermediate stations. The Central and Intermediate stations are electrically identical but at the central station the Incoming Set (Blue Box in diagram) isn't used. In the diagram opposite, only four "Outgoing Sets" are shown (White Box) but in practice the number would be adjusted to meet the needs of the area The Main Control Centre may well be a Police Station where Home Office apparatus (Autowailer) would be installed to generate the signals to the Post Office equipment.

The Terminal Station, has an "Incoming Set", (shown as a blue box) and an Autowailer provided to give local control should the GPO lines fail. 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.

The System E Control Station apparatus (See Image Gallery) is mounted on a wallboard, with a test panel at the bottom for isolating line faults and has buttons to silence the alarm bell and provide "Hand Control" of the siren activation. Above this local control and fault alarm set. Above this is the Incoming Set, which is identical to that used at a Siren Station. Above this are mounted the Outgoing Sets, each strip can hold three sets but they can be sub-equipped with a single or two sets to meet the demand in the area.

Normal Condition

Not Operated Line Signals

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

Siren Operating Line Signals

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

Siren Street Cabinet

Layout Diagram of Siren Street Cabinet

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 Cabinet" could be used to house the equipment.

A Home Office autowailer was included at the siren site to allow local operation of the siren should the Post Office lines fail or the control room be put out of action. This feature was retained during the subsequent upgrades.

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