Post Office 'System E'

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'. The General Post Office provided all UK landline communication at this time.

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.

System E Apparatus

System E Apparatus Click for Enlargement

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 Lefthand Image) 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.