By Kevin Sanderson


From the early sixties until the early nineties at precisely 9 o’clock, the frequency of the tones used to supply the standard GPO time service TIM‘s 3 'pips'– ('at the third stroke') – changed from its normal 900hz to 2500hz.

This relatively small and normally unnoticed change in the tones was part of the engineering test and monitoring process of Britain’s 'semi secret' telephone line based Nuclear Early Warning System.


The system was not military however and its sole purpose was to warn the general populace (you and me) of impending doom! The government publication UKMWO stated "As far as the system described in these pages is concerned, experts have calculated that its contribution could amount to the saving of as many as six to ten million lives - simply by warning enough people in time!"

Whilst the system and its workings are no longer 'secret'– (the entire system was dismantled in 1992 at the 'end' of the Cold War) – I do not intend to cover any political aspect but merely try to explain its workings and hence its part in our telecommunications heritage .

The System consisted of two distinct parts, that which communicated the attack warning from RAF Strike Command to the control points (Police Stations) and that which passed the warning to the thousands of warning points across the country.

Following World War 2, the early basic system was known as SYSTEM E. It used DC signaling along private wires from control points (normally Police Stations) to the terminal Points (Sirens) .

Each control point housed a Home Office supplied device the 'Autowailer' this was a motor driven device used to supply the timing of the 3 main signals 'Air Raid' 'Raiders passed – all clear' and 'stop' The GPO lines were solely used to communicate these signals to the Siren equipment.

In its normal state the A and B wires are connected to earth at the control point, at the terminal end the A wire is connected to earth and the B wire to negative 24 volts. The line relay 'C' on the B wire remains operated and contact C1 holds signaling relay 'B' in an un-operated state. Relay 'A' at the control point is also operated and this is used to detect any line faults or power failure of the equipment at the terminal end.

In its operated state the earth on the A and B wires at the control point are replaced with positive 24 volts and negative 24 volts respectively, this releases the 'A' and 'C' relays operates the 'B' signaling relay at the terminal end a contact of which is then used to switch the 3 phase mains supply to the siren. Wherever possible the equipment at the terminal end was located inside a building, normally with the siren on the roof , however in some locations a 'street cabinet' was used , painted green similar to GPO cabinets but looking more like 'power' cabinets they were 4 ½ feet high by 5 feet wide and 1 foot deep.

The photo shows a siren control cabinet still existing at the side of the road in South London

Within the cabinet can be seen various items, top left is the siren control panel, top right is the GPO equipment and at the bottom of the cabinet is the main power equipment and switchgear a mains powered heater was also installed. Above the siren controls can be seen a retro fitted 'WB1400 receiver' (a WB600 would have been mounted here until the early eighties) as this cabinet remained in use right up until 1992! (more details later).

In the early sixties SYSTEM E was phased out and replaced with a carrier based system (carrier systems use signals not normally audible to the human ear 'superimposed' over the top of normal speech signals so as not to interfere with normal conversation or signaling). The early carrier systems used on 'line' links were known as 'WB' or 'Wire Broadcast' systems.

The System for alerting the 'Carrier Control Points' (the same Police Stations used by SYSTEM E) of which there were 250 spread across the country, was known by the code word 'HANDEL' and was cleverly designed to be sent 'over the top' of the existing network around the country that was used to distribute the 'TIM' time signal , the reason for this was two fold, one: the network already existed and would therefore save time and money and two: it was a monitored network and would therefore need no separate fault monitoring or reporting should anything go wrong.

The System was then effectively split into two the latter part of the system connecting the carrier control points to the some 18,000 warning points over 'normal' subscribers lines using the same philosophy of fault reporting as above – i.e. the customer would report any faults on their line, therefore negating the need for any monitoring system (unless they worked for the GPO the unsuspecting subscriber had no inkling their line was connected to such a system).

The Carrier Control Points were also connected to some 4000 'warning recipients' (Civil Defence HQ’s, Hospitals and the public utilities etc.) and nearly 900 ROC (Royal Observer Corps) underground posts all of which had 'carrier receivers' in order to receive the broadcast warning.


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Last updated 9th January 2005

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