Within the WB600 receiver, the 72Khz carrier and associated tones were amplified and filtered by electronic circuitry, the filtered 'G' pulsed tone was then detected by a cleverly designed 'Pendulum Relay' the pendulum action prevented the system being inadvertently set off by speech signals on the line. Approx. 6 seconds of pulsed 'G' allowed the pendulum relay to swing enough to make contact, this operated the 'S' tone circuit. A relay in the 'S' tone circuit then subsequently operated the siren mains supply contacts.

Mounted adjacent to the WB600 receiver was also (as per SYSTEM E) a Home Office supplied 'Autowailer' for manual operation of the siren .The tones generated by the WB600 system could also be heard on the WB400 receivers at warning points. Before the lines from the Police Station reached the remote sites, they of course passed through the local Telephone Exchange serving the Station .The CCE (Carrier Control Exchange) equipment, consisting of Filters and Combiners for the 'TIM' speaking clock feeds, amplifiers, fault monitoring equipment and ongoing distribution units were housed in a large grey or light straw cabinet. The bottom half of the cabinet had a separate section containing batteries for back up use. The cabinet was constructed of heavy steel plate with an airtight seal around a bolted down cover, all cables fed into the cabinet in a large cast iron pipe – strangely however once the cabling reached the cable 'grid' atop the Strowger (later TXE 4) equipment the unprotected cables then wended their way across the grid to the MDF, power feeds and the 'TIM' network! – many engineers never could see the irony of this arrangement.

WARNING POINTS
At each of the remote sites ( Warning Points, Warning Recipients or ROC Post/Control) the line terminated onto a filter and then onto the 'Receiver Carrier WB400' This grey metal cased unit was the part of the system visible to the public.

RECEIVER CARRIER WB1400
(ALONG WITH A RECEIVER SIGNALING WB 1400/1401) THIS REPLACED THE RECEIVER CARRIER 400A

It was also of robust construction, with a cork seal and plastic membranes protecting the speaker, the interior contained a desiccant. (Although how this would have stood up to a nuclear blast was thankfully never put to the test!!) The unit had a drawer (similar to 200/300 type telephones) that contained an instruction card to be used in time of war! The reverse of the card gave routine testing details . When turned on but not receiving any signals/messages the unit gave a regular pulsed 'tick' known as the 'confidence tick' basically to let the recipient know it was on and working.

LATTER DAYS
In the early Eighties the entire system of WB600 and WB400 was replaced and updated with a combined system the 'WB1400'. The earlier WB600/400 equipment had never been designed to be protected against an effect of a nuclear explosion called the EMP (Electro Magnetic Pulse). This pulse could wreck all electronic equipment within a large radius by destroying all semiconductors (transistors) within.

Equipment could be protected from harm by a 'Faraday Screen' (basically an earthed metal case). The combined WB 1400’s electronics were contained in large sealed metal cases (shown in the photographs). All the equipment mentioned earlier from the Police Station, both the WB401A/400A and all the cabinet mounted apparatus in the equipment room were replaced with one single unit the 'Equipment carrier WB1400"'

At the warning points the WB400 Carrier Receiver was replaced with a separate metal cased receiver and loudspeaker unit 'Receiver Speech WB1400 and Loudspeaker unit WB1400'.

At remote siren sites the WB600 unit was replaced with an identical looking 'Receiver Signaling WB1400' but no loudspeaker.

WB1400 The later WB1400 system used the same 72 kHz carrier and signaling as the earlier WB400/600 so a relatively seamless changeover could occur. The main differences were the aforementioned EMP protection, line powering to eliminate the need for constantly changing batteries and extra signaling, (including "flood" warnings).

The WB400 receiver was powered by a battery, which often went flat, the units having been left on in error, (the units should only be turned on for use or testing ).

The WB1400 receiver however was permanently 'on' in standby mode, the internal battery being 'trickle charged' by a line current. An extra tone signal (pulses of 620 Hz) sent from the WB1400 control point would 'alert' and turn on the receiver ready to receive further signals or speech.

RECEIVER SIGNALING WB1400 MOUNTED SIDEWAYS IN CABINET

ROC POSTS
Lines installed into ROC (Royal Observer Corps) posts also had means by which the observers could communicate directly to their Group control and other posts. The equipment which shared the same pair of wires as the WB equipment was known as 'Teletalk'. This was an 'intercom' system permanently on receive that required the pushing of a button on the unit to speak.

Early WB400/600 era units were known as 'Unit Intercom AD 3460' (AD stood for Air Defence) and used two batteries 67.5v for signaling and 6v for speech. The later WB1400 era version 'Loud speaking Telephone AD 8010' was line powered. Both units required the use of a filter between itself and the WB carrier receiver.

EQUIPMENT CARRIER WB1400
THIS REPLACED EQUIPMENT CARRIER 401A AND 400A

Many of the lines to ROC posts were only live when required and during testing. Known as EC or Emergency Circuits the lines were switched or turned on by either engineers or traffic staff. Each exchange that served a 'post' was equipped with a 'Unit Observer Post Switching', a written form THQ 2775 was submitted to the GM (general manager) of the respective telephone area by the ROC in order to request the lines to be turned on!

This believe it or not was a 'cost exercise' the ROC not being charged for the lines when not in use !

FLOOD
In areas that so required it the system was also equipped to warn of flood. The originally separate WB601 Flood warning system was incorporated into the WB1400 system, although in some areas (especially London and the Thames) 'WB1401' carrier receivers were used where the flood siren was unconnected to the Air Raid system (confusingly '1401' was also used to describe a more robust speaker unit 'loudspeaker unit WB1401' used in damp areas in place of the normal 'loudspeaker unit WB1400').

The flood signaling used the same network and carrier, with differences to the signals and timing (of the siren) In flood alerts the 'G' signal was sent for a pulsed 115mS on and 115mS off. This part of the system was controlled from the Police Station 'Carrier Control Point'.

For more information see Steve Scanlon's web site
The UK Nuclear Attack Warning System

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