This topic describes the first incarnation of the HANDEL Nuclear Attack Warning equipment known as Wire Broadcast System WB400 and WB600 Siren Control. The introductory page "HANDEL Warning System" should be read beforehand to gain an understanding of the context for WB400 and its successor WB1400.
HANDEL for WB400
HANDEL Control Circuits
At the time of the introduction of WB400 and HANDEL, the speaking clock was sourced from telephone exchanges in London and Liverpool with two duplicate clocks at each location. Each clock was a mechanical device based around multi-track magnetic recordings, replaying small sections of the time message all strung together to form a complete message every ten seconds.
In the first generation of HANDEL used with WB400, any warning message would be prefixed by P+Q tones of 2400Hz and 2600Hz to set off the alarm in the police station control panel. For engineering test purposes, at 08.30 GMT precisely every morning, the usual 1000 Hz pips of the speaking clock were replaced by ones of 2500 Hz, chosen to be midway between the P and Q frequencies. Telecomms Instruction E9 E2091 states these special pips should measure -5dBm on a peak program meter to ensure the apparatus functions correctly.
In 1963, each of the two control centres, the ADOC and UKWMO Preston, had a Private Wire ( PW ) to each of the two speaking clock sources located at Kelvin House, Judd Street, London WC1H 9NP and Liverpool Central, Lancaster House ATE, Old Hall Street, Liverpool L3 9PY This was later increased in 1967 to two PW's to each clock source as shown in the diagram. The GPO circuit diagrams for AT60281, AT60282 and AT60283 refer to them as being part of the speaking clock distribution, but with no mention of HANDEL (for security reasons).
To make a broadcast, a control centre operates a speak key, [AT60282] sending a burst of P+Q tone along each the Private Wire ( PW ) to activate the receiver [AT60281] starting a sequence in the switching equipment [AT60283] to disconnect the clock and connect the control centre to the distribution rings. Before switching over to the control centre PW voice circuit, the equipment sends a burst of P+Q out over the distribution rings to trigger the police station alarm.
At the end of the broadcast, releasing the speak key, sends a burst of P+Q tone followed immediately by a burst of Q tone along the PWs. Neither of these burst of tone make it out to the distribution rings as the transmission path is cut by the receiver [AT60281] in the clock source switching equipment. The speaking clock is then restored.
The switching control receivers used for HANDEL, are modified receivers originally designed for use on overseas telephone circuits in the Trans-Atlantic cable ( TAT ) which used the same tones for signalling, they didn't reinvent the wheel.
Carrier Control Point (CCP) - WB400 Era
Police Operating the WB400 CCP
This photograph taken from the UKWMO booklet shows the control panels at the Police Station being operated by a sergeant. This was the business end of the equipment comprising the Carrier Control Point (CCP)
The control panels in the office are wired back to the telephone apparatus room in the police station. In the apparatus room were two steel cabinets, each 1ft 2in deep x 1ft 10in wide x 6ft high, these are shown further down in this topic and contain the electronics shown CCP Block Schematic also below.
One cabinet contains a spare Voice Broadcast Message Controller, which can be moved into the office should the working unit go faulty. If the office is damaged by the attack, it can be operated from the cabinet if necessary.
Carrier Control Point Control Panels
Operating Unit WB401A - HANDEL, ROC Group Telephone and Siren Control
This exhibit from the British Imperial War Museum, is the Operating Unit WB401. The unit has X and Y HANDEL handsets with their associated alarm lamps and warning sounders, for listening to the national warning messages. The Black handset is for two way conversation with ROC Group Headquarters and has white alarm lamps and sounder, with a white flick switch for calling group.
The centrally mounted plate, the Control Unit WB600, has the siren master key lock, three illuminated siren buttons and a test button. Behind the grille is an earpiece so the siren control signal tones can be heard by the operator.
Operating Unit WB400A - Voice Broadcast Message Controller
This photograph of the Operating Unit WB400 Mk2 was kindly provided by Gavin Saxby, is one of two units at each CCP, the spare being housed in one of the equipment cabinets. This unit has two keys to control the Call and Warning signals that precede a warning message.
A Speak key, activates the microphone which is normally housed behind the drop down cover. The meter indicates the correct voice level when broadcasting messages.
The Mk1 version of this unit had some extra switches on the front panel, but in this Mk2 version they are in a covered recess on the rear.
Block Diagram of the CCP
CCP Block Schematic
The CCP is connected to the Control Point Exchange (CPE) by two pairs of wires, shown leaving the right hand side of the schematic. These are identified as the X and Y paths. Each pair carries audio messages from Strike Command, distributed via the speaking clock system and returns the Carrier signal back to the CPE for distribution around the area.
The two grey (with Red Tape) HANDEL handsets, mounted on the "Operating Unit WB401A" are connected via amplifiers to the two audio feeds from the Control Point Exchange. To alert the Carrier Control Point, two simultaneous tones of 2400Hz and 2600Hz known as P and Q tones, precede a warning message, the P+Q detector triggers the lamps and sounder above the HANDEL phone. The alarm continues until one handset is lifted. Being a broadcast system there is no facility to talk back to Strike Command or between Carrier Control Points. In peacetime, the speaking clock can be heard on the X and Y path HANDEL phones.
Not shown on this block schematic is the third Black handset connecting the CCP with the UKWMO Group Headquarters warning switchboard. Fallout messages from the Group are relayed onwards to the warning points. The police station can initiate a call to the ROC by operating the call button. In either case, the police can have a two way conversation with Group.
Warning messages are broadcast over the WB400 system from the "Operating Unit WB400A", consisting of a microphone, modulation meter and a number of keys on a sloping fronted box. The keys send pulsating tones before each spoken message. One sound was known as "Call" and the other "Attack Warning" the later must not be confused with the attack warning siren control signal which is something completely different. A "Monitor Tick" sound, like a ticking clock, is generated when not in use. Further down this page, a recording of a routine test message carries all these signals.
The signals are specified as : Monitor Tick, 12 cycles of 1 kHz with a pulse repetition rate of 90 per minute. Call Signal : A Dot of 12 cycles and Dash of 50 cycles of 1 kHz tone repeated at the same speed as the monitor tick. Attack Signal : 1 kHz signal varying in frequency 140 cycles per minute.
The "Operating Unit WB400A" is normally plugged into the "Working" multiway connector wired back to the working side of the carrier generation equipment housed in an "Equipment Carrier WB404A" located in the police station's telephone apparatus room. Another multiway connector marked "Reserve" is wired back to the reserve side of the carrier equipment. In case of a fault develops, the office's operating unit can be unplugged from the working and plugged into the reserve side connector. The spare "Operating Unit WB400A" housed in the "Equipment Carrier WB404A " is kept plugged into the reserve side. This can be used in situ or moved into the police station office.
The output from both Working and Reserve carrier generators are monitored, if the Working side fails, the Reserve will be switched in automatically to maintain the carrier. This protected carrier feed is divided and sent out on the two separate paths (X&Y) to the Control Point Exchange.
To sound the power sirens, it is necessary to turn a key-switch on the Siren Control Unit (Control Unit WB600A) mounted on the wall mounted Operating Unit WB401. The master key is usually kept in a 'break glass' box by the office. The circuitry is designed so each time a siren button is pressed each of the two tone sequence generators will take it in turns to generate the WB600 control signal. A small sounder behind the grille on the unit allows the user to determine if the sequence was running. Therefore if on pressing a siren button, a faulty generator means the tone sequence is not heard, pressing the button a second time will engage the other sequence generator. The active generator signal is fed both the Working and Reserve WB400 Modulators.
As a safeguard against failure of the single unduplicated siren control panel in the Police office each of the duplicated sequence generators in the equipment cabinet had its own set of buttons and key-switch for local control from the police station's telephone apparatus room. A full description of the siren control sequences can be found in a separate topic further down this page.
CCP Equipment Cabinets
The electronics needed to make the CCP control panels work were very bulky and heavy by today's standards, the total weight being approximately 900 lbs. (8 cwt / 400 Kg).
The two cabinets were usually located in the telephone apparatus room although in the case of the CCP featured here, they were next to the CCP Control units. The left hand cabinet 'Equipment Carrier WB404A' housed the WB400 speech broadcast plug in units and the right hand cabinet 'Equipment Signalling WB400A' contained the HANDEL incoming call detector and the WB600 siren signal generation, plug in units. The bolt on covers have been removed and stood against the right hand cabinet.
Equipment Carrier WB404A and Equipment Signalling WB400A Cabinets at a CCP
Roll your mouse over the photograph for a description of the item.
I am very grateful for a copy of this slide showing the insides of the WB400 and WB600 equipment cabinets as I believe this to be the only photograph of WB400 plug in units on the internet.
Police Routine Functional Test
Every six months the police performed a test of the speech broadcast system. Before the test each Warning Point was sent a card advising them of the date and time of the test. This had to be completed and returned to the police liaison point who reported any faults to the telephone fault repair service. The link below has a recording made by Bob Pickwoad at Newick ROC Post during one such test in 1966/7. I am very grateful to Bob for offering me a copy of the recording for this site in 2005.
Recording of a Routine Test
[Monitor Tone] tick-tick-tick
[Call Signal] pip-pip pip-pip
This is the Tunbridge Wells Carrier Control Point making a routine test broadcast.
You are now to hear a short test of the Warning Signal.
[Warning Signal] wow-wow
Now please take your question form and enter the word Domino at item number four.
Please complete the form and forward it to the Chief Constable of your District today.
[Monitor Tone] tick-tick-tick
Remote Siren Test
Regular testing of the siren receivers were performed by the police, using the 'Test' button on the Siren Control Unit at the CCP. This did not sound the siren but lit a small lamp. Over the next few days, the 'War Duty' police officer would visit the siren locations and check that the test lamp was illuminated and report any faults.
Contractors, usually the electricity board, visited the siren switch panels to test them and 'flick test' the siren. By briefly applying power the siren would be heard to growl, but it was not allowed to get up to warning speed.
Control Point Exchange (CPE) - WB400 Era
The Control Point Exchange (CPE) is located in the public telephone exchange that serves the Carrier Control Point (CCP). I do not have a photograph of this hardware in the exchange, known as an Equipment Carrier WB405A and Box, Battery WB401A. It is a very uninteresting grey steel cabinet with a screw on front cover. It is a larger version of the one shown in the photograph in the distribution topic below. The cabinet is mounted on a steel battery box, with a matching screw on cover, containing large dry cells that can power the equipment if the exchange's 50 volt battery supply fails.
The actual dimensions of the CPE hardware: Equipment Carrier WB405A, 1′-10½″ wide, 1′-0½″ deep, height above battery box 4′-1″. Weight empty 175lb, plug in units 35 - 55lb. Box Battery WB401A, 1′-10½″ wide, 1′-2″ deep, 1′-11″ high. Weight empty 40lb, batteries 185lb.
CPE Block Schematic
The CPE equipment is connected to the CCP at the Police Station by two pairs of wires, shown leaving the left hand side of the schematic. These are identified as the X and Y paths. Each pair of wires to the CCP transmit audio messages from Strike Command in one direction and received the carrier signal for amplification and distribution.
The exchange receives two feeds from the Speaking Clock distribution rings. Each is split to serve both the Civil Defence CPE equipment and the public speaking clock. In the public speaking clock each of the two feeds are amplified with one working (serving customers) and the other on standby (to a dummy load). Should either feed fail an exchange alarm will be raised and the other amplifier used instead. Should both fail then Number Unobtainable Tone is be connected to anyone dialling the clock so they are not be charged for the call. During the Cold War period the speaking code was 8081 and 123 in large cities. This was standardised nationally to 123 in April 1995 when the speaking clock rings were decommissioned.
The audio from the speaking clock feed is amplified in the CPE equipment and sent down the X and Y path towards the CCP at the police station. The same X and Y path wires bring the 72kHz carrier signal back to the CPE. In case one path should fail the two carrier signals were monitored in a change over unit. This protected feed was amplified and fed the local warning points and also be sent over inter exchange junction circuits to other remote exchanges for distribution to their warning points too.
Carrier Distribution Network Between Exchanges
The Carrier Control Point (C.C.P.) generates two identical carrier feeds sent over the X and Y path to the Control Point Exchange (C.P.E.). In the diagrams these are shown in Red and Blue.
In the 1960-70 era, many villages only had a single underground cable feeding the exchange, so there was little point in duplicating their carrier feed. In more remote locations, one cable passed through a number of exchanges. In the left hand diagram, village C is supplied from village B. There were design rules that limited the number of tandem links that could be used. Quite often a small village may have only have had one or two warning points to feed.
Swindon Carrier Area
If it is desired to extend the dual paths beyond the C.P.E., perhaps to other large towns within the warning district, then two additional amplifier are fitted on the dedicated X and Y paths. These amplifiers feed distribution panels to extend the X and Y path to other dual fed exchanges. In the left hand diagram only one such exchange is shown here at the 'Large Town' in the diagram. To guard against damage to the underground cables, the X and Y paths are kept physically as far apart as possible.
In the Swindon Carrier Area, the dual path runs from Swindon through two small exchanges to Marlborough. All other exchanges are single fed. The way the exchanges are fed in tandem reflects the route of the cables in the area. The two or three letter group inside each exchange circle is the THQ1141 code for that exchange.
At all dual fed exchange including the C.P.E., a changeover unit monitors the two incoming carriers. Should one fail, it selects the other. This switched supply (shown in Black) is amplified and distributed to the Warning Points in the town. If it is necessary to extend the distribution onto further exchanges, these too are single fed from the switched supply.
The photograph below (taken at the Avoncroft Museum, Bromsgrove) shows the Equipment Carrier WB400A and Box Battery WB400A.
This protective cabinet, used at single path dependent exchanges, houses the amplifiers and distribution units.
Protective Steel Cabinet
The actual dimensions of the Equipment Carrier WB400A is, 1′-10½″ wide, 1′-0½″ deep, its height above battery box is 2′-8″. An empty weight of 115lb, with the plug in units adding 15 - 25lb.
Box Battery WB400A is, 1′-10½″ wide, 1′-2″ deep, 1′-0¼″ high. Weight when empty 35lb, the dry batteries adding a further 85lb.
In the mid-eighties when the WB1400 replaced the WB400, the protective cases were retained and refitted with shelves holding the new smaller electronics.
Local Distribution to Warning Points
Basic Receiver Station
The telephone line going to the warning point was used to convey the WB carrier to the receiver or siren. In the 1960-70 period, lines were often in short supply, waiting lists for phones was not uncommon and two customers may have shared the same pair of wire back to the exchange in what was known as shared service or party line. Sending the carrier over the existing phone line removed the need for an extra pair of wires.
Exchange Feeding the Receiver
In the WB400 equipment at the exchange, the carrier was amplified and divided into a number of individual supplies, one for each warning point. A connection is made from the carrier distribution unit to the line towards the customer. To prevent the exchange Line Circuit from shunting the carrier, the wiring is routed via an inductor unit then out to line. Inductors block the passage of the higher frequency of the carrier whilst still allowing the voice, direct current and ringing for the telephone through.
In the early sixties when Bakelite telephones were in use, a filter unit made from Bakelite to match the phone was used to separate the carrier signal from the telephone circuit. The gallery shows the components of a typical receiver station, comprising of the filter unit, the carrier receiver and the basic Bakelite table top telephone.
When coloured plastic telephones replaced the old fashioned black Bakelite models, the opportunity was taken to remodel the filter unit into a plastic case. The older Filter Unit WB400 was electrically identical to the Filter Unit 3A and may have remained in use if the customer decided to upgrade their old black telephone to a modern coloured model. For new installations of plastic telephones, the Filter 3A would be fitted.
WB Filter Unit 3A
WB28680, Filter Unit 3A Circuit
At the warning point, the telephone (telephone instrument as they were called) is wired via a 'Filter Unit 3A'. This separates the speech, line current and ringing from the LINE and sends it to the INSTrument connection whilst blocking the carrier, this stop the telephone from shunting the carrier.
The filter's other connection point RECeiver has the opposite effect, it only allows the carrier to pass and blocks everything else, so the presence of the receiver does not affect the operation of the telephone.
Distribution within Buildings
Bypassing around a Switchboard
In large industrial premises the siren or speech receiver may be situated in a part of the building remote from the point where the telephone lines enter the building. In these type of premises there would be a number of incoming exchange lines to the company’s switchboard or Private Automatic Branch Exchange ( PABX ). The Warning Receiver may need to be located in a permanently staffed room such as a security office away from the switchboard. It was necessary to convey the carrier signal to that receiver. Although the use of a dedicated pair of wires was a possibility it represented a fault liability so a nearby extension telephone line was utilised.
WB Filter Unit 4A, Diag. WB28681
The carrier could not pass through the switchboard or PABX so it had to bypass it. This was done with a 'Filter Unit 3A' separating the 72kHz carrier being sent over a designated exchange line. The carrier signal was reintroduced back onto the selected extension line with a Filter Unit 4A as shown in the diagram on the right. This would wend its way through the building. A further Filter Unit 3A located near the telephone extension separated the carrier to feed the receiver. In the diagram, the wires carrying the carrier only are marked Green, those with telephone and carrier are Red, Telephone only are coloured Black.
The 'Filter Unit 4A' allows easy passage of the carrier from the CARRier to the extension ( EXTN ) whilst blocking it from going towards the EXCHange (The switchboard). Audio, ringing and direct current to energise the telephone extension pass easily between EXCH and EXTN but are blocked from going to the CARR terminal and causing overhearing between the exchange line and extension.
WB400 Carrier Receiver
Battery Dry No. 27
The WB400 receiver was powered by a Six volt battery (Battery Dry No.27) in the base. Spare batteries were provided and it was expected the user would change them when necessary by removing the baseplate with a coin.
When correctly adjusted it would emit the quiet tick of the monitor tone sent by Carrier Control Point, the volume increased automatically for a calling signal or speech message.
In peacetime the receiver was normally switched off and only turned on for tests. The red knob on the front is a combined On/Off and Volume control. Below that is a small pull out draw containing an user instruction card. This card is reproduced in the Topic below, it told the user what to do in response to a message and how to change the battery.
A number of large images can be found in the Gallery, illustrating the exterior and interior components of the Receiver Carrier WB400.
To protect the electronics from moisture, the chassis plate has a cork seal and a plastic membrane covers the speaker grills. A desiccator unit helps absorb any moisture ingress, this is fitted to the chassis plate so it may be viewed from the battery compartment. Normally Blue, the colour changes to Pink if damp gets in.
The receiver has two printed wiring boards, the radio frequency board which amplifies and detects the audio on the 72kHz carrier can be identified by the four square aluminium cans. The other is the audio board which has a pre-amplifier and suppressor to reduce the output when the monitor tone is being received. Next to the audio board is the output transformer with the two output transistors mounted on its tags.
On many occasions the user would forget to switch it off after the six monthly test message and flatten the battery. When the replacement system was designed, changing batteries were no longer necessary as the power was fed from the telephone exchange. It seems strange though, it was still necessary to turn the receiver on when required as this was not done automatically.
WB400 Warning Point User Instructions
WB400 Instruction Card 1968
Instructions for the Warning Points are printed on a small folded card held in a drawer in the base of the WB400 receiver unit.
The four pages of the card are reproduced in the gallery. A printable version suitable for collectors may be found in the Document Library - WB Documents section by using the tabs at the top of this page.
Hand Operated Siren
Fallout Warning Maroons
Electric Mains powered sirens were usually confined to towns and cities. The mains operated sirens are described in detail on the siren page under the HANDEL tab.
Warning points in rural areas without power sirens had a hand operated siren like the one photographed to sound the Attack Warning and the All Clear signals.
In this picture of a policeman preparing to give a fallout warning, using a maroon, a firework that shoots up a shell which explodes around 100ft with a very loud bang. The maroon in the photograph is held in place with sandbags and operated remotely with a battery. Once triggered it sends up three maroons spaced a few seconds apart to give the BANG, BANG, BANG fallout warning. The policeman may also give three blast with his whistle too.
WB400 Carrier Receiver Circuit Diagram WB28013
The receiver employs Germanium junction transistors, reflecting the technology of the early nineteen sixties, when it was designed. The RF board has three CV7005 (OC71) transistors, the audio board one more CV7005 with two CV7006 (OC72) output transistors and two CV7004 (OC45) in the squelch. Transistors VT1 and VT2 amplify the incoming signal. VT3 is the AM detector that feeds the volume control. The audio push pull output stages comprise VT4, 5 and 6. A small amount of audio is fed into what is called the suppressor circuit. This reduces the gain of the audio output stage when the monitor tick is present so its like a clock ticking. When a spoken message is received the gain is increased so it may be heard clearly.
There was no Electro-Magnetic Pulse (EMP) protection in this receiver. This was due to the WB400 equipment being designed and manufactured before the phenomena of the EMP was discovered. The EMP effect was first noticed in 1962. The American " Starfish Prime " test on 9th July 1962 affected telephone and power systems 900 miles away in Hawaii. A Russian test on 22 October 1962 when a 300kt warhead was detonated at 290km altitude apparently affected 570km of telephone lines and started a fire that burnt down the Karaganda electricity generating station.
The receiver is simply an AM radio tuned to 72kHz that gets its signal from the telephone line rather than an aerial. Those deployed in areas near to Very Low Frequency (VLF) transmitters operating close to 72kHz could be affected by interference. A special line balancing circuit could be fitted to eliminate weak radio interference. The VLF transmitter at Rugby on 68kHz was one source. In some carrier control areas near to Decca Navigation VLF transmitters (Purple slave) the interference was stronger, it was necessary to change the carrier frequency from 72kHz to 70.5kHz to overcome the problem. These CCP's were Lockgilphead, Campbeltown, Dumphries, Lanark, Motherwell, Stirling, Oban and East Kilbride.
This corresponds to Decca on 70.538 kHz at Sterling, however http://jproc.ca/hyperbolic/decca_chains.html shows transmitters at Wormleighton, Warwickshire on 70.833 kHz; Burton Fleming, Yorkshire on 70.379 kHz and Llancarfan, Vale of Glamorgan on 70.233 kHz. I don't understand why these sites didn't appear to cause problems too. Decca VLF in the UK closed March 31, 2000.
Siren Control System WB600
Police Station Equipment
Control Unit WB600A
Mounted on the front of the "Operating Unit WB401A" which houses the HANDEL phones, is a small siren control panel. Known as "Control Unit WB600A", consisting of interlocking push buttons each containing a small lamp, and a key-switch to prevent accidental or malicious operation. A break glass unit, located by the CCP, kept the activating key safe. Once the key was inserted and turned on, pressing either the top right "Attack Warning" button or top left "All Clear" button would activate the remote power sirens. Operating the "Stop" button, located between the two siren buttons, will immediately stop activation signal.
In the apparatus room cabinets, duplicate "Unit Signalling WB600A" generate the tone signals used to modulate the WB400 carrier. Each "Unit Signalling WB600A" has siren control buttons and key-switch mounted on its front panel, enabling the sirens to be controlled from there. Remote sirens receivers were controlled by two audio tones, a guard or "G-signal" of 1500Hz and siren activation or "S-signal" of 2160Hz. To prime the siren receiver, the G-signal was pulsed 0.4 seconds on and 0.4 seconds off for 10 seconds. This was followed by the S-signal whose duration depended on the warning being given. It would be pulsed 4 Seconds on and 4 Seconds off, for the Attack Warning, or 60 Seconds continuously for the All Clear.
Siren Point Receiver
At the siren point, a "Receiver Carrier WB600" looking like a grey plastic box mounted on a steel back plate, is approximately 16 inches wide by 9 inches high and 6¾ inches deep. The otherwise featureless box has a test lamp in an integral reset switch. The receiver is mains operated and connects to a telephone line via a "Filter Unit WB600A". The receiver output is wired into the Home Office, Siren Switch Panel.
Remote Siren Test
To test the siren remote control box the " Test " button would be pressed on the Siren Control Unit (Control Unit WB600A) on the CCP. This button can be used without operating the Master Switch. Over the next few days, the 'War Duty' police officer would visit the siren locations and check that the test lamp on the control unit was illuminated and press the lamp cover reset it and extinguish the lamp. The officer would note any location where the lamp wasn't illuminated and report these faults to the Post Office telephone repair service.
WB600 Carrier Receiver
The circuitry contained a 72kHz carrier receiver similar to the WB400 receiver. The audio output can be monitored on the back panel checking the WB400 monitor tone tick is present. This audio is amplified and filtered to extract the two remote control signal tones known as the 1500Hz Guard 'G' signal and 2160Hz Siren 'S' signal. To prime the siren circuit, the guard signal is pulsed 0.4 Seconds on and 0.4 Seconds off, for 10 seconds. This was immediately followed by the siren signal, its duration controlling how long the siren motor was energised.
All this was done with Germanium Junction transistors available in the nineteen sixties when the system was designed. Unusually the guard signal pulses were detected by a galvanometer relay. This device is tuned to a period time of 0.8 seconds, after a few seconds of 'G' signal pulses, the pendulum travels far enough to touch an electrical contact. This sequence gave the necessary immunity to prevent the receiver being accidentally triggered by the human voice during speech broadcasts.
Approximately 6 seconds of pulsed G-signal primed the S-signal circuitry. The subsequent S-signal operates a small relay, extending mains line voltage back into the siren switch panel to further operated the siren 3-phase Mains contactor.
None of the WB400 or WB600 equipment circuitry contained any EMP protection. After a thunderstorm, it was in my experience, often necessary to replace faulty WB600 receivers especially those in water towers and other high points. After changing the receiver, it was good fun to run the siren for a couple of seconds. Thank goodness this was never heard for real.
As the WB600 audio control signals modulate the WB400 carrier, warning points can hear these on their receiver, which is mentioned on the warning point instruction card. (See earlier topics) The siren switch panel and sirens have a dedicated page. This may be found as 'Power Siren' on the navigation tabs at the top of this page.
Demonstration of Siren Control Signals
This is a Stereo M P 3 recording, with the WB600 Control tones on the left channel. So that you can appreciate how these worked, the siren sound is on the right channel.
Roy has kindly sent this photograph of a Tester WB400A at the 'Ulster Aviation Society Museum'. This tester was used to set the correct carrier signal levels in the CCP and Exchange apparatus and locate faults in the distribution network. It is a Level Measuring Set ( LMS ) tuned to 72KHz that displays the signal strength on the meter in the range of -65 to +25 dB. The unit also contains a speaker unit so the audio modulation could be heard just like listening to a warning receiver. The input terminals are on the bottom left hand side in this photograph. The designation plate shows it was made in 1962 to diagram WB28029.
The tester is in a rugged steel box approximately 24" x 18" x 6" and has a detachable lid. It weighs 5 or 6 Kg and I can recall climbing up a telephone pole with mine to locate a fault. I shudder to think what would have happened if I had dropped it on someone!
I am not aware of any other piece of test equipment being available to local exchange and field maintenance staff. This shortcoming was rectified with the introduction of the new generation of HANDEL.