The Public Warning System is a network of civil defense sirens installed by the Singapore Civil Defence Force on over 2,000 strategic points in Singapore to warn Singaporeans of impeding dangers and air raids. Thus far, the siren network has only been used for occassional public awareness drills, monthly noon chimes and for commemorating the island's Total Defence (February 15) and Civil Defence Days (September 15).
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When it was a British colony, Singapore had mechanically-powered air raid sirens over the island, but the system did not work perfectly and was defunct after World War II. Then, in the 1990s, the SCDF planned a network of powerful electrical sirens throughout Singapore to warn the people of natural disasters and air raids. The first of these sirens were completed in 1991 and tested in 1992 on certain parts of Singapore, and by now nearly every part of Singapore has the sirens installed on the selected buildings' rooftops or on the ground if there are no buildings.
There are a total of four installation types, they can be installed with:
Standard: eight speakers joined together on the left and right sides of a thin medium length steel pole (for buildings with short, medium or no rooftops). This is the original design from 1992 still used today. Also built as on ground sirens on certain areas of Singapore: Yew Tee Industral Estate, Lim Chu Kang Road, Adam Road Mid-Sub Power Station, Mandai Air Base, a unknown factory in northern Woodlands and on the compounds of Woodlands Fire Station.
Half-size: four speakers joined together on the left right sides of a thin medium length steel pole (for buildings with medium-high rooftops) or a long steel pole (for buildings with high rooftops). First completed in mid-1990s.
Double: Double the speakers, but not double the decibels of the signals with sixteen speakers as standard type and eight speakers for half-size. First completed in late 1990s and installed in early to mid 2000s. Also built as an on ground siren (standard sixteen speaker type) on the Assumption English School compound few meters away from the school.
"Zig-Zag": four speakers joined together in a "zig zag" pattern on both left right sides of a thin short steel pole, completed in 2009 it is currently on the rooftop of Clementi Fire Station.
Sirens attached on appropriate rooftops of buildings can have a custom-made mini server box (which is white on color) beside the siren and in on ground sirens to eliminate remote control of them from the nearby fire stations turning them into sourcing sirens.
There are a total of four signals sounded by the sirens, they are:
Alarm signal: Wailing blasts that warn people of a danger or threat and it means for them to move a SCDF air raid shelter. Used only once on the island's Total Defence Day in 2006.
All clear signal: Continuous blasts of a clear sound to allow the people to move out of the shelter if the threat is over.
Important Message signal: Pulsating blasts sounded to alert the people of important messages broadcast by Singapore FM radio stations. This is the most used signal as to celebrate Total Defence and Civil Defence Days in Singapore.
Testing signal: It is not even a warning signal at all. It is a testing chime sounded on the 1st day of every month on 12:00PM (GMT+8:00 SGT). Since 2009 this chime has also been used on National Day on August 9 to signal the Pledge Moment when Singaporeans recite the Singapore Pledge and sing the National Anthem. In 2009 it was held at 8:22 pm and in 2010 it was held at 8:10 pm (or 2010 hrs in the 24-hour clock, coinciding with the year 2010) and known as One Voice 2010. In 2011 it was held again at 8:10 pm
in 2009, Hörmann Sirens and Sircom AG of Germany has thought of using the SCDF's Public Warning System's siren as a base to make their own sirens, made two types of sirens based on Singapore's PWS designs: the ECN 1200, which is based on SCDF's 'Standard' build but with an unusual arrangement of its eight speakers and the ground type ECN 2400 also with its unusual arrangement of its speakers and Sircom's version with its sixteen speakers attached to front and rear of the pole.