Emergency Warning Systems - Principle of Operation

For many of us, the first time we learn about an emergency, is when we hear the telltale signs of an emergency warning system in operation.

A emergency warning system is a life safety system installed to safeguard occupants from illness or injury by warning them of a fire or emergency and to safeguard occupants during the orderly evacuation of a building in an emergency. They assist in these two functions by;

  • providing mass notification of an emergency; and
  • providing a method to communicate with and direct building occupants in the event of an emergency.

The term emergency warning systems is used in this article as a generic term used to describe a three types of systems described in the Building Code of Australia (BCA) as follows;

  • Building Occupant Warning System
  • Sound and Intercom Systems for Emergency Purposes
  • Emergency Warning & Intercommunication System (deprecated)

Emergency warning systems alert occupants in an emergency by broadcasting a warning message or tones over a network of monitored loud speakers. Speakers are distributed throughout a building to ensure;

  • warning messages and tones satisfy a specific sound pressure level (volume) and are distinctly audible throughout all required areas of the building; and
  • warning messages intelligible (clearly understood) by the occupants.

Building Occupant Warning System

A building occupant warning system (also known as an occupant warning system) is a type of emergency warning system that is designed and installed in accordance with Clause 6 of Specification E2.2a of the BCA and Clause 3.22 of Australian Standard AS1670.1.

A building occupant warning system can be one of the following;

  • A sound system for emergency purposes designed and installed in accordance with Australian Standard AS1670.4; or
  • Electronic sounders or and amplified sound systems producing an evacuation signal.

These systems may be supplemented by additional visual or tactile signals to augment the emergency evacuation signal.

Sound Systems and Intercom Systems for Emergency Purposes

A sound system and intercom system for emergency purposes in different to a building occupant warning system. These systems are typically installed where the building satisfies one of the following;

  • in a building with an effective height of more that 25m; or
  • in a Class 3 buildings having a rise in storeys or more than 2 and used as a residential part of a school, or accommodation for the aged, children or people with disabilities; or
  • in a Class 3 building used as a residential aged care building; or
  • in a Class 9a building having a floor area of more than 1000m2 or a rise in storeys of more than 2; or
  • in a Class 9b building used as a school and having a rise in storeys of more than 3, or used as a theatre, public hall or the like having a floor area of more than 1000m2 or a rise in storeys of more than 2.

Historically these systems were also described as an emergency warning & intercommunication system (EWIS). This term and the relevant Australian Standard AS2220 have however been deprecated in favour of a “sound system and intercom system for emergency purposes”.

A sound system and intercom system for emergency purposes must be designed in Accordance with the requirements of Australian Standard AS1670.4 and AS2220

Typical Features & Functions of a Emergency Warning System

An emergency warning system comprises either electronic sounders (applicable to Building Occupant Warning System only) or a network of loudspeakers and/or visual warning devices distributed throughout the building. Loudspeakers and/or visual warning devices are connected to control and indicating equipment (c.i.e.).

Typical Features of an Emergency Warning System

The control and indicating equipment that forms the ‘brains’ of an emergency warning system generally comprises the following components;

  • Cabinet
  • Primary Power Supply Unit & Battery Charger
  • Secondary Power (Batteries)
  • Control Electronics
  • Illuminated Indicators
  • Input Interface & Control
  • Input Termination & Monitoring
  • Output Termination & Monitoring
  • Evacuation Zone Amplifiers

Each manufacturer in of a sound and intercom system for emergency purposes differentiates themselves from other manufactures by the features and layout of the user interface. The following illustration shows a typical interface with two distinct sets of controls;

  • Sound System controls on the left; and
  • Intercom System controls in the right.

Typical Emergency Warning System Interface

The horizontal rows in this configuration illustrate en evacuation zone.

According to Australian Standard AS1851.4-2004, an evacuation zone is a subdivision of the premises that can be evacuated separately from any other subdivision.

This separation is achieved by providing one unique amplifier and speaker circuit for each evacuation zone. An amplifier for an evacuation zone is rated to provide a defined amount of power, measured in watts (w) to the speaker circuit.

Loudspeakers can be purchased with a two or three power settings, also measured in watts. The greater the number of speakers, the greater the power requirements (demand) on the amplifier.

Example:
An amplifier is rated at 10w there are 20 speakers fitted, each speaker is rated at 0.33w each. The combined speaker load is 20 speakers x 0.33w equals 6.6w load. This is within the available capacity of the 10w amplifier.

Typically loudspeakers are permanently fixed into position and distributed throughout the evacuation zone within a building typically at ceiling height in quantity and distribution to satisfy two criteria;

  1. a warning signal sound pressure level exceeding 10dB above ambient sound pressure level and not less than 65dB and not more than 105dB; and
  2. at all places within the evacuation zone where the ambient noise figures are less than 85dB the speech intelligibility shall be equal to or greater than 0.5 STI (speech transmission index)

At this point a little science is required to explain the terms used such as sound, sound pressure, sound pressure level, noise and intelligibility.

  1. Sound is the pressure variation caused by a sound wave.
  2. The magnitude of the variations of the air pressure from the static or normal air pressure is a measure of the sound pressure level, measured in decibels (dB).
  3. The number of cyclic pressure variations per second is the frequency of sound, measured in hertz (Hz).
  4. Noise is considered to be sound caused by an array of random activities of different sound pressure levels and frequencies.

Speaker Wiring

Loudspeakers connected to an emergency warning system are generally connected in a parallel to the control panel with an end of line (circuit) device (typically a resistor).

Typical Emergency Warning System Speaker Circuit

Each loudspeaker is also fitted with a “step-down” transformer and a capacitor as described in the following illustration. The “step-down” transformer provides an ability to adjust the power to each speaker, therefore adjusting the local sound pressure at each loudspeaker.

Speaker

Measuring Sound Pressure

In measuring sound pressure levels, the human ear is not equally sensitive to all frequencies, meaning that humans perceive some sounds louder than others depending on the frequency. As a result the measure of sound pressure is frequency weighted so that the measured sound pressure level correlates more closely to perceived sound pressure level by a human.

In the measurement of loudness, an A-weighting frequency filter is commonly used to emphasize frequencies around 3–6 kHz where the human ear is most sensitive. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA.

Measuring Speech Intelligibility

Intelligibility is a measure of the degree to which speech can be understood. Intelligibility is affected by many factors including spoken clarity, explicitness, lucidity, comprehensibility, precision, quantity & quality of speakers, area acoustics and background noise. Developed in the early 1970’s, the Speech Transmission Index (STI) is an machine measure of intelligibility whose value varies from 0 (unintelligible) to 1 (excellent intelligibility).

Speech Intelligibility

Operation of an Emergency Warning System

An emergency warning system is generally configured to be activated automatically on an alarm signal activated by;

  • an automatic fire sprinkler system; or
  • a fire detection and alarm system; or
  • an emergency control point.

On receipt of an alarm signal, the emergency warning system is typically activated as follows;

Building Occupant Warning System

Typically a building occupant warning system is configured a single amplifier/circuit and on the receipt of an alarm the entire loud speaker circuit is activated.

Sound Systems for Emergency Purposes

A sound system for emergency purposes is more often than not a system that has two or more evacuation zones fitted. In this case, the typical operation is for the system is that the evacuation zone corresponding to the source of the alarm signal is operated first followed by a delay where the next adjacent zone is then operated. The cascading sequence is then repeated until all evacuation zones have been operated or the system has been manually disabled.

Sound systems for emergency purposes may also be fitted with verbal messages to provide additional warning to occupants. These messages may also be configured to assist occupants in multi-lingual environments.

These systems may also be used for non-emergency purposes such as public address, or background music. In these cases, the systems is designed in such a way as to ensure the CIE can override these features in an emergency condition.

Maintenance

The maintenance of these two types of evacuation system does vary slightly according to Australian Standard AS1851:2005 depending if the system was designed to AS1670.1 or AS1670.4. Please consult the relevant Australian Standard and the manufacturers specifications for detailed maintenance obligations.

In summary, emergency warning systems are designed and installed to alert and evacuate building occupants of an emergency. This is typically conducted in accordance with a defined set of emergency response procedures. Emergency warning systems are versatile systems than can also be used for other purposes such as public address and background music.

References

  • Building Code of Australia
    • Specification E1.5, Clause 8 - Building Occupant Warning System for required sprinkler systems.
  • Australian Standards
    • AS1670.1-2004 - Fire detection, warning, control and intercom systems—System design, installation and commissioning
    • AS1670.4-2004 - Fire detection, warning, control and intercom systems—System design, installation and commissioning Part 4: Sound systems and intercom systems for emergency purposes

About the author

Russ Porteous is the CEO and one of the founders of Maintenance Essentials, he has over 22 years experience in the installation and maintenance of fire and essential safety measures. Russ is an active contributor to a variety of Australian Standards including AS1851 for the Maintenance of Fire Protection Systems and Equipment. Russ speaks regularly at conferences and trade shows as a subject matter expert. You can follow Russ online via Twitter @rport or his popular fire safety blog.

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