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Alerting people in the event of a fire

Author : Neil Porter, sales and marketing director, European Safety Systems

24 September 2010

Audible and visual warning devices are used for many different purposes in hazardous area environments including fire and gas alarm, process alarm, instrumentation and disaster warning.

Alerting people in the event of a fire
Alerting people in the event of a fire

Because of this variety of applications and the fact these are used for critical safety warnings, some considerable thought needs to be given to product selection. For example, at one end of the spectrum may be an offshore oil rig, which is manned by highly trained workers who operate under strict safety procedures at all times; at the other end is an onshore petro-chemical site where there is a duty to warn the general public outside of the site boundary. Clearly, the technical requirements for the warning devices and the nature of the warning signals themselves will vary greatly. The key consideration for the system designer is how to produce clear, unambiguous audible warnings throughout the protected areas in the event of a fire or explosion.

Many countries, such as Germany, France, Holland and Australia have a national 'evacuate tone'; most other countries do not. However, for specific applications such as offshore use, specific tones are defined, known as PFEER, Prevention of Fire, Explosion and Emergency Response. As a minimum, there must be provision for three types of alarm on the installation or platform:
1 Prepare for evacuation is a continuous constant amplitude signal with varying frequency of 1200Hz to 500Hz over 1 second then repeated.

2 Toxic gas alarm is a constant 1000Hz tone reinforced where necessary with a red beacon or strobe.

3 Other cases for alarm (fire) is a 1000Hz, 1 second on, 1 second off tone reinforced where necessary with a yellow beacon or strobe.

In normal commercial environments such as offices, hotels, hospitals and public buildings, the ambient background noise will typically be around 65dB, and most individual areas are relatively small. Such environments may be covered with multiple sounders with typical outputs of approximately 100dB(A) at 1 metre; even the traditional 6" bell is effective in small installations, although not particularly compatible with today's low-current control systems. It is generally accepted that the effective distance of a sounder is when the calculated dB(A) is at least 5 dB(A) above the known ambient background noise, so the effective distance of a sounder in an ambient of 65 dB(A) is the distance at which the output level reduces to 70 dB(A). Using the inverse square rule / rule of thumb that the output falls by 6 dB(A) each time the distance doubles, the output will reduce by 30 dB at a distance of 32 metres.

In high background noise industrial environments, it is essential to select alarms that have an output above the background noise rather than use multiple units of a lower level. The overall alarm level should be a maximum of 10 to 15 dB(A) over the ambient background noise and in extremely high noise areas such as close to compressors or where ear defenders are worn, supplementary beacons should also be installed as audible alarms will be ineffective.

Electronic sounders are increasingly replacing the traditional electromechanical sirens, hooters, buzzers and bells that have been the mainstay of the wide area warning device market for many years. Many sites however, still prefer to retain the existing signals as these are instantly recognisable and part of the safety "culture". Now, complex digital to analogue conversion software and the latest in SMD class D amplifier technology enables E2S Hootronic sounders to mimic, in one product, an industrial hooter, high and medium frequency mechanical sirens, a buzzer and a bell with amazing fidelity. The operational advantages of replacing electro-mechanical devices with electronic equivalents are the savings in power consumption and weight; traditional sirens and hooters can be extremely heavy and often need three-phase mains power.

Unlike electro-mechanical devices, the Hootronic range is continuously rated, requires zero maintenance, three remotely selectable stages are available and signal quality and performance will not degrade with age. Available in both industrial and explosion-proof versions it can be utilised in almost any location.

It is becoming increasingly common in both commercial and industrial applications for visual signals to be required to reinforce the primary audible warning device. A visual signal should never be used by itself as the primary warning mechanism in a life safety system, although they are widely used by themselves in industry to indicate machine state or environmental condition.

Advances in lighting source technology have generated a number of alternatives to the traditional Xenon tube as the basis for strobes. In particular, high output brilliant white or monochromatic LEDs provide the benefits of low current draw, long life and simple electronic configuration and control.

The Xenon tube uses a very high voltage, generated by an inverter circuit, to break down the Xenon gas in the tube, creating an instantaneous brilliant flash of light, normally enhanced by using a 'Fresnel' lens. The light energy of the flash is a function of the Xenon tube size, the voltage across it and the capacity of the capacitor discharging into it. The Xenon strobe beacon has the best light output to power input ratio and is the most widely used and versatile technology currently available. Tube life is critical: it may be as little as 1 million flashes in cheaper devices but specifiers should typically expect 5 to 8 million flashes from higher quality units. However, traditional Xenon tube beacons cannot perform to their full potential when managed through intrinsically safe barriers for use in hazardous areas; the input energy that is allowed to pass through the Zener barrier or galvanic isolator is limited and consequently the performance of the Xenon tube is severely compromised, significantly reducing its light output. In such cases, the solid-state LED (light Emitting Diode) unit has far greater potential. An array of ultra bright LEDs produces a bright flashing warning signal when powered through a Zener barrier or galvanic isolator, overcoming the restrictions associated with a Xenon strobe.

Traditionally, beacon output has been quoted in Joules but increasingly Effective Candela is becoming the main unit of measure. It is generally accepted that beacons of at least 10 Joules are used in outdoor locations with exposure to bright sunlight in order to provide an effective warning device.

Hazardous areas
Hazardous areas are defined as areas where concentrations of flammable gases, vapours or dusts may occur, either constantly (Zones 0 and 20), under normal operating conditions (Zones 1 and 21) or unusually (Zones 2 and 22). A whole series of additional conditions relating to the temperature classification and the auto-ignition temperatures of the type of gas or dust to be found to ensure that any equipment will not initiate an explosion or fire. Hazardous areas are to be found in a very wide range of manufacturing industries, far beyond the obvious petrochemical plants. Food, pharmaceutical and cosmetic manufacture all involve processing potentially explosive substances, while the problems of explosions in grain silos and sugar processing plants are very well documented.

There are two ways of ensuring that the sounder or strobe does not initiate an explosion when operated in a hazardous area:

Intrinsic Safety
Simplistically, the input energy entering an intrinsically safe device is constrained so that any arcing or sparking within the unit cannot generate enough heat to start ignition. Intrinsically safe warning devices have a relatively low output so are mostly used for instrumentation or indoor warning such as on fire alarm systems in pharmaceutical plants and chemical storage areas.

These house the equipment in an enclosure that is sufficiently robust to prevent any internal explosion from reaching the outside. While bulkier, heavier and more robust than intrinsically safe ones, they can achieve much higher outputs and are therefore more likely to be installed in external applications, particularly as they will be environmentally sealed to IP66 or IP67 in order to achieve the degree of protection required.
Wide area sounders, with an output at 1m in excess of 140dB, significantly higher than the human threshold of pain, have an effective warning range of between 250m and several kilometres depending on the atmospheric conditions. Used in quarries, on large industrial and petrochemical sites and for civil defence requirements, electronic wide area sounders will normally generate multiple internationally recognised alarm tones including fire, security, civil defence, alert, COMAH (SEVESO II) toxic gas alarms and disaster warnings for flood, tsunami, tornado and other severe bad weather conditions. Typically, systems consist of a pole-mounted speaker array designed to give either full 360° coverage or directional warning to a specific place. Because this type of alarm is often located away from the control centre and in order to minimise cabling costs, these sirens can also be controlled using many different methods including radio control (VHF/UHF, GSM and Tetra), TCP/IP and RS485.

Disaster sirens can be linked into other safety systems such as fire and gas detection systems but because this is such a critical alarm (and a last resort) they often have their own activation panels which can be push button, LCD or use a touch screen PC based solution. Battery back is essential for this type of alarm as power supply integrity in the event of a major incident cannot be guaranteed. Units should operate for up to 7 days in standby mode and up to 30 minutes in alarm.

One of the key features of electronic disaster sirens is their ability to offer fault diagnostics. As this is a critical alarm and usually only sounded as a last resort, it is very important that the plant manager knows at any time that this vital piece of safety equipment is fully functional. Electronic sirens can carry out a “silent” test every few hours to ensure that the alarm is working and a log of these tests can be held either as a hard copy or a computer file.

The choice of alarm tone is also very important for long distance warning. Standard electronic type tones can be confused with plant and process alarms that often sound the same, so the traditional “air raid” type tone is still the preferred choice as it is instantly recognisable whatever the distance, especially by people off site with little or no training.

Sounders, reinforced by strobes, are the primary mechanism for alerting people in the event of a fire or explosion. Whatever the warning system chosen, the primary function of the system designer is to ensure that the audible and visual output levels are adequate throughout the protected area.

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