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Written by: Keith Norgate and Vaune Everington

Category: Detectors and Positioning

Published: 04 April 2025

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FIRE DETECTORS-5

LINEAR HEAT FIRE DETECTORS

The FSIB introduces the fifth in a series, on types of detectors available for the many and varied applications one might face.

 

Linear Heat Fire detectors

Linear heat detection cables are specialized fire detection devices that detect heat continuously along their length. Each linear cable length is connected to a control panel that determines a temperature change has occurred. These cable systems are often well suited to environments where point-based detectors might not be ideal. They are highly versatile and effective in areas with challenging conditions or where large coverage is required.

  

Linear Heat detector types.

There are distinct types of linear heat detectors, tailored to meet various fire detection needs:

1. Digital Linear Heat Detection Cables - Specialized fire detection that triggers an alarm when a specific temperature is reached determined by the length of cable.
2. Analog Linear Heat Detection Cables - These monitor the actual temperature along the cable and can trigger alarms based on temperatures programmed from a range of levels within the controller. An alarm is raised before the cable is damaged.

 

Applications

Linear heat detection cables are ideal for locations where traditional fire detectors may not be best suited, such as:

• Cable trays in industrial environments.
• Tunnels and underground installations.
• Conveyor belts.
• Installed around items of plant.

Areas where high airflows, significant dust accumulation, or other environmental conditions occur and where standard point-type detectors may not be suited is where linear heat cables come into their own.

Linear heat detection is also used for monitoring certain items of plant. Cables are laid in close proximity to the machine or piece of equipment to monitor for overheating.

These systems are also used over long lengths such as cable runs in cellars and where the linear cable can be laid in close contact with the other cables.

 

How do they work?

Digital Linear Heat Detection Cable

The digital type of linear heat cable consists of two conductors within a plastic sheath. Several types of sheathing are available.

Digital linear heat detection cables are designed to trigger a response when a specific temperature is reached. If the temperature surrounding the cable reaches the alarm temperature, the two cores quickly come into contact and trigger an alarm. These cables are known as “digital” because they are “off” below the activation temperature and irreversibly switch “on” when the activation temperature is reached

The cable will be damaged at alarm temperatures and must be repaired or replaced.

 

Analog Linear Heat Detection Cables

Analogue linear heat detection systems comprise of a control or interface unit, a zone of a four core analogue linear heat detection cable and an end of line unit. The system identifies fires or overheat conditions by continuously measuring changes in electrical resistance along the detection cable. An alarm is triggered once the resistance along the cable falls below a set threshold, which is determined by the desired alarm temperature (chosen by the user).

Analogue systems can be reset after an alarm is generated, without damage to the cable.

 

Where should we NOT use it?

While linear heat detection cables are often dependable, they are not suitable for situations requiring detection of smoke rather than heat such as stairways, computer environments or for the operation of gas suppressions systems.

 

Where can it be installed?

Linear heat detection cables are perfect for areas where point-type detectors cannot provide adequate coverage. Examples include long conveyor belts, cable tunnels and industrial sites with extensive cable runs.

 They are also invaluable for hazardous areas that require reliable, explosion-proof detection systems.

 

Installation Guidelines

When installing linear heat detection cables, consider the following general guidelines:

• Spacing: Adhere to manufacturer guidelines and national standards for spacing between runs and distance from walls or other structures.
• Testing: Perform regular testing to ensure the system is functioning as expected and clean the cables to remove dust or debris that could impact performance.
• Routing: The cables should not be installed where they are expected to be damaged.
• Environmental Considerations: Account for environmental factors such as temperature, humidity, and exposure to chemicals, and select cable types that are suitable for those conditions.

 

Advantages of Linear Heat Detectors

Linear heat detection cables can offer numerous benefits:

• Wide Coverage: They detect heat along their entire length, ensuring no "blind spots." Long lengths of coverage.
• Durability: Resistant to harsh environments, some including extreme temperatures and humidity.
• Early Detection: Advanced models offer early warnings, mitigating fire risks before they escalate.
• Localised detection: Linear heat cables can be mounted close to machinery, equipment, or cables for early detection of overheating.

 

False alarms and maintenance

While linear heat detection cables are less prone to false alarms, environmental factors like heavy dust accumulation or chemical exposure can impact their performance.

Mechanical damage to cables can occur if the cables are loosely slung. Ensure the cables are securely fixed in areas of limited traffic

Regular inspection, testing, and cleaning can help maintain their reliability.

 

Conclusion

Linear heat detection cables are a great tool in fire detection systems. Their adaptability, reliability, and ability to cover extensive areas make them a preferred choice for various applications. Whether protecting industrial sites, tunnels, or storage facilities, linear heat detection cables can provide dependable protection.

 

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Written by: Keith Norgate and Vaune Everington

Category: Detectors and Positioning

Published: 21 February 2025

Hits: 381

FIRE DETECTORS-4

FIRE BEAM DETECTORS

 

The FSIB introduces the fourth in a series, on types of detectors available for the many and varied applications one might face.

 

Fire beam detectors

Beam detectors are specialized fire detection devices that use a projected beam of light to detect smoke. These detectors are ideal for large, open spaces where traditional point detectors might not be as effective or practical and can be used at greater heights than that of point type detectors.

 

Fire Beam detector types.

There are several types of beam detectors, each with unique features and applications

1. Projected Beam Detectors: These use a beam of light projected from a transmitter to a receiver. Any smoke or heat that interrupts the beam triggers the alarm.

2. Reflective Beam Detectors: In these systems, the beam is reflected back to the transmitter by a reflector. The presence of smoke reduces the intensity of the returned beam, triggering the alarm.

3. Open-area Smoke Imaging Detection (OSID) combines advanced dual-wavelength projected beams and optical imaging technology for early warning smoke detection. Multi-emitter solutions provide a true 3D arrangement. This type of beam detector supports up to seven emitters with a single receiving Imager giving the advantage of covering unusual, shaped areas.

4. The Motorised beam detectorself-aligns to the centre of the reflector during commissioning and will automatically keep alignment when building movement occurs. The motorised beam head continually self-aligns during its installed life. This intelligent motorisation can result in fewer false alarms.

  

Applications

Beam detectors are commonly installed in locations where large, open spaces or high ceilings exist, such as:

Warehouses        Atriums     Auditoriums    Industrial facilities    Tunnels

Beam detectors can be installed above the height that traditional smoke detectors can operate.

 

How do they work?

Beam detectors function by emitting an invisible beam of light (usually infrared) across a protected area. When smoke enters the beam path, it scatters the light, reducing the amount of light received by the detector. This reduction triggers the alarm. In some advanced models, the system can differentiate between smoke and other particles, reducing false alarms.

 

Where should we NOT use it?

Optical beams are subject to movement and building structures that expand, and contract causing the beams to go out of alignment.

Optical beams require line of site, where obstruction from high stacking of goods or where birds may fly around will cause the detectors to false alarm.

 

Where must it be installed?

Point type conventional and addressable smoke detectors cannot be installed above 10.5m from the ground, here is where the beam detector comes into its own, as it can be installed at heights of up to 20m from the ground.

Ideal for high open plan areas such as warehouses. It only has one competitor at elevated levels.

 

Installation Guidelines

When installing beam detectors, consider the following guidelines:

1. Mounting: Install the detectors at the highest point where smoke will accumulate, typically within 600mm of the ceiling or ten percent of the overall roof height. (considering further recommendations of SANS 10139).
2. Alignment: The beam must be accurately aligned with the receiver or reflector. Special instructions or tools are provided for this. Some units have the capability to auto-align, which can help compensate for the movement of buildings.
3. Spacing: Adhere to the manufacturer’s recommendations for spacing, while following the guidelines provided in SANS 10139. Typically, these guidelines allow spacing of up to 15m apart with a maximum distance of 7.5m from each side wall. These distances can be increased in conjunction with the slope of the roof as specified in the national standard.

The beam detectors can cover distances up to 100 meters, though some kits may provide protection of even greater distances. One beam detector can cover a maximum of 2000m²

4. Beam path clearance: A clearance radius from 500mm up to 1000mm of the beam path is often required by the manufacturer’s instructions.

5. Maintenance: Regular cleaning and testing the detectors will ensure optimal performance. The lens of the beams can accumulate dust so should be cleaned regularly.

 

Advantages of Beam Detectors

Beam detectors offer several advantages in fire detection, particularly in large or high-ceiling environments:

1. Coverage: They can cover large areas, significantly reducing the number of detectors needed.

2. Cost-Effective: Fewer detectors can mean lower installation and maintenance costs.
3. Early Detection: They provide early warning by detecting smoke along the entire length of the beam path, this can sometimes span the length of warehouses.
4. Minimal Aesthetic Impact: Beam detectors are less obtrusive, helping maintain the aesthetic look of the building.

 

Have you suffered false alarms?

While beam detectors are less prone to false alarms than some other types of detectors, they can still be affected by dust, fog, physical obstructions and building movement. Regular maintenance and proper installation can help mitigate these issues.

 

Conclusion

Beam detectors are an excellent choice for fire detection in large, open spaces. Their ability to cover extensive areas, coupled with their cost-effectiveness and early detection capabilities, makes them a valuable addition to any fire safety system

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Written by: Keith Norgate

Category: Detectors and Positioning

Published: 24 January 2025

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FIRE DETECTORS-3

MULTI CRITERIA DETECTORS

 

The FSIB introduces the third in a series, on types of detectors available for the many and varied applications one might face.

 

Multi criteria detectors

A multi criteria detector is one with two or more different fire detection elements within the same casing.

The use of multi-sensor detectors can help to reduce false alarms from common causes such as steam, dust, cooking, and aerosol sprays.

The more sophisticated the optical / heat multi-sensor, the less prone they are to common causes of false alarms, whilst their ability to detect real fires is not compromised.

 

 

Multi criteria detector types.

Multi Criteria, or multi sensors as they are sometimes called, are available in many configurations. Both addressable and conventional types are available.

The most common is the optical smoke sensor partnering a heat detector, but many other variations can be found by adding a CO element or infra-red device.

Some of these devices work independently, acting as a smoke detector or heat detector but others need a combination of smoke and heat to operate. Some provide a facility to programme the various options available including elevated levels of smoke or low levels of smoke.

 

Applications

Multi criteria detectors are installed where one may expect installation at places where abnormal environmental conditions exist e.g. where smoke, steam or dust occurs.

One may not be sure if a heat or smoke type fire will occur.

Sometimes one may want to be able to adjust the sensitivity of a device but still sense a real fire.

Multi criteria detectors can be used to combat troublesome alarms.

SANS 10139 recommends the use of multi criteria sensors for smouldering fires.

Typical applications would be electrical substations, laboratories etc.

 

How do they work?

The multi criteria detection device is capable of generating multiple alarm signals from any one of the sensors employed in the design, independently or in combination.

The sensor output signals are mathematically evaluated to determine when an alarm signal is warranted. The evaluation can be performed either at the detector or at the control unit for addressable devices.

All sensor signals will be continuously weighted and combined by the internal electronic evaluation system. If a signal combination matches the stored fire signal pattern within the detector, an alarm will automatically be triggered.

The method of combining the signals from each sensor varies between different proprietary multi sensor fire detection systems.

With some multi-sensor fire detectors, it is possible to disable an individual detection sensor. For example, with a heat and optical multi-sensor detector, the optical sensor might be disabled during the day, if the room is to be used as a smoking room.

It must be noted that some multi criteria detectors will activate with a large amount of smoke only or a large amount of heat only.

Due to the large amount of variations provided with multi sensors it is advisable to discuss your application with the product supplier.

 

Where should we NOT use it?

The standards do not recommend using multi sensors in electronic equipment environments due to the heat element.

Using multi sensors in corridors is also not recommended as it is smoke that is the danger therefore optical detectors will perform more efficiently.

 

Where must it be installed?

It must always be installed at the highest point where the heat or smoke will always accumulate.

This can be any ceiling or roof and must be installed within 150mm of the roof or ceiling due to the presence of the heat detection element.

For conventional detectors they work on a OR basis, either smoke or heat will activate the device therefore they must be within 3.8m of the walls and spaced 7.5m apart providing up to 56m² coverage.

For multi criteria sensors that are being used for smoke detection only, one can revert to maximum 100m² coverage and 5m spacing. i.e. 10m apart.

 

Have you suffered false alarms?

As multi criteria sensors are used where false alarms occur, false alarms are rare for multi sensors and unwanted alarms should be thoroughly investigated to determine the cause of the alarm.

 

Conclusion

A good detector for use where abnormal environments occur and are generally used in electrical substations or laboratories where diverse types of testing take place.

 

 

Details

Written by: Keith Norgate

Category: Detectors and Positioning

Published: 03 December 2024

Hits: 518

FIRE DETECTORS-2

POINT TYPE HEAT DETECTORS

The FSIB introduces the second in a series on types of detectors available for the many and varied applications one might face.

 

Heat detectors

Heat detectors are a valuable detector in the fire detection designer’s armoury. They come in various forms including conventional, addressable, linear and heat probes. For this edition we are concentrating on point types of heat detection. Conventional and addressable operate in the same manner.

 

 

 

Heat detector types.

There are two main types of heat detectors on the market Rate of Rise heat detector or Fixed Temperature detector. Both being available as conventional or addressable types.

If you order a heat detector from your supplier, he will in most cases issue you with a rate of rise heat detector.

 

Rate of Rise Detector

Rate of rise detectors are designed to detect a fire as the temperature increases, but they also have a fixed upper limit at which the detector will go into alarm if the rate of temperature increase has been too slow to trigger the detection earlier.

A rate of rise detector monitors a gradual build-up of heat in the protected space i.e.: 15^oC / minute.

Should the temperature in the room rise at a rate of 15^oC in one minute or faster the detector will

operate.

 

Fixed temperature Detector

Fixed heat detectors only change to the alarm state at their preset temperature.

 

Applications

Heat detectors are installed to monitor for high temperatures at the ceiling/roof level. They are normally used where smoky, steamy, or dusty environments can be experienced.

SANS 10139 also recommends the use of heat detectors for flaming fire where little smoke will be experienced.

Typical applications would be alcohol stores, kitchens, parking garages, electrical substations, oil stores.

 

How do they work?

The rate of rise detector operates by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature, the other is sealed. In normal conditions, the two thermistors register similar temperatures, but, on the development of a fire, the temperature recorded by the exposed thermistor will increase rapidly, resulting in an imbalance of the thermistors and causing the detector to change to the alarm state.

Addressable detectors use a single thermistor to sense the air temperature at the detector position.

As temperatures rise the fire panel monitors the change until it reaches the preset parameters and then raises the alarm.

The fixed temperature detectors use a single thermistor set at a specific temperature. When the temperature surrounding the detector reaches this level, it will raise the alarm.

There are a range of detectors monitoring for temperatures between 25^o to 90^oC.

 

Where should we not use it?

The standards do not recommend using heat detectors to protect corridors due to its late alarm compared to a smoke detector.

Heat detectors should not be installed in bedrooms, hospitals, clean rooms, IT environments or for actuating gas protection systems.

 

Where must it be installed?

It must always be installed at the highest point where the heat will always accumulate.

This can be any ceiling or roof and must be installed within 150mm of the roof or ceiling.

The detector can cover an area up to 56m² but must be within 3.8m of the walls and spaced 7.5m apart.

  

Have you suffered false alarms?

The heat detector is less prone to false alarms than smoke detectors. If the device itself goes faulty you will receive an alarm at the fire panel.

Although false alarms are rare, heat detectors can be subject to nuisance alarms especially if positioned near ovens or furnaces.

 

Conclusion

A good detector for dirty. dusty or humid environments, commonly used in kitchens and garages.

As the heat detector is recognised as slow to operate, designers should only choose a heat detector if there are no other suitable alternatives.