NEWSHEET5

Linear Detection System Comparison

To most International specifiers and end users, LINEAR HEAT DETECTION remains the most effective method of detecting overheat and fire conditions within a number of heavy Industrial risk areas. The point of risk installation (not reliant upon favorable airflows -as with remotely installed point type and " line of sight" detectors), its ability to operate with limited maintenance in adverse environmental conditions (including corrosive, hazardous and sub-zero ambient temperatures), within areas of limited access and surveillance, the provision of alarm point location facility (digital and fibre optic linear detection systems only), and in an increasing number of applications - the ability to detect overheat conditions BEFORE a fire develops - all provide justification for linear heat detection system specification.

During the past decade, the " heavy Industrial" reference list of installations for linear detection has been complimented by an increasing number of light Industrial, commercial and even domestic project installations. Included within such a list are:

• Multi -storey car parks
• Warehouses (bulk, rack storage and freezers)
• Airports
• Railway Stations (incl: Metro systems)
• Ships
• On + Off road vehicle protection
• Shopping Malls
• Hospitals
• Hotels

One of the most difficult decisions many specifiers and design engineers face, is which system to use and where to install the heat sensitive cable. To this end, PROLINE PROTECTION SYSTEM LTD regularly posts Newsheets to this website - covering a diverse range of applications and design considerations.

In this newsheet, we are providing the practical (not theoretical) facts that will make specification of the most appropriate linear detection operating concept easier.

THE FACTS

PROLINE PROTECTION SYSTEMS LTD is able to offer ALL of the operating systems described below. The product concept assessment is therefore not based upon any biased considerations or specific product manufacturer influences!

Many new generations of linear heat detection systems have been introduced - each claiming new features, benefits and operational superiority over its predecessor. However, most of the claims have again been proven to be more theoretical than practical. For example:

ANALOGUE

In theory: a recoverable sensor cable system with adjustable alarm thresholds and more sensitive than digital??

In practise: with all overheat or fire conditions starting as localised events, only a small % of the zone length of sensor cable will be subjected to the temperature rise - resulting in a high alarm temperature requirement - far in excess of its digital counterpart alarm temperature rating, and also above the destruct temperature of the pvc extruded analogue sensor cable.

Example: - 500m of sensor cable connected to its dedicated interface, calibrated for a risk area max: ambient of 40C has to be set to switch position - 9. In alarm temperature terms, this means a) heat the total 500m for an alarm temp; of 58C or b) heat 1m of the 500m to 150C before an alarm will be signalled!!

The 150C is SIGNIFICANTLY HIGHER THAN THE MAXIMUM SENSOR DESTRUCT TEMPERATURE SPECIFIED BY THE MANUFACTURER OF ANALOGUE SYSTEMS - The analogue system - practically speaking - CANNOT therefore claim to offer a recoverable cable.

The alarm calibration (system sensitivity) cannot be adjusted without shortening the zone length of sensor cable or reducing the max: ambient specified - which of course is not a feasible option!

In addition, specialist tooling is required for sensor cable jointing and/or terminating - a heat shrink sleeve or cap. Apply excessive heat during the jointing process and the sensor is destroyed. Apply insufficient heat and moisture ingress can occur resulting in fault or false alarm signalling!

FIBRE OPTIC

In theory: a flexible system (mechanically and design wise) with the ability of pinpointing the exact location of the alarm signal along a multi - kms zone length!

In practise: a VERY high cost system with complicated and time consuming design and set up procedure. A loop connection configuration potentially inflating the sensor cable installation length /price requirement by 100%! A product manufacturers specialist training attendance - required before commissioning or routine maintenance can be carried out by end user site engineers. In addition to which the manufacturers of these systems protect their markets by insisting upon in house design and project identification at the enquiry stage - removing confidentiality and market protection for the individual or Corporation making the initial enquiry!

PNEUMATIC / ELECTRONIC PNEUMATIC

In theory: a simple non-electrical device with high temperature withstand, rate of rise detection capabilities and increased mechanical handling flexibility.

In Practise: Limited zone length capability system, but offering recoverability (even after short time exposure to direct flame), sensitivity adjustment, high ambient temperature operation, but with a higher risk of false alarm signalling than other linear heat detection systems - due to the potential for leakage from the pressurised tubing that forms the systems detection medium. A drop in sensor internal pressure = alarm.

DIGITAL

The most price competitive, responsive, flexible in design and compatible (with Central Fire Alarm Control Panels) of all linear detection systems. The most tried, tested and Internationally approved of all systems, and in the view of the author and many International end users and specifiers, the most effective of all systems available . for the following reasons

"DIGITAL linear heat detection - the method by which abnormal temperature rises associated with overheat and / or fire conditions can be locally monitored and signalled - regardless of environmental conditions.

The combined "detector and cable in one" concept provides ease of installation, integration with central fire alarm control panels (supplied by others), the advantages of line coverage with point sensitivity and the potential for detection at incipient stages of fire or even at a pre-combustion stage (non flash fire risk situations). INDEPENDENT INTERNATIONAL RESEARCH HAS PROVEN THAT 80% OF INDUSTRIAL FIRES ARE UNDETECTED OVERHEAT CONDITIONS THAT PROGRESS TO THE COMBUSTION STAGE - DIGITAL linear heat detection may be able to PREVENT the fire, not DETECT its presence - as with most other heat + smoke detection systems!

The linear heat sensor cable - which forms the basis of the system - is comprised of two steel conductors, each individually insulated with heat sensitive polymer. The insulated conductors are twisted about each other during manufacture to impose a spring pressure and then wrapped with a protective tape and finished with an outer extrusion of ABRASION AND CORROSION resistant polymer.

The sensor cable is a fixed temperature digital switching device - which is not reliant upon length exposed to a temperature increase in order to define its operating alarm level.

At the pre selected alarm level (please specify from 70C, 90C, 105C, 138C, 180C, 220C alarm temperature rated sensor cable options) the heat sensitive polymer insulation yields to the pressure upon it, permitting the inner conductors to move into contact with each other thereby initiating an alarm signal.

This action takes place at the first heated point anywhere along the detector's length.

No system calibration is required.

Where a single zone length of sensor cable has to be installed within an area where different maximum ambient temperature levels have been recorded, it is possible to "series" connect different alarm temperature rated sensor cable in a single zone length to increase operating sensitivity.

After alarm condition has been signaled and overheat / fire condition has been rectified, sectionalized replacement (only the short damaged section of sensor cable not the complete zone length) should be carried out using junction box, crimping or similar end user approved jointing method.

Continuous monitoring of heat sensor (and interconnecting) cabling for open circuit - fault or short circuit - alarm conditions can be carried out via an existing or "supplied by others" central fire alarm control panel - removing the need for additional dedicated monitoring or signaling hardware. However, the panel zone card MUST be able to operate with the sensor cable 650ohms / km electrical resistance levels.

Where the central panel is NOT able to operate with such resistance levels and/or where "alarm point location" facility is required (i.e. the identification of exactly where along the length of heat sensor cable the alarm has occurred), a dedicated PROLINE alarm interface must be specified - which includes multiple output signaling arrangements to allow ease of integration with Central Fire Alarm Control Systems!

The alarm point location facility is of particular advantage to end users, where overheat conditions are detected by the heat sensor cable -no visible flame.

Any mechanical damage to the sensor cable will result in the two twisted conductors springing apart and generating an open circuit fault condition. As the heat sensitive extrusion around the sensors two conductors will be continuously maintained until an alarm temperature is reached, it is highly improbable that any abnormal external pressure applied to the sensor cable will result in a short circuit false alarm condition.