Newsheet 1

Cable Trays:- Fire and/or Overheat Detection

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The early 1940's, saw the development and introduction of the concept of electrically operated "digital " Linear Heat Detection systems, and its successful employment for the protection of cable trays against fire and / or overheat conditions - predominantly within Power Generation and distribution plants. This application is in fact considered by many to be the most "traditional" of all applications for cable based linear detection systems.

This detection concept - as a combined cable and detector - installed at the exact point of risk and not the most convenient roof or ceiling height - provides rapid LOCALISED detection of abnormal temperature rises. Such conditions can be generated from overloading of high voltage power cables, short circuit fault conditions on the same cables, cable jointing overheats and fires generated by extraneous sources - local maintenance work ( ie soldering tools, halogen lights etc carelessly located on / adjacent to the trays and cables ) or ignition of rubbish left on the tray or under the bottom tray on multi-tier tray installations.

Since the 1970's - with the introduction of "analogue" linear heat detection and formation of more Companies specialising in this area of detection - the specification of linear detection for cable tray protection has become much more International. Today, linear detection systems are regularly specified by end users, consultants, Fire engineering Companies etc throughout all the major continents across the globe. With literally thousands of installations now also protecting cable trays at petro-chemical and chemical plants, airports, railway stations ( above and below ground ), hospitals, shopping centres, office blocks - to name but a few!

The environmental conditions that are found at such sites and installations - dirty, damp, humid, dusty, abrasive, corrosive etc, severely restrict the detection options available to the end user. Attempts at using alternative detection methods have repeatedly proven troublesome - with high maintenance costs, greatly reduced operating lifetimes and frequent false alarm conditions.

Linear Heat Detection, is able to provide a minimum 10 year Installation life, with a limited or maintenance free requirement (more preventive maintenance is recommended to ensure fixings remain in place and the heat sensor cable is not subject to mechanical damage from abrasion against edge of trays etc). The installation of the heat sensor cable 150 - 200 mms above the tray / risk it is protecting (see enclosed sketch) allows access, without obstruction, to the tray for power cable repair, replacement or addition of new cables by site engineers, whilst retaining the ability to detect abnormal temperatures generated by cable overheat conditions. In many cases, providing the customer with an alarm BEFORE combustion temperatures are reached - thereby preventing a) loss of service, b) the cost and inconvenience associated with replacement or repair of power cables c) risk to human life and capital investment.

System design procedures that should be adopted for linear detection include :-

Zoning 1 - use of fire barriers or fire doors in tunnels to provide easily identified zone locations.
Zoning 2 - Where associated extinguishing systems are specified or installed, the detection zone should match the area covered by the extinguishing zone.
Cable Tray width - a single run of heat sensor cable will provide adequate coverage for trays up to 600mm width. For wider trays, a second run of sensor cable per tray should be installed.
Always install a sensor cable run beneath the bottom tray - the incidence of rubbish fires are MUCH greater than many believe. Even on those plants that exercise a stringent "good housekeeping" policy.
Alarm Temperature selection - A minimum of 20C should be allowed between the specified maximum ambient temperature and minimum linear heat detection alarm temperature.
Sensor cables should always be terminated at the start of the designated zone - using non heat sensitive / short term fire-proof cabling for connection to the remote interface / control panel.
Trays carrying communication / instrumentation cables ( rather than power cables ) may present less of a fire / overheat risk, but still provide an essential service to the site. Fire damage or overheating of these cables DO still occur and can prove costly to the service provider should they become inoperative as a result of a situation that could have simply and economically been detected by linear heat detection - and prevented!

Linear Heat Detection has a proven track record for the protection of cables that can be verified by many major International end users. For additional information on the most effective system to suit your particular requirements, please contact us.

A - MULTI - TIER CABLE TRAYS - INTERNATIONAL DESIGN FOR PROTECTION - PARALLEL RUNS

B - MULTI - TIER CABLE TRAYS - U.S.A. DESIGN FOR PROTECTION = "SINE WAVE " ON TRAY

Direct contact between sensor and cables presents obstruction to site engineers during routine maintainance, repair or replacement of power / instrumentation cables. In many events, sensor cable removed to allow access, is NOT replaced after completion of work. Resulting in loss of protection of essential cable services.