The debate on cable types pre-dates the draft revision by a number of years. The norm has been that the alarm circuit cables should meet the criteria laid down in BS 6387 CWZ. These criteria have given satisfactory service, with no evidence of cables failing to perform in a manner that protects life and property. Further, this has allowed designers and installers a choice of cable types (pliable or mineral insulated) to achieve the most appropriate solution for a particular set of conditions. Much of the discussion has revolved around test protocols, with proponents putting forward particular groups of tests and ignoring others.
Hardly surprisingly, in preparing the revision of BS 5839 Part 1, BSI Technical Committee FSH/12/1 has reconsidered the suitability of the test protocols in BS 6387 as well as BS 7629-1, the constructional standard for fire alarm cables. Its majority conclusion is that: "It is now considered that the temperatures, duration and methodology of the tests are inappropriate for cables of fire alarm systems complying with this part of BS 5839".
The draft proposes two levels of fire performance, but gives no indication of why the parameters have been chosen or any reason to ignore the very satisfactory performance of current standards. In fact the only 'justification' seems to be that there may be products already available to meet the new requirements – an interesting speculation when the new test protocols are not yet ratified.
With water or without?
In the revision of this part of BS 5839 a Euronorm, BS EN 50200:2000, has been introduced as the basis for assessing both of the fire resistance requirements, although it still requires the addition of a water application testing protocol.
BS EN 50200 was first published in February 2000. The test protocol integrates fire and mechanical tests; mechanical shocks are applied throughout the test duration at five minute intervals. Importantly, no water application provisions were included in the preparation of this Euronorm since this requirement was only relevant in the UK. The CENELEC TC20 WG10 standards working group has developed a proposed water application protocol to supplement EN 50200. Primary input to this work has been by the UK. This proposal provides an integrated fire, mechanical shock and water test condition; the proposed integrated test protocol will be subject to vote within CENELEC.
Standard or enhanced
Another major development in the revision of BS 5389 is the introduction of two levels of performance for fire alarm cabling: standard and enhanced performance.
In the case of standard performance, a sample is required to maintain circuit integrity when simultaneously exposed to flame at a temperature of 830(-0+40)°C with mechanical shock for 15 minutes, followed by simultaneous exposure to water drench and mechanical shock for a further 15 minutes. All the elements for this test protocol have been prepared.
For enhanced performance, a sample of cable is simultaneously exposed to flame at a temperature of 930(-0+40)°C for 60 minutes with mechanical impact, followed by a further 60 minutes of fire, mechanical shock and water drench. It is important to note that full details of this test are yet to be developed by BSI Committee GEL20 and this additional requirement has not yet been made fully known to the rest of Europe via CENELEC. However, experience during the development of these test protocols by UK cable manufacturers has shown them to be satisfactory.
During the drafting of this revision of BS 5389 it has been noted that cables that have been used for many years, performing satisfactorily without causing problems, are likely to continue to meet the new 'standard' requirements. Experience in testing cables thus far would tend to confirm that prediction. However, it should not be taken for granted that all of the products currently available in the market will meet the integrated testing requirements.
The definition of the two performance requirements for cable fire resistance has to an extent been determined by product benchmarking.
While some mineral insulated copper sheathed cables to BS EN 60 702 Part 1 will meet the enhanced requirements, the specifier, designer and user will not be limited to using only this type of cable. Conscious of the market demand for a cable with better installation characteristics, developments are continuing to enable the supply of just such an alternative to mineral insulated copper sheathed cables for this enhanced requirement.
However, it is almost certain that the adoption of enhanced performance requirements, as defined in the appropriate areas of application, will increase the installation costs of fire alarm systems installed in the future. This is not just a consideration of the cable requirements.
The draft standard effectively acknowledges that a chain is only as strong as its weakest link, and notes that other elements of the system need to be constructed of materials that will withstand a similar temperature to that of the cable. A footnote in the draft states that no definitive guidance can be given, but it can be anticipated that this situation will not be allowed to rest there.
Source
Electrical and Mechanical Contractor
Postscript
David Oldfield is product development manager with Draka Industrial Cables.
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