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Fires

Fires are responsible for causing hazards to human life and the property because of the heat generation (thermal hazard), toxic effluent, corrosive effluents and smokes (non-thermal hazard). Fire hazard increases with the burning area leading in some cases to flash-over and a fully developed fire. This is a typical fire scenario in buildings.

The best method for testing electrotechnical products with regard to fire hazard is to reproduce exactly the real conditions. In many cases this is not possible. Consequently, for practical reasons, the fire tests of electrotechnical products are carried out by simulating as closely as possible the actual effects occurring in practice.

Fire tests made carried out on the end-product are to be preferred; however, sub-assemblies, components or parts of a system sometimes may be used for this purpose even for practical reasons.

The National Fire Regulators and the National Standardisation Bodies dedicated lot of their efforts to show that the test conditions (the fire model) defined in standardized test method are relevant to, and allow to replicate/simulate a particular stage of a fire.

During recent years big steps have been made in the analysis of fires and ISO has published a general classification of fire stages (see Table 1 of ISO/TR 9122-1).

Tests conditions used in laboratories, defined “scale tests” can be derived from table 1 of ISO/TR 9122-1 in order to correspond, as far as possible, to the full scale fires.

However, fires involve complex and interrelated physical and chemical phenomena and, as a consequence, it is difficult to simulate all the aspects of a fire during a laboratory test. The problem related to the fire model validity is perhaps one of single most difficult technical problems associated with the fire testing.

After the ignition, the fire development may occur in different ways depending on the environmental conditions, buildings and the chemical compositions of combustible materials.

However, within a compartment or a room, a general may be established showing a three stages diagram (temperature-time) plus a final decay stage (see Figure 1).



Tabella 1

Table 1



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Figure 1

 

Stage 1 (non-flaming decomposition) is the incipient stage of the fire prior to sustained flaming, with little rise in the fire room temperature. Smoke and toxic effluent production are the main hazards during this stage.

Stage 2 (developing fire) starts with ignition and ends with an exponential rise in fire room temperature. Spread of flame, heat release, and the production of smoke and toxic effluent are the main hazards during this stage.

Stage 3 (fully developed fire) starts when the surface of all of the combustible contents of the room has decomposed to such an extent that sudden ignition occurs all over the room, with a rapid and large increase in temperature (flashover).

At the end of Stage 3, the combustibles and/or oxygen have been largely consumed and hence the temperature decreases at a rate which depends on the ventilation and the heat and mass transfer characteristics of the system. This is known as the decay stage.

In each of these stages, a different mixture of decomposition products may be formed and this, in turn, will influence the toxicity of the fire effluent produced during that stage.

Cooperations with other Technical Committees

Within these frameworks, IEC TC 89 is the horizontal committee with a safety pilot function assigned to write basic safety publications concerned with the fundamental aspect of fire.

TC 89 prepares safety standards and test methods for fire hazard testing for some Technical product Committees, e.g. TC 14 “Power transformers”, TC 20 “Cables”, TC 34 “Luminaires and related equipment”, TC 59/61 “Household and similar electrical appliances”, etc.

The tests developed by TC 89 are classified in order to fit to the purpose of the TC.

Struttura CT89 IEC.

The last ten years has witnessed fundamental improvements in TC 89 test methods and guidance. Each test method has been thoroughly re-analyzed and improved where necessary to achieve greater repeatability and reproducibility. This has, in part, been accomplished in cooperation with ISO/TC 92 and ISO/TC 61 and the CTL (Committee of Testing Laboratories).

TC 89 produced a number of standards and technical specifications/reports intended to be used by product committees for updating in their standards the requirements related to the improvement of the safety products with respect to the fire risks and introducing methods for measuring some characteristics of reaction to fire of the products (e.g. heat release, production of smoke, etc) to be indicated in the technical information of the product.

Those data give the project engineers and the safety authorities elements for evaluating the fire scenarios of the different locations in which electrical products are installed/used, in view of a correct adoption of the measures for preventing fires or for reducing the relevant risks.

The requirements contained in a large number of present IEC product standards are taken from IEC TC 89 publications edited before 1988-1990. The product Committees concerning the above mentioned standards should be encouraged to update/complete them by taking into account the more recent TC 89 publications, in which the increased knowledge acquired in the last decade on fire safety techniques (e.g. FSE) and tests is taken into account.

Moreover IEC Guide 108 (“The relationship between technical committees with horizontal functions and product committees and the use of basic publications”), and the recent IEC Administrative Circular 29/AC recommend the use of basic publications by product Technical Committees with intend to contribute to coherency of requirements, avoidance of overlapping and duplication of work.

The use of relevant basic standards by product Technical Committees in preparation of their standards is a requirement to:

  • ensure the consistency of IEC publications in areas common to a number of technical committees by avoiding duplication of work and contradictory requirements;
  • improve the coherency of the IEC publications system;
  • improve the mutual understanding of engineers of different technical disciplines.

Knowledge and understanding of basic standards by product committees is best achieved through providing their input from the start to projects of TCs/SCs holding a horizontal function and TCs/SCs holding a partial horizontal function.

1) USE OF HORIZONTAL STANDARDS BY PRODUCT COMMITTEES

Product Technical Committees are strongly recommended to use basic publications wherever possible. Exceptions from this general principle are given in clause 5 of Guide 108, as follows:

  • When the appropriate basic standards are not referenced in product standards, secretaries of Product TCs/SCs are requested to report exceptions to the Standardization Management Board (SMB) indicating the reasons for not making reference to those standards.
  • In the event that it may not be practicable for a product TC to adapt its standards immediately to a new or revised basic standards, this adaptation should, nevertheless, be performed when the product standard is next amended or revised (item 5.2.2 Guide 108).

2) EXCHANGE OF INFORMATION ON NEW PROJECTS BETWEEN HORIZONTAL COMMITTEES AND PRODUCT COMMITTEES

Improving cooperation between horizontal committees and IEC product Technical Committees is recommended to contribute from the start to the development of standards of horizontal committees and to assure transparency of the process of the development of basic standards. In particular:

  • Horizontal committees should indicate on NPs those product committees concerned and should seek out appropriate candidates for participation.
  • Product Technical Committees are strongly encouraged to review NPs and revisions submitted by horizontal Committees, available on the IEC web page.
  • Officers and concerned participants in product committees are requested to use the feature “my IEC” which will notify them of documents from horizontal Committees, which are of interest to them.
  • Product Committees/Subcommittees are strongly encouraged to establish liaisons with horizontal Committees, and to appoint experts, confirmed by their National Committees, to the relevant Working Groups of these TC/SCs. It is recommended that the size of the Working Group be limited.
  • TC/SCs product experts to WGs of horizontal TCs/SCs shall be included in the circulation list of documents of these Working Groups.
  • For coordination purposes experts shall report back to their TCs/SCs and highlight potential problems.

3) FIRE HAZARD TERMINOLOGY

IEC 13943:2000 deals with the current terminology used in the fire safety sector.

It is recommended, mainly for product Committees, to use this standard together with the subjects included in the present dedicated area of CEI CT 89.

4) ASSIGNMENT OF HORIZONTAL FUNCTIONS AND PROMOTION OF BASIC STANDARDS

IEC Guide 108:

  • indicates how horizontal functions are assigned by the SMB;
  • gives precise advice to horizontal committees and product committees with regard to their relationship;
  • gives general advice on horizontal matters;
  • the list of TC/SCs and their horizontal functions, with links to the TC/SC publications and their work programmes, is available on the IEC WEB page.

5) INDUSTRY PARTICIPATION IN HORIZONTAL TECHNICAL COMMITTEES

It is recognized that not all industry sectors actively participate in the work of the horizontal Committees.

Nevertheless, industry when developing products needs to be aware of the requirements given in basic standards. Involvement of industry in horizontal work by providing input through product committees at the initial stage of the development of horizontal TC work would help to satisfy the needs of product Committees.

CONCLUSIONS

There will be a Global increase of electrotechnical products in all application areas (e.g. Information Technology systems, buildings, transport …), and Electrotechnical products present two potential hazards : they convey energy and may be a source of ignition, they may be an avenue of fire spread.

Together with other standardisation bodies, TC 89 aims to establish basis for Fire Hazard Assessment (FHA) and for test methods which provide performance based data in a format suitable for use in Fire Safety Engineering FSE.

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