Fire in buildings due to the usage of electricity is common since the day of the invention of electricity. The transmission, distribution, storage, and utilization of electrical energy can have the potential to contribute to fire hazards. In India, statistics show a large number of fires in buildings are caused due to electrical short-circuiting in low voltage (230 - 440 V) systems.
It is a common belief that most electrical fires are caused by a short-circuit, there are many other possible causes of ignition as well. These can include improper installation, improper usage, and inadequate maintenance (e.g. operation under overload, operation under unsuitable conditions, inadequate heat dissipation, faulty ventilation).
In this article we primarily focus the fire hazard Fire hazards associated with Electrical Equipment.
Fire hazards & Electrical Equipment, an overview from IEC 60995-1-10 & 11
Electrical products, when operating, generate heat. In some cases, arcing and sparking are normal phenomena. They should not lead to hazardous conditions if they have been considered initially at the design stage, and subsequently during installation, while in use and during maintenance. The likelihood of ignition will depend on the product and system design, the use of safety devices and systems, and the type of materials used. Common causes of ignition encountered in electrical products are listed in table 1, out of which the most frequent causes are overheating and arcing.
Fires involving electrical products can also be initiated from external non-electrical sources. Fire which do not arise from the use of the electrical product itself, can damage that product. When designing products, the prevention of ignition in normal and abnormal operating conditions requires a higher priority compared to minimizing the eventual spread of flames. After ignition has occurred, for whatever reason, the effects of the subsequent fire must be assessed. Factors to be considered include:
a. Fire growth and flame spread.
b. Heat release.
c. Smoke generation (visibility).
d. Production of toxic fire effluent.
e. Production of potentially corrosive fire effluent.
f. The potential for explosion.
Quantification of fire risk form electrical products.
It is required to quantify the effects of the fire that is being evaluated to measure fire risk. The repercussions might be loss of life or property damage due to dangers like heat, low oxygen levels, or a build-up of incapacitating fire gases or loss of property due to damage. To determine estimates of total fire risk, a wide variety of probable fire situations may be quantitatively analysed.
If c is the consequence of the fire, and p is the probability of the fire occurring within a period, then the fire risk in that period is calculated as the product of p and c:
Fire risk = p × c
Mitigation of fire risk in electrical products.
There are two ways of mitigating fire risks. One is to reduce the probability of occurrence (reduction of p). The other is to reduce the consequence (reduction of c). Fire hazard testing is concerned with the reduction of p.
There are several distinct ways in which the probability of fire can be reduced. The most important are:
a. Product design and selection, including the selection of appropriate materials,
b. Containment using fire resistant enclosures and compartment boundaries,
c. The use of appropriate assembly and installation methods,
d. The incorporation of circuit protection devices,
e. The use of detection and suppression systems.
Fire scenarios differ in fire stages, the oxygen content, the CO/CO2 ratio, the temperature, and the irradiance.
Types of fire test on electrical products.
Assessing the fire hazard of electrical products is accomplished by performing fire tests which, dependent on the maximum dimension of the test specimen, they are,
a. Small-scale fire test where the fire test is performed on a test specimen of small dimensions of less than 1 m in size.
b. Intermediate-scale fire test where the fire test performed on a test specimen of medium dimensions between 1 m and 3 m.
c. Large-scale fire test cannot be carried out in a typical laboratory chamber and is performed on a test specimen of which the maximum dimension is greater than 3 m.
d. Real-scale fire test that simulates a given application, considering the real scale, the real way the item is installed and used, and the environment.
Due to the test criteria, all types of fire hazard tests applied to electrical products are divided into QUALITATIVE FIRE TESTS and QUANTITATIVE FIRE TESTS.
Types of fire tests are classified into five and are.
Fire simulation tests, also known as real-scale fire tests which examine the reaction to fire of electrical products and are a representation of the use of the product in practice. Since the real conditions of use of a product are simulated as closely as possible, and the design of the test procedure is related to actual risks, such tests assess the relevant aspects of the fire hazard associated with the use of the product.
Fire resistance tests are intended to assess the ability of a product or a part to retain its functional properties under specified conditions of exposure to fire, for a stated time. They are intended to provide data on the behaviour and performance of a product or a finished assembly under a particular condition of heat, fire, or test flame exposure.
Reaction to fire test is carried out on standard test specimens under defined conditions and in most cases are used to give data on properties related to burning behaviour and for comparative evaluation. Properties such as ignitability, flammability, flame spread, heat release, smoke production, toxic gas production, and corrosive gas production are measured.
Preselection fire tests is used in the process of assessing and choosing candidate materials, components, or sub-assemblies for making a product.
Basic property tests are designed to ensure that, on measuring a basic physical or chemical property of a material, they yield information that is independent of the testing method such as thermal conductivity, thermal capacity, density, melting point, boiling point, heat of vaporization, and heat of combustion.
The relationships between the steps of the fire hazard assessment and the resulting tests are shown schematically in figures 1 to 4 in flowcharts.
The fire scenario's primary purpose is to identify the product's potential contribution to each undesirable effect of the developing fire. Once the key contributors are established, methods for their quantification or measurement must be identified as illustrated in figure 1.
The first fire scenario evaluated using figure 1 will yield a list of the product's fire attributes, which relate to its contributions to the undesirable consequences arising from that fire scenario. Analysis of subsequent fire scenarios will often identify similar undesirable consequences and, therefore, many of the same fire attributes will be important. Hence, the list of required measurements will grow more slowly, or perhaps not at all, as the analysis of the fire scenarios proceeds.
Following this, the fire scenarios are ranked in order of their importance. Ranking can be done either on the basis of frequency or severity, or a combination of both. Once a fire scenario ranking is established, it becomes apparent which aspects of product fire performance are most important.
Interpretation of test results
After identifying the parameters which are to be used and how they are to be calculated, but the interpretation of the results may still pose additional technical questions.
In a fire hazard assessment, one should specify the procedure to be used in calculating an overall comparison between products, or to a baseline, such as a rule that one product is better than another only if it is better in all measures.
If more than one fire scenario has been used, it is necessary to specify the procedure to be used in making an overall fire hazard assessment.
If the fire hazard assessment is not expressed directly in terms of death, injuries or monetary loss, guidance on the other quantitative units and measurements should be provided (e.g. available safe escape time, extent of flame spread, or the size of the fire).
Ideally every electrical equipment should be qualified to the respective fire tests. In the case of air conditioners, an average of 250 fatal fire accidents are reportedly ignited from air conditioners, killing more than 300 people annually. Most air conditioners sold in the market are not subjected to the fire tests except the one’s having voluntary ISI mark.
The above article is based on IEC 60595: Guidance for assessing the fire hazard of electrotechnical products. Further details will be included in the next article.
S. Gopa Kumar is an Electrical engineer, founder, and promoter of
Cape Electric Pvt Ltd,
Cape Electric Power Production pvt ltd,
OBO Bettermann India Pvt Ltd (JV of CAPE & OBO Germany)
LP Consultants International Pvt Ltd.
He is a member in the technical committees and working groups of BIS and IEC.
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