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Power consumption and the number of gadgets using electricity in a building have increased. Whenever a fire is reported the first question or the hypothesis for the cause of the fire will be an Electrical short circuit. While we can neither dispense with electricity nor reduce the gadgets using electricity, we have to give a serious thought and also analyze whether electricity is the real culprit for causing fires. Most of the fires are initiated from other causes. But even then at one or the other intermediate stage of fire growth the electrical equipment will be involved or attacked by the fire and the electrical installation will unavoidably contribute its energy for the growth of the fire.
Electricity respects only those who respect it. Electrical Installations in proper state of repair do not cause fires.
The normal protective devices will provide adequate protection, unless improper additions are made and defective appliances are connected.
Today the designer has at his disposal a few new products which can help in reducing the possibility of contribution of electrical energy to the enhancement of fire.
This article brings out some of the new device available. An analysis of the types and causes of electrical fires is also made. An attempt is also made to correlate areas where the new devices available can be of use.
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Power consumption and the number of gadgets using electricity in a building have increased. The general increase in power generation and distribution systems automatically has brought in an increase in the power that would be fed to a fault. It is practically impossible for a fire to be initiated by an electrical installation. But any fire initiated by any other cause will very soon involve the electrical installations.
Once the electrical installations are affected by the fire, the improper and uncontrolled flow of electrical energy will speed up the growth of fire. In the post fire scenario it may not be possible to find the actual primary cause of the fire and electrical short circuit will be the whipping boy to take the blame.
Electrical fires do occur. But electrical installations will get involved whether it is an electrical fire or otherwise. An analysis of the types and causes of electrical fires is also made.
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One of the major changes that has been taking place in the built environment is the increase in the number of electrical gadgets that are in use and the rate at which it is growing. Higher power density in buildings has in turn led to higher transformer capacity, both at the building substation as well as at various points in the system right from the generating station. The increased capacities naturally contribute more energy in the event of a fault also.
Fire incidents caused by & having their origin at any electrical installation are ‘Electrical Fires’. Fire incidents initiated by other ignition sources such as those caused by
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The maximum fault current that would have
been available within a typical building about 25
years ago was in the range of 3KA to 5KA,
depending on the proximity of the building to the
substation.
Typical fault current available in a multi–
storied building today is of the order of 25KA to
50KA. The system voltage referred is the same in
both cases. Such an increase has been unavoidable
due to the tremendous increase in demand for power.
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As an example if we take Shastri Bhavan as a typical building the maximum power demand has increased
from 700KVA in 1966 to 2660 in 1996. The system
fault level of the NDMC system, which was 60MVA
(requiring 150MVA HT switchgear) has gone to
300MVA (requiring 350MVA switchgear).
Energy fed into a fault is
Efault = Vfault X Ifault X t,
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With progressive increase in capacity of generation and distribution ‘Ifault’ has been on the increase. There is no control over the ‘Vfault’ as it is dependent on the fault resistance. Our general design aim is to keep the fault resistance as low as possible, as a high fault resistance tends to increase the shock and electrocution hazards.
As such Energy fed into a fault
Efault = Vfault X Ifault X t
=(Ifault X Rfault ) X Ifault X t
= Ifault2 X Rfault X t
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A fault with a high resistance is called leakage and a fault with a low resistance is called a short circuit in common reference or parlance. The fault resistance should be very low to ensure low shock risk and also to cause early operation of the protective device through a high ‘Ifault’ , thereby reduce the fault duration ‘t’. This has been the guiding principle for distribution system design to keep the energy fed into a fault low while keeping the shock hazard also at the minimum. The fault resistance changes dynamically, almost always varying to our disadvantage. A high resistance fault (or a leakage), generally exhibits a progressively reducing resistance and consequent increasing energy feed to the fault. A low resistance fault (or a short circuit), exhibits continuous swings in the fault resistance, but rarely reaching zero resistance, causing continuously varying, but progressively increasing energy feed to the fault. As such the control over the fault can be exercised only be by reducing the duration of the fault. The term ‘let through energy’ is used to refer to the energy that passes into a fault before it is isolated by the protective device. The ‘let through energy’ is proportional to (Ifault) 2 & t. Methods to reduce (Ifault)2 & t are the means of protection against ‘Electrical Fires’.
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Reduction or control of (Ifault) 2 had been attempted in the past by use of series reactors or/and by using a number of transformers of small capacity instead of large transformers in the building substations. But this has been counter productive as the voltage regulation, energy efficiency suffer and the distribution cost goes up. The loss in regulation may also cause more problems due to voltage stabilizers which will in turn increase the current. Control can be exercised only on ‘t’, the duration for which the fault is fed.
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The common protective devices in electrical circuits protect the systems by disconnecting or isolating the faulty section automatically on the basis of sensing abnormal current, or voltage or power flow.
Rewirable fuses were an adequate means of protection in systems where the fault current was of the order of 3KA. These rewirable fuses become the source of an electrical fire when they are used in a system with fault currents exceeding 10KA.
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The subsequent development was the HRC Fuse (High rupturing capacity fuses). In HRC fuses the fuse wire element is encased in a refractory material and even at the time of fusing and arcing the heat dissipated is held in a totally non–combustible space. Further the filler materials surrounding the fuse wire generate gasses (under the effect of the high current), which help in cutting off the current in lesser time. Higher the prospective fault current shorter will be the time for cutoff. This feature of HRC Fuses makes them an ideal protective device from the aspect of Fire control. But unfortunately in practice there is tremendous mismanagement leading to fitting external or internal fuse wires in HRC Fuse cartridges, which makes them worse than a rewirable fuse in its environment.
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HRC Fuses are a boon in the context of protection against electrical fires. The ‘let through energy’ is the minimum when compared to any of the other protective devices. The greatest advantage is that the ‘let through energy’ in fact comes down in case of large fault currents.
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The only serious disadvantage of HRC Fuse is the requirement of spare fuses readily for replacement; and they are expensive in large current sizes. In the event of a HRC replacement not being available readily the HRC Fuse will be wired up unscrupulously leading to a very high level fire hazard.
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Miniature Circuit Breakers or MCB`s are switches with the feature of automatic switching off in the event of an overcurrent either by its magnetic element (which operates in a short time on large fault currents) or by its thermal element (which operates with a graded time delay related to the intensity of the current).
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MCB`s can handle only limited fault currents typically 3KA or 6KA or 9KA. In view of the ever increasing fault levels due to increase in generation & distribution capacities general choice is for the 9KA MCB`s.
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MCB`s will be satisfactory in most of the built environments (prospective fault current less than 9KA). Where the fault level is higher the MCB or groups of MCB`s will have to be backed up by HRC Fuses.
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MCB`s give the advantage of dependability as they are sealed and do not require any field adjustments or calibration. As such they are very beneficial in eliminating the possibility of fire by short circuit. (Even when in a location with a prospective fault current higher than its rating the device is still reliable and will destruct itself in the event of a fault beyond its capacity). However in such cases the MCB should have a HRC Fuse backup.
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MCB`s & HRC Fuses practically eliminate the possibility of the electrical installations initiating a fire by a short circuit.
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Current flow in any electrical circuit has to be in a defined path. As such the current in the forward path has to be equal to the return path current. Residual current circuit breaker (also called ‘Earth leakage circuit breaker’) monitors a circuit from this aspect of balance in the current flow in the two paths and isolates the circuit in the event of an unbalance exceeding the set limit of 30mA. (RCCB`s are also set for limiting unbalance currents of 100mA & 300mA for different applications). Flow of current to earth due to leakage through weak insulation or due to an intended high resistance connection will cause tripping or the ELCB (or RCCB).
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RCCB`s practically eliminate the possibility of the electrical installations initiating a fire by a leakage.
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The common misconception that fires are caused by short circuits has been discussed above and it has also been brought out that with the present day protective equipment there is absolutely no risk of initiation of a fire by electricity. The protective devices such as MCB`s and HRC Fuses have become a common feature of present day installations.
Leakage of electricity and possibility of the same developing into a hot spot and later leading to a fire had been a point of discussion in the past. But with the availability of RCCB`s (or ELCB`s) and their successful deployment in electrical installations possibility of initiation of a fire by leakage is also eliminated.
There are however other elements of an
electrical installation such as cables (or wires) and
control switches which are also to be examined from
the aspect of initiation of a fire.
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Due to shortage of copper in the country the government took a decision during 1964 to ban the use of copper conductor for other than control and communication applications. This led to extensive use of aluminum conductor cables for small wiring for power and light in buildings. Though aluminum is a good conductor of electricity its mechanical properties such as ductility, cold flow under thermal cycling and flow under pressure as well s the chemical properties of its surface oxidation were problems which were neither appreciated nor were solved before wide scale usage of aluminum conductor started.
Another related problem has been the lack of development appropriate to aluminum and the absence of modification of terminals in switches and connectors to suit the properties of aluminum.
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The contact of aluminum conductor with the brass terminal would deteriorate due to cold flow of aluminum. This coupled with surface oxidation of aluminum leads to increase in contact resistance and generation of more heat. Higher temperature leads to further flow of aluminum at the terminal completing a vicious circle. Higher temperature would later lead to smoldering of the PVC insulation progressively extending from the terminal till the naked aluminum conductor is brought in contact with another conductor or earthed metal, leading to a fault and quite often resulted in the combustion of PVC of the insulation before developing into a short circuit to be cleared by a fuse or circuit breaker.
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Clamp type terminals instead of screw type terminals have been developed. These hold the contact with lesser pressure to avoid cold flow, while providing a larger area of contact to reduce contact resistance and heating due to current flow. But even with these there cannot be total satisfaction of termination of aluminum conductors. The only satisfactory solution is to revert back to copper conductor at least for sizes below 10mm2. The government ban on use of copper conductor cables has been removed. As such copper conductors only should be used for smaller size cables. Aluminum can be used for larger size and long runs of multicore power cables.
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Another improvement desirable is the use of FRLS (fire resistant low smoke) cables, which cost about 10% more than the PVC insulated cables, contributing a mere 0.05% to 0.25% increase in the cost of building. The extra investment is worth the advantage as there would be total freedom from the chlorine rich dark smoke in the event of a fire whether it is of electrical origin or be of any other origin. FRLS cables are also less prone to smoldering and fire even in the event of heat production at the termination.
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Techniques for jointing aluminum are quite different from those that were in vogue for copper. Joints and terminations made with the old techniques would not last long because of the different properties of aluminum explained earlier. In high current applications, particularly in cases where current sharing was necessary by parallel cables the joint failures were very common. The common remedy for a copper cable termination was tightening, which when applied to an aluminum cable termination made matter worse by cold flow leading to sparking and sometimes fires.
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Today’s electrician is trained to handle aluminum and he treats aluminum as aluminum, whereas in the past due to lack of appreciation of the properties of aluminum, aluminum was being treated with methods and forces which were harmful.
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Cable terminations are no more a cause of worry from the aspects of fire.
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Another alternative is to use XLPE insulated cables instead of PVC insulated cables (conductor in both cases being aluminum in sizes 10mm2 and above and copper for smaller sections). XLPE cables have a better thermal performance and do not produce chlorine rich dark smoke like PVC. The extra cost of about 20% in the cost of power cable should not discourage the use of XLPE cables as ultimately the increase in building cost will be of the order of only 0.25 to 0.30 percent of the building cost and freedom from fire is assured.
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Use of copper conductor cables in sizes below 10mm2 with FRLS insulation and XLPE insulated cables (could be with aluminum conductors) for major power cables handling more than 200 A should be a strict practice to eliminate the possibility of fire originating in cable terminations.
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While fires caused by equipment do not strictly fall under the purview of this paper it would not be incorrect to analyse fires originating in the components of electrical equipment such as chokes of fluorescent tube fittings, capacitors in window type air conditioning plants, electrical heating devices etc.,
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Fires originating in these components was quite common about 15 years ago. But today considerable improvement in quality has taken place. Further there has been a tremendous change in the choice of raw materials that go into these devices. Raw material selection and advancement of material science has drastically reduced the possibility of fires from these devices.
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Chokes for fluorescent tube fittings Choke design has undergone a significant change over the last two decades.Chokes are today designed with the temperature rise to be within the polyester withstand limit even when the fluorescent tube has failed. (Earlier designs invariably led to heating of the choke beyond the limits if the tube failed to light up). As a result of the design changes chokes have become more compact, use less copper and iron and have lesser thickness of the polyester filler leading to better heat disposal.
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Even for these chokes a built in thermal cutout has been developed. The additional feature of the built in cutout increases the cost marginally by only about 5% of the cost of the choke.
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Chokes are soon going to be superseeded by the electronic ballasts which consume much less power (about 2W as against about 14 W for the choke) and provide better utilization of the fluorescent tube and are absolutely safe from the risk of initiating a fire. Electronic ballasts incorporate a fuse within their circuit for the protrection of the semiconductor devices used. This feature makes them absolutely safe. (Their widespread application has not picked up due to their high cost and the unsatisfactory harmonic current charecteristics).
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There have been many cases of capacitors bursting in the window type air conditioning units in the past. On many occasions of capacitor failure (bursting) there would be smoke due to the evaporation of the dielectric material which could be called a fire eventhough there would not be any sustained fire. But present day capacitors have been satisfactory and there are no reports of capacitors bursting.
The air conditioner designs were copied from the designs applicable to the temperate regions of the west and were basically difficient for the duty required in the hot dry summer climate of the tropical region. (Even today the Indian Standards also call for the rating of the AC unit at an external ambient temperature of only 35 C, which is absurdly low as the typical summer ambient temperature would be 42 C).
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The capacitor design technology has also changed. Today’s polypropeline capacitors are classified as ‘self-healing’, because of their ability to handle surge voltages. A very small portion of the electrode film is sacrificed in the event of an overvoltage and the capacitor is back in its normal operation after the surge passes off. (A very small loss in the rating of the capacitor takes place whenever a surge is handled. This leads to a progressive reduction of the capacity of the capacitor). But from the aspect of originating a fire today’s capacitors are safe.
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Heating devices attract the maximum attention when we are considering the possibility of fire and it is also natural that the fire hazards associated with heating devices have got to be high.
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Radiation type room heaters have been responsible for a nuber of fires. They can ignite any material such as curtain cloth or upholstry etc., if the heater is placed in close proximity. But we cannot classify such fires as of electrical origin and have to be attributed to negligence of the user.
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Fan type room heaters are relatively safe in as much as they will not directly ignite any combustible material in the visinity.
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Electrical strip heaters provided in the window AC units either for monsoon dehumidification or for winter heating are a positive hazard as they can reach abnormally high temperatures inte event of restrictions on the air flow from the fan. Thermal insulation materials in the adjoining spaces of the AC units are likely to catch fire. Use of heaters in window AC units has been banned by CPWD in view of these hazards.
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There have been a few instances of the strip heaters provided in package AC units having reached abnormally high temperatures due to the failure of the fan and consequent stoppage of air flow around the heater strips. Early designs of the package units did not provide adequete safety. Further the ducts even in the viscinity of the heater bank were provided with thermal insulation such as polyurethane foam. There have been cases where plywood sheets have been used for ducts even in the viscinity of the heater bank. Foam insulation and wood have contributed to increase the fire once the ignition took place due to an overheated strip heater.
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Another design improvement to avoid the possibility of fire from a package unit which has heaters is to use non combustible thermal insulation for the ducts in the section upto 1.5 meters from the location oh the heaters.
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Food warming has been considered an essential neccessity in the northren parts of the country during winter. In the absence of hot cases food is warmed using hot plates with plain open Nichrome coil elements. Such heaters are also used for preparetion of tea etc., A number of cases of fire caused due to these heaters, which for convienience are kept in work and storage areas of offices, instead of being kept only in areas such as the canteen, pantry etc., Quite often such devices which should be used only on a 16A rated power socket are used in a 6A socket or even in asocket meant for office equipment such as Computers, Copiers etc.,
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Current limiting semi conductor varistors have long been in use for limiting current in many electronic circuits. But these devices were available only at very low power levels of upto 10 watts and for milli ampere current applications. Asea Brown Bowari of Sweeden and Togami of Japan are reported to have brought out such current limiting semi conductor cartridges for currents in the power application range upto 200 A. These devices have the property of a steep increase in resistance when a current above the design threshold flows through them. The fault current value will be restricted to a high but not unreasonably large compared to the normal full load current of the device. The energy fed to the fault is drastically reduced by reducing the Ifault This reduces {(Ifault)2 x t }, the feed into a fault and allows the isolating protective device such as MCB or Fuse to act and stop the feed. These devices can be so planned that even the duty on the protective device is made less stringent.
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These devices are today made only by two manufacturers in the world, are expensive and are yet to be used on a wide scale.But they are claimed to have the potential of totally eliminating the possibility of electrical energy feed beyond a set limit.
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Two types of Thermal cutouts are available.
One type consists of a small semi conductor device which exhibits a significant change in its resistance due to a change in temperature. They are used in equipment such as switch boards, equipment terminal boxes etc., and they operate in conjunction with the normal protective devices such as circuit breakers to trip the breaker in the event of an abnormal temperature in the enclosure. These devices are not current dependant. The load current does not flow throught them. Only the control current flows through them.
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The second type is based on a bi–mettalic strip which exhibits a bend in the strip under abnormal temperature and mechanically trips the equipment. Such devices are common in good quality household gadgets such a mixers–grinders chokes for flourescent tubes etc., But these are limited to low current applications less than 5A.
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Fire survival cables which can withstand (means continue to be in functional service even if there has been a change in the properties of the insulation & the conductor materials) a temperature of 950oC for 30 minutes or 700oC for 90 minutes are available. In view of their high cost they have not been used much.
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This
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There