An electrical short circuit is the most common cause of accidental fires in domestic, commercial and industrial buildings. Every year, fire takes a huge toll in terms of both human lives and properties. Several recent fires in garment factories in Bangladesh drew the attention of the world media. Foreign garment buyers are becoming increasingly critical about the safety of garment workers in Bangladesh. Many of us are not aware that most accidental fires in buildings can be prevented if a few precautions are taken while laying the electric cables and in uses of electricity.

Let us start with a simple example to explain how short circuits occur and how they cause fires. To light a normal filament bulb, we connect it to two electric wires at two different voltages. When we switch on, a current passes through the filament of the bulb which is heated to a point that it emits light. The filament has a resistance which restricts the flow of current through the wires.

We should remember that any current flowing through a resistance will produce heat. Even a conductive cable has a small resistance and is heated when a heavy or excessive current passes through it. You are likely to burn your fingers if you hold a bare cable and connect it across the two terminals of your car battery. A heavy current flowing through the cable will heat it up very quickly. It is interesting to note that the heat produced in a cable is proportional to the square of the current flowing through it. So, if the current doubles, the heat produced in the cable will increase by four times.

If we now connect another similar filament bulb in parallel with the first bulb, the current passing through the wires will be double. Similarly, if we continue to connect more bulbs in parallel, a heavy current will flow through the wires. The current will heat up the wires at a much faster rate as explained earlier and their insulation will soon melt. The two wires may thus be directly connected to each other or with another conducting material connected to earth. At this point, an excessive current will pass through the wires. This is an electrical short-circuit. It is defined as an abnormal connection of two wires at different voltages of an electric circuit resulting in excessive current. At one point the wires may be sufficiently heated to catch fire.

Like bulbs, we connect fans, motors, heaters and different appliances and machinery to an electric circuit. These are called loads. Every time a new load is added to a circuit, an additional current passes through it.

Had there been a fuse or a circuit breaker in the system to disconnect the bulbs from the main line when the current exceeded a certain safe limit, the flow of current would have automatically stopped and there would have been no short circuit or fire. The fuse or the circuit breaker is a protective device to isolate a load from the circuit when the current exceeds a safe limit

A short circuit could also occur even with a single bulb if it is connected with defective wires i.e. with insufficient or damaged insulation. Insulation can be damaged if too many cables are drawn through a narrow conduit or sharp bends. Often a defective switch, a loose contact in the circuit or a break in a wire results in electric sparks which can cause fires. On many occasions, fuses or circuit breakers fail to operate and cause fires. Fires can result from leakages of current if a live wire with poor insulation is connected to a wet wall, metal or any conducting material connected to the ground. All metal parts of a building should be earthed. Unearthed metal parts in contact with live wires may electrocute people if they touch such metal parts. Lightning, which is another form of short circuit, can also cause fires in a building unless it is fitted with a lightning arrester.

From this discussion, we learn several important lessons. First, an electric circuit must have a protective system. Second, the load at any point of the circuit must never exceed a safe limit. Third, the wires, the switches, protective devices etc must be of good quality. Fourth, there should be no loose contacts in the circuit. Fifth, no live wires, switches, sockets etc with poor insulation should come in contact with any conducting material connected to the ground. Sixth, all metal parts of buildings should be earthed. Seventh, all buildings must be fitted with lightning arresters.

We should always remember that electrical wiring is the job of a professional. It is, therefore, important to employ an electrical engineer or a professional/certified electrician to design, lay, test, modify, extend or repair electrical distribution systems in any house, office or factory. Care should be taken in procurement of cables, switches, circuit breakers, sockets, conduits and other electrical items so that they adhere to the highest safety standards. No sub-standard material should be used in electrical circuits. Applicable codes of electrical wiring must always be followed. Bangladesh National Building Code (BNBC-1993) adequately covers all aspects of electrical wiring of buildings.

Once a new distribution system has been laid, it is proper to get it tested by an independent electrical engineer or certified electrician. Under no circumstances, a novice or an apprentice electrician should be allowed to fiddle with electrical distribution systems of a building.

Special care should be taken in laying electrical lines in areas where combustible gases/materials are used, stored or manufactured. A small electric spark can cause a huge explosion if there is leakage of combustible gases. All electrical connections must be made very carefully to eliminate the existence of loose contacts in the circuit. Combustible materials, including natural and synthetic fibres, should not be stacked close to electrical cables, switches, circuit breakers etc.

Leakages of current can also cause accidental fires. Poor insulation or damp walls usually cause current to leak. The walls, the conduits, switches, circuit breakers etc must be dry at all times. The dampness of walls and conduits should be checked regularly. If any dampness is detected, the main switch should be switched off and the causes of dampness removed.

All houses, offices and factories should be equipped with fire detectors, fire fighting devices and fire exits. If there are frequent trips of circuit breakers, heating of cables, smokes or any smell of burning cables in any building, the main switch must immediately be put off and should not be put on unless the whole circuit has been checked by a professional electrician and the causes of the fault or faults have been removed. Electrical wiring including the protective devices, switches and sockets should be checked at regular intervals. Routine insulation tests, continuity and conductivity tests, cross-sectional area check of cables are helpful to detect faults in the electrical wiring.

We should remember that precautions cost very little but negligence can cost a fortune. The probability of fires caused by electrical short circuits in buildings can be significantly reduced if the simple instructions mentioned above are strictly followed.

The writer is a former chief engineer

of Bangladesh Atomic Energy

Commission.