What Every Fire Fighter Should Know
Compiled and Arranged
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SOME COMMON CAUSES OF ELECTRIC FIRES
Electric fires are usually due to defective workmanship or to lack of knowledge of ordinary electric phenomena Overheating of conductors, arcing, or sparking of electric appliances are directly responsible for the large percentage of fires.
While the development and universal adoption of the National Electrical Code have eliminated most of the practices which in the early days of the electrical industry caused many fires (and were blamed for many more they did not cause) the fact remains that in electrical energy still reside all the early potential dangers which should be sought for and overcome in all new applications of electricity—especially to domestic use. Electric wiring for lighting has been so successfully protected that we no longer look for fires and accidents from such installations in the home or office; but installations for power and heating are yet new enough to demand attention from both manufacturer and user
The electric flat iron is a constant cause of fires in homes, tailor shops, etc., no thoroughly reliable and practical safeguarding or indicating device having yet been successfully applied to it. Certain devices designed for use on electrical circuits and which may obviously cause fires if not properly constructed and operated are described below. (From a report of the Underwriters’ Laboratories.)
The use of electricity in dentistry is of special interest and its development extends over a period of about twenty years. The present-day dental panelboard for use in dental operating rooms and mounted on the wall is arranged to supply and control current at a low voltage for such istruments as mouth lamps, root canal dryers, antrum lamps, ball points, wax spatulas, cauteries, hot-air syringes and similar devices. Such equipment as dental engines, lathes, automatic air compressors, furnaces, gold annealers, atomizers, sterilizers and water heaters are furnished with current at ordinary voltage from special connections on the board.
Panelboards for this purpose are of marble and are mounted on a cast-iron frameUpon the front of the board are pilot lamps, switches, fuses, rheostats, the different air regulating attachments and the low voltage terminals. Methods of arranging these parts vary with different manufacturers, but the controlling resistances and small transformers, if any, are found either on the back of the board or in a special cast-iron case on the floor.
The wiring on the back of the board should be and usually is done either with rubber-covered or with slow-burning wire, depending upon the temperature attained here under extreme conditions.
From time to time electric heater pads, foot warmers and electrically heated so-called blankets have been submitted to Underwriters’ Laboratories for examination and test, with request for approval and favorable listing.
Such devices, when tested in the open and freely exposed to the air, might operate in an apparently safe and acceptable manner It can readily be understood that when placed in practical service, heater pads, foot warmers and the like are covered by a mass of heat-insulating material and with continued operation the heat must necessarily rise until the radiation and conduction from the heated material surfaces dissipate the energy as rapidly as it is being supplied. Before such an equality is established, the rise in temperature is sufficient to cause the ignition of readily combustible material surrounding the heater device, and a fire is probable.
Recognizing the extremely hazardous conditions presented by the use of heater pads, various attempts have been made to provide thermostats or automatic cutout devices which will cause the supply circuit to be opened at a predetermined temperature, and which would prevent the ignition of materials and the starting of a fire. Automatic appliances of this character are somewhat sensitive in operation and are quite subject to sticking and arcing at the contacts. They are also likely to be accessible, thereby making it easy to change the adjustment and possibly raise the temperature at which the circuit will be opened.
Heater pads must be more or less flexible and cannot conveniently be of bulky or cumbersome size, ad the thermostats or thermo cutouts are generally not suitable for the relatively rough usage met with under conditions of actual service
The illustrations in Figure 8 show beater pads which actually caused fires while in use The circumstances attending one of these fires are as follows: The pad was used to heat a bed during unusually cold weather The pad was left in service for the night. At a later hour the person was awakened by the smell of smoke, when it was found that the bedding had been charred completely through immediately over the heater, and the mattress underneath also injured The edges of the charred section were aglow and ready to burst into flames, but prompt action with a pitcher of water and turning oil of the heater prevented what might have resulted in fire and personal injury.
Danger of Poorly Made Electric Toasters
The increasing popularity and use of electric household heating appliances has resulted in the appearance on the market of cheap electric toasters in whi-ch the amount of-material is reduced to a minimum, and the construction to the simplest form. One of such toasters is shown in Figure 9. This particular toaster is of the horizontal type and is simply a rectangular sheet-metal frame supporting several wires across the top, spaced about 11/2 inches apart, and a length of resistance ribbon terminating in a pair of ordinary dry battery binding screws and looped between two narrow strips of asbestos board. It will be evident from a consideration of the features of construction and assembly that little, if any, attention has been paid to the hazards incident to the use of a heating device. Among these hazards may be mentioned the exposed and unprotected heating element; lack of protection from heat reflected downward, on the surface under the toaster; absence of upturned lugs or equivalent means for preventing the current supply wires from becoming disconnected at binding nuts; liability of loose strands of the supply wires coming in contact with and making the frame “alive,” and the use of supply wires having insulation not designed for electric heaters-
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What Every Fire Fighter Should Know
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The photograph points out some of the effects to be expected from the use or abuse of an appliance of this character.
The toaster, with the current turned on, was placed on an asbestos pad with a single sheet of white note paper beneath and with a square of sheet asbestos the size of a slice of bread on top. The result on the paper at the end of ten minutes is shown on the left.
The toaster was next placed on a pine board with a layer of cheese-cloth beneath and a square of sheet asbestos on top. At the center of the photograph the appearance of the cheesecloth at the end of five minutes is shown.
A third test was made by placing the toaster on the uncovered varnished surface of a pine board with a square of sheet asbestos on top. The varnish blistered in one-half minute and at the end of four minutes the board appeared as shown on the right of the illustration.
With the toaster placed on a plain, uncovered pine board, with a square of sheet asbestos where a slice of bread would be placed, flames enveloped the toaster in six minutes.
In the 25-minute period over which the foregoing tests were continued, the insulation on the lead wires adjacent to the terminals was entirely destroyed by the heat for a length of about two inches This destruction of insulation extended far enough along the wires so that the bare strands of both wires could come in contact with the edge of the frame of the heater.
The incidents above are simply cases of overheating of conductors, either through improper design or through lack of radiation of the generated heat It must be remembered that a body accumulates heat, and if the heat is supplied to it at a greater rate than it is taken away, the temperature of the body is bound to rise.
Arcing may be caused by breaking a circuit in which current is flowing, as when a switch is opened while the load is on the line. In this case an arc is apt to follow the tjlades of the switch until they are so far away from tbc’-javvs that the resistance of the air between is too great to permit the current to continue to flow. Arcing may also be caused by high voltage causing current to leap a gap in a conductor. Lightning is one of the sources of exceedingly high voltages, which produces such arcing.
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