INDUSTRIAL FIRE SAFETY
FIRES in electrical apparatus are known as Class C fires. In reality they are no different than other fires, except that the extinguishing agent must be a nonconductor of electricity. In other words, a fire in braided or rubber insulation (solid materials), in the winding of a motor or on a switchboard is treated like a Class A fire, while a fire in the transil oil of a transformer or circuit breaker (flammable liquid) is treated like a Class B fire, except that in both instances a nonconducting extinguishing agent should be used.
The extinguishers approved for Class C fires are the vaporizing-liquid type using carbon tetrachloride or chlorobromomethane, carbon dioxide and dry chemical. Water in the form of high-velocity fog can also be used on live electrical apparatus if a distance of 5 or 6 feet is maintained. However, in any electrical fire shut off the power, if possible, before attempting to extinguish the fire. In any electrical fire the damage was done at the instant of electrical breakdown, and the motor or transformer will have to be completely rewound anyway, so a delay while the power is being turned off may not be objectionable.
While it is not recommended that a solid stream of water be unnecessarily directed onto a live conductor, it is safe to direct a stream of fresh water from a 1⅛inch nozzle, at 50-psi pressure, onto 13,000-volt primary wires on the top of the poles in front of a burning multistory building—provided the nozzle is not less than 25 feet from the wires. Wires at this voltage will be at least 25 feet above the street.
If a fireman should accidentally come into contact with a live conductor he may receive a severe shock and/or bums. Arcing between electrical conductors near to a fireman may cause him severe ultraviolet bums of the eyes.
Let us study some elements of electricity, and compare it with water flowing through a fire hose.
Pressure Rate of flow
Pressure drop ohms in a length of hose
It is the amperage which injures or kills a man. However, as may be seen in the above formula, if the resistance remains the same, then the doubling of the voltage will double the amperage. Hence, we commonly say that the higher the voltage the greater the hazard. The effect of current on the average adult human being is about as follows:
1—3 ma—Barely perceptible 8—9 ma—Maximum voluntarily endured
50 ma—At, or beyond, the limit of human endurance 100 ma—About the minimum required to kill
The electrical resistance through a human being is made up of (1) the body resistance, which is low due to the body fluids; (2) the skin resistance which varies greatly, depending upon the area of contact with the conductor, and upon how moist the skin is. The total resistance between an arm and a leg ranges from 300,000 ohms for the best conditions to 300 ohms for the worst. But just because a conductor is at low voltage is no reason to have disrespect for it. Let’s consider a man whose skin is very dry (say 46,000 ohms) and who brushes against a 2300-volt wire:
—t— = 0.050 ampere, or 50 ma 46,000 ohms
Even if knocked unconscious, immediate resuscitation should restore this man to life. However, a man who is perspiring freely (say 1000 ohms) might fall against a 120-volt knife switch. ^
120 volts 1000 ohms
— 0.120 ampere, or 120 ma
This shock probably would prove fatal. The blood returning through the veins enters the right auricle of the heart, and passes into the right ventricle, which together form the first-stage pump. The blood then passes through the lungs, and enters the left auricle and finally the left ventricle, which together form the second-stage pump, from which it is delivered into the arteries.
These blood pumps work in unison, but when a low-voltage shock in excess of about 100 ma is received, the heart action is not stopped, as in a high-voltage shock, but the two pumps are thrown out of step. This condition of a quivering ventricle of the heart is known as “ventricular fibrillation.” The chances of a victim being resuscitated are very slim.
There is little or no danger of electric shock due to the conductivity of the extinguishing agent in any ordinary building fire where the voltages are not over 440. However, it is good policy to de-energize any electrical circuit which is involved in a fire before trying to extinguish it. But don’t open the main switch, for you may shut off all lights and ventilating fans, stop elevators and trap people between floors, and possibly stop the fire pump.
If the fire is in a transformer vault, don’t be in too big a hurry to go into the room. Remember that the damage has already been done. If necessary to enter the room, wear an oxygen or air mask. And remember not to operate any electric switches in an area.
It has been proven that there is little or no danger of electric current flowing from a live conductor to a hose nozzle through the fog curtain. There may be, however, a serious hazard not realized by many fire fighters if fog is played onto live equipmeat so as to form a film of water across the insulators and possibly down the building wall to the floor where a puddle is formed. If a fireman holding a bronze hose nozzle (which may be electrically grounded through the water in the hose) should step into such a puddle, his body might be the closing link between the live conductor and ground, and he could receive a shock which might be fatal.
Likewise, a similar hazard may exist if a fireman, even without holding a hose nozzle, should step into a charged puddle between the live conductor and a location where it may be in contact with a water pipe riser or other good electrical ground.
Let us consider a variable resistance coil-type rheostat, such as may be used for dimming building lights, or for similar purposes. As the sliding contact is moved to the right, say 10 inches along the coil, the voltage between the coil and ground (which is the voltage applied to the lamps or motors) gradually reduces from 120 volts to 20 volts. In other words, there is a constant voltage drop along the length of the coil, at the rate of about 10 volts for each inch that the slider is moved. If the lead wires from the voltmeter are touched to the coil 1 inch apart, anywhere along the coil, the reading will be approximately 10 volts.
Now let us consider a practical case which is almost identical except that the voltages are 100 times as great. Several years ago, during a hurricane, a man and his wife came out of a supermarket and started to walk across the street to their parked car. Both were electrocuted when they stepped into the street. They were unaware that a high-tension wire had been blown down into the street some distance away. The electric current flowed from the wire through the heavy accumulation of rain water on the pavement to some good electrical ground, such as a hydrant.
If the wire was energized at say 12,000 volts, and if its distance from the grounded object was say 60 feet, then the voltage drop was about 200 volts per foot. If they walked with a stride of 2 feet, and got into the path between the wire and the ground, then the voltage up one leg, through the body and down the other leg was about 400 volts. Keep in mind that their feet were probably wet so that they were making good contact.
If you ever find yourself in a situation where there is a current leakage such as this, remember to walk with very short steps to minimize the voltage through your body. Also, firemen’s boots are not intended to give electrical protection, although better than wet shoes.