Knowing the safely hazards of electrical construction can help reduce accidents and fatalities.
Electric current can injure and even kill directly. It sometimes is the trigger that initiates a chain reaction of other physical problems. As such, it's important that you understand the nature of electrical hazards. To do so, we'll discuss the nature of the three most significant sources of injury: shock, arc blast, and suffocation/poison.
There are two types of electric shock: macro- and micro-shock. Macro-shock is a type of shock originating from the outer skin, micro-shock originates from an electrode that has penetrated through the outer skin. For our discussion here, we'll concentrate on macro-shock. Before proceeding, we should review some electrical basics and how they pertain to electric shock.
As we all know, a basic electric circuit has three components: voltage, current, and resistance. The killer component is current.
Basics. Per Ohm's Law, the magnitude of current is directly proportional to voltage. (This is the main reason why the fatality rate on medium-voltage (MV) accidents is 10 times higher than for low voltage accidents.)
Also per Ohm's Law, the current is inversely proportional to resistance. The resistance of human skin varies as a function of moisture content in its external and internal layers. This moisture content changes with ambient temperature, overwork, high humidity, fright, or anxiety. Typical skin resistance values range from 500 ohms (moist skin) to 300,000 ohms (dry skin).
The effect of electric current passing through a human body is also a function of the current's magnitude and the duration of current passage. As the current flows through tissue, it causes heating. Tests have shown that human tissue is destroyed at 122 [degrees] F.
Shock tolerance studies have shown that 99% of healthy males can tolerate a magnitude current ([I.sub.b]) through the heart region as defined by the equation [I.sub.b] = 0.116/T, where T is the time duration in sec. The duration of contact affects the speed at which the skin is destroyed. As this occurs, skin resistance drops and current increases.
Current path through body. The current path is also important because of the organs that may be affected. For example, a hand-hand path, as shown in Fig. 1, causes the current to pass directly through the heart. A left hand-left foot path, as shown in Fig. 2 (on page 38), again causes the current to pass through the heart. A right hand-right foot path, as shown in Fig. 3 (on page 38), or hand to elbow path is not quite so critical. A foot-foot path, as shown in Fig. 4 (on page 38), results in current passing through organs that are vitally important.
Response to electrical shock
Recent medical studies have begun to uncover the mid- and long-term effects of electric shock. Previously, a survivor of an electrical shock was thought to be "healthy" as well as lucky. Now, however, available evidence indicates that side effects of an electrical shock can show up several years after the accident. These side effects are as follows.
* Immediate effects: confusion, amnesia, headache, breathing stoppage, heart beat stoppage, and burns.
* Secondary effects: paralysis (most frequently in the legs and lasting hours to days), muscular pain, vision abnormalities, swelling, headache, and cardiac irregularities.
* Long range effects: paralysis, speech or writing impairment, loss of taste, and numerous other disorders. Most are the result of nerve tissue damage, which does not regenerate.
Accident report data
The following items are important to any investigation of an electrical injury. As such, they should be determined and recorded as soon as possible.
* Possible status of injured party prior to accident.
* Length of time party worked prior to accident.
* Ambient temperature.
* Condition of soil or surface moisture if accident occurred outdoors.
* Type of clothing worn by injured party.
* Last bath or washing of hands.
* Type of system involved (AC or DC).
* Potential of exposure (120V, 480V, 4160V, etc.).
* Duration of exposure in seconds and minutes.
* Contact points on body (right hand, left hand, etc.). Human body diagram as shown in Fig. 5 (on page 42) should be used.
* Make, model, and location of equipment involved in accident.
* Preliminary determination as to whether equipment or operator was primary cause of injury.
* Observed results of accident.
The last item noted above should contain answers to any or all of the following.
* Loss of consciousness: immediate or delayed?
* Any secondary injuries from falls or fire?
* Did respiration stop? How long before it was restarted and by what method?
* Was there stoppage of the heart and, if so, how long before it was restarted?
* Were there any burns? If so, exactly where on the body?
* How fast did help arrive to treat the injured party?
* Was the injured party taken to a hospital?
Electric arc blast
Electric arcs, if not confined and properly interrupted, create enormous volumes of hot, ionized gases that throw molten metal and blow open doors and equipment. These arcs can reach temperatures of over 15,000 [degrees] F. In addition to the gases, ultraviolet rays created by the arc will detrimentally effect human skin and tissues of the eyes. These types of noncontact burns are known as flash bums and can easily become third degree burns.
The extent of arc blast is often a function of the power distribution system's system duty. This is the amount of power that will flow through a short circuit and is usually expressed in MVA or short-circuit current. The actual amount of energy released is a function of the type of fault as well as the system duty.
There are different types of faults: phase-to-phase-to-phase, phase-to-phase, phase-to-ground, and the combination of all three. These faults are also classified as bolted and arcing faults. A bolted fault rarely occurs and, while the current levels are high, the energy is distributed through the conductor system. Arcing faults, on the other hand, are quite destructive because all of the energy is liberated at the arcing point.
Fault energy levels
Energy is the product of power and time and is expressed as [I.sup.2]t, watt-sec, or kW-CY (kilowatt-cycle). To equate relative energy levels to physical destruction, several manufacturers have performed arcing fault tests and observed the resulting destruction. In the tests, the kW-CY term was used and is expressed by the following equation.
kW-CY = (V x A x T)/1000 where
V = arc voltage (volts)
A = fault current (amperes)
T = time duration (cycles)
The results of these tests are shown in Table 1 on page 38. To equate the respective kW-CY with resultant damage, let's look at an example. Suppose we had an arcing fault of 6000A that lasted for 1/2 sec (30 cycles). The fault energy produced would be more than 20,000 kW-CY. At this level, the blast would probably blow through a 10-gauge metal enclosure and injury anyone in the adjacent area.
How can you help prevent arc blasts? There are a couple of straightforward [TABULAR DATA FOR TABLE 2 OMITTED] guidelines that you should follow.
* Use personnel safety equipment (arc blast suits, face shields, rubber gloves, etc.).
* Follow recognized safety procedures when working on or near energized equipment.
* Make sure that proper preventive maintenance has been done on all distribution protective equipment.
* Verify that proper settings have been made on all distribution protective equipment.
Suffocation and poisoning
Natural air consists of 78% nitrogen, 21% oxygen, and 1% other gases. The minimum safe level of oxygen is 16%. OSHA requires a minimum of 19.5% oxygen before an enclosed space can be entered. Excess oxygen, in levels above 25%, can also be hazardous.
Manholes and enclosed vaults can present suffocation and poisoning hazards to workers entering them. These hazards can be present in many different forms.
Combustible gas. It's not uncommon for process gases, sewer gases, or natural gas to accumulate in enclosed spaces. These potentially explosive gases represent a major hazard to those entering such spaces.
Adequate oxygen supply. Many manholes have heavy concentrations of nitrogen or carbon dioxide. These gases displace oxygen, making breathing difficult or worse, impossible.
Carbon monoxide. This gas accumulates as a result of cable faults or combustion engine exhaust. Breathing carbon monoxide can be fatal and levels above 50 ppm (parts per million) are not acceptable.
Hydrogen chloride. This gas usually is the result of a fault or fire involving PVC (in plastics, insulation, and conduit) or insulating fluids containing PCB. Levels above 15 ppm cause irritation of the mucous membranes; higher levels cause pulmonary edema and suffocation.
Suffocation/poisoning prevention guidelines
Follow these guidelines to prevent suffocation and poisoning.
* Test the atmosphere of the enclosed space prior to entering with an appropriate testing device and record the test results.
* If necessary, ventilate the enclosed space. Then, retest the atmosphere to verify satisfactory levels of gases.
* Continuously or periodically test the atmosphere in the enclosed space while it is occupied by personnel to ensure that air contaminants do not build up to dangerous levels.
* Deactivate equipment when ready-access is not possible from an enclosed space having fire suppression equipment.
* Station a standby employee at the entrance to an enclosed space under the following circumstances: whenever, due to an emergency, it's not feasible to ensure removal of dangerous air contamination or oxygen deficiency; or whenever a safe atmosphere cannot be ensured. The standby employee should have approved breathing apparatus available for immediate use and should have either a direct view of or an adequate communication system with the person in the confined space.
RELATED ARTICLE: CLASSIFICATION OF BURNS
Burns are classified as first, second, or third degree based upon the depth of skin damage.
First degree burns are characterized only by redness of the skin.
Second degree burns are characterized by blistering of the skin, either right after or somewhat later of the accident. The complete thickness of the skin is not destroyed. This type of burn, however, is the most painful.
Third degree burns are characterized by complete destruction of the skin, with charring and cooking of deeper tissues. This type of burn is the most serious as it usually produces a deeper state of shock and more permanent damage and disfiguration. Third degree burns are not as painful as second degree burns because the sensor nerve endings are destroyed