HOW ELECTRIC SHOCK IS RECEIVED?

Whenever you work with power tools or on electrical circuits, there is a risk of electrical hazards, especially electrical shock. Anyone can be exposed to these hazards at home or at work. 

Workers are exposed to more hazards because job sites can be cluttered with tools and materials, fast-paced, and open to the weather. Risk is also higher at work because many jobs involve electric power tools.

Electrical trades workers must pay special attention to electrical hazards because they work on electrical circuits. Coming in contact with an electrical voltage can cause current to flow through the body, resulting in electrical shock and burns. Serious injury or even death may occur. 

As a source of energy, electricity is used without much thought about the hazards it can cause. Because electricity is a familiar part of our lives, it often is not treated with enough caution. As a result, an average of one worker is electrocuted on the job every day of every year!

An electrical shock is received when electrical current passes through the body. Current will pass through the body in a variety of situations. 

Whenever two wires are at different voltages, current will pass between them if they are connected. Your body can connect the wires if you touch both of them at the same time. Current will pass through your body.

In most household wiring, the black wires and the red wires are at 120 volts. The white wires are at 0 volts because they are connected to ground. The connection to ground is often through a conducting ground rod driven into the earth. The connection can also be made through a buried metal water pipe. If you come in contact with an energized black wire—and you are also in contact with the neutral white wire—current will pass through your body. You will receive an electrical shock.

If you are in contact with a live wire or any live component of an energized electrical device—and also in contact with any grounded object—you will receive a shock. Plumbing is often grounded. Metal electrical boxes and conduit are grounded.

Your risk of receiving a shock is greater if you stand in a puddle of water. But you don’t even have to be standing in water to be at risk. Wet clothing, high humidity, and perspiration also increase your chances of being electrocuted. Of course, there is always a chance of electrocution, even in dry conditions.

You can even receive a shock when you are not in contact with an electrical ground. Contact with both live wires of a 240-volt cable will deliver a shock. (This type of shock can occur because one live wire may be at +120 volts while the other is at -120 volts during an alternating current cycle—a difference of 240 volts.). You can also receive a shock from electrical components that are not grounded properly. Even contact with another person who is receiving an electrical shock may cause you to be shocked.

ELECTROCUTION AND ELECTRICAL FATALITIES BASIC INFORMATION

The term electrocution refers to an electrical event with electrical current exposure that results in death. The implication is that the current flow has caused an electrical shock with subsequent death.

“Electrical accident fatality” is a general use phrase seen in news reports meaning either electrocution, or death resulting at the time of the electrical accident. This phrase may include fatalities associated shock or other forms of energy released at the time of the electrical accident, in particular those causing physical changes including burns, blast effects, and radiation damage.

“Electrical injury mortality” is a medical statistics phrase which suggests that persons who were injured in an electrical accident lived long enough to receive medical care for their injuries, but the medical care was not followed by survival.

It’s important to appreciate that an electrical event can produce a fatality or injury even when there is no electrical current flow to the victim or electrical shock. This might be the situation, for example, when a victim is caught in an electrical ignition fire, explosion, or blast.

In this type of scenario, the “root cause” of the accident is electrical, but the mechanism of death or injury is from thermal, acoustic, radiation, or blast exposure related to electrothermal chemical (ETC) combustion.

Another way employees can be killed or injured after an electrical event is that they are surprised by an energized source, either through a spark, like a static “zap” to exposed skin, or through a noise, like a sharp “gunshot” type sound close to the head.

The surprise can lead to an unintended body movement which might be characterized as a “startle response.” If the startle occurs at the top of a ladder or scaffold, the direct mechanism of death or injury can be through a fall.

If the startle occurs in proximity to other energized equipment that is moving, the direct mechanism of death or injury can be with a body part being caught in or by the moving equipment.

Fatal and non-fatal electrical incidents share three characteristics:

1. The unintentional exposure of employees to electrical energy;
2. Compliance failure in at least one aspect of electrical design, installation, policies, procedures, practices, or personal protection; and
3. Energy transfer to exposed employees in some combination of electrical, thermal, radiation, acoustic (pressure), mechanical, light, kinetic, or potential energy.

What is the difference between fatal and non-fatal electrical incidents? The answer depends in part on whether the question is asked hypothetically, like in a “what if ” planning scenario; or whether the question is asked retrospectively after a traumatic accident has occurred.

Hypothetically, based on human physical and biological characteristics, we know that a fatal electrical event transfers a greater amount of energy to its victim than a non-fatal situation. This knowledge about the fatal risk of energy transfer underlies the use of equipment designs (for example, required doors, specified space clearances, venting systems on equipment to discharge combustion products, “umbilical corded” controls, infrared monitoring ports for doors closed heat monitoring) and barrier protection (such as PPE, including leather gloves, flash suits, safety glasses, face shields, long sticks, extended handles, and flame resistant clothing).

By reducing the amount of possible energy transfer during an unintentional electrical exposure, strategies including equipment design and barrier protection can increase the likelihood of survival after an electrical incident.

Retrospectively, if two people are present in an electrical incident when one dies and the other survives, the difference in survival may come down to nuances in the victims’ innate individual differences and their spatial and temporal relationship to the electrical hazard at the time of the energy release, transformation, and transfer. Medical and legal privacy protections tend to reduce accessibility to accident details, so systematic information is lacking about how various scenarios unfold.

Generally, there is a lethal exposure “dose” for different forms of energy that can result in death. When multiple forms of energy are involved in an electrical event, multiple lethal or sublethal doses of energy may flow from the event, transformed from the electrical hazard source, and transferred to nearby employees may result in highly variable damage to the body.

EFFECT OF CURRENT and ITS DURATION TO THE HUMAN BODY DURING ELECTRIC SHOCK

WHAT KILLS A PERSON? CURRENT OR VOLTAGE?

To answer the question, we need to put things in context. That means, there is no absolute. Current kills, but it needs to be present for a certain period of time.

The amount of energy delivered to the body is directly proportional to the length of time that the current flows; consequently, the degree of trauma is also directly proportional to the duration of the current. Three examples illustrate this concept:

1. Current flow through body tissues delivers energy in the form of heat. The magnitude of energy may be approximated by:

J = I²Rt

where J = energy, joules
I = current, amperes
R = resistance of the current path through the body, ohms
t = time of current flow, seconds

If sufficient heat is delivered, tissue burning and/or organ shutdown can occur. Note that the amount of heat that is delivered is directly proportional to the duration of the current (t).

2. Some portion of the externally caused current flow will tend to follow the current paths used by the body’s central nervous system. Since the external current is much larger than the normal current flow, damage can occur to the nervous system.

Note that nervous system damage can be fatal even with relatively short durations of current; however, increased duration heightens the chance that damage will occur.

3. Generally, a longer duration of current through the heart is more likely to cause ventricular fibrillation. Fibrillation seems to occur when the externally applied electric field overlaps with the body’s cardiac cycle. The likelihood of this event increases with time.

Also, we need to understand how much current is significant.

The magnitude of the current that flows through the body obeys Ohm’s law, that is,

I = E/R

where I = current magnitude, amperes (A)
E = applied voltage, volts (V)
R = resistance of path through which current flows, ohms (Ω)

Parts of the Body. 
Current flow affects the various bodily organs in different manners. For example, the heart can be caused to fibrillate with as little as 75 mA.

The diaphragm and the breathing system can be paralyzed, which possibly may be fatal without outside intervention, with less than 30 mA of current flow. The specific responses of the various body parts to current flow are covered in later sections.

Nominal Human Response to Current Magnitudes

ELECTRIC SHOCK HAZARD ANALYSIS

WHAT HAPPENS WHEN WE GET ELECTRIC SHOCK?

Electric shock is the physical stimulation that occurs when electric current flows through the human body. The distribution of current flow through the body is a function of the resistance of the various paths through which the current flows. The final trauma associated with the electric shock is usually determined by the most critical path called the shock circuit. The symptoms may include a mild tingling sensation, violent muscle contractions, heart arrhythmia, or tissue damage.

Common effect are the following:

Burning.
Burns caused by electric current are almost always third-degree because the burning occurs from the inside of the body. This means that the growth centers are destroyed. Electric-current burns can be especially severe when they involve vital internal organs.

Cell Wall Damage.
Research funded by the Electric Power Research Institute (EPRI) has shown that cell death can result from the enlargement of cellular pores due to high-intensity electric fields. This research has been performed primarily by Dr. Raphael C. Lee and his colleagues at the University of Chicago. This trauma called electroporation allows ions to flow freely through the cell membranes, causing cell death.

HOW YOUR PHYSICAL CONDITION IS A FACTOR DURING ELECTRIC SHOCK

Physical Condition and Physical Response. 
The physical condition of the individual greatly influences the effects of current flow. A given amount of current flow will usually cause less trauma to a person in good physical condition.

Moreover, if the victim of the shock has any specific medical problems such as heart or lung ailments, these parts of the body will be severely affected by relatively low currents. A diseased heart, for example, is more likely to suffer ventricular fibrillation than a healthy heart.

EMERGENCY LIGHTING SYSTEM DESIGN CONSIDERATIONS CODES AND STANDARDS

Emergency lighting is required when the normal lighting is extinguished, which can occur for any of three reasons:

1. General power failure
2. Failure of the building’s electrical system
3. Interruption of current flow to a lighting unit, even as a result of inadvertent or accidental operation of a switch or circuit disconnect.

As a result of the third reason, sensors must be installed at the most localized level—that is, at the lighting fixture (voltage sensor) or in the lighted space (photocell sensor).

Codes and Standards
Because emergency lighting is a safety-related item, it is covered by various codes, several of which may
have jurisdiction. In addition, there are widely accepted technical society and industry standards whose recommendations normally exceed the minimal required by codes.

1. Life Safety Code (NFPA 101, 2009). This code defines the locations within specific types of structures requiring emergency lighting and specifies the level and duration of the lighting.

2. National Electrical Code (NFPA 70, 2008). This code deals with system arrangements for emergency light (and power) circuits, including egress and exit lighting. It discusses power sources and system design.

3. Standard for Health Care Facilities (NFPA 99, 2005). This code deals with special emergency light and power arrangements for these facilities.

4. OSHA regulations. These are primarily safety oriented and, in the area of emergency lighting, discuss primarily exit and egress lighting requirements.

5. Industry standards. These include the publications of the IESNA and the IEEE, in particular Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications (IEEE Standard 446-1995).

Because codes and standards are constantly being revised and updated, the designer for an actual project must determine which codes have jurisdiction, obtain current editions, and design to fulfill their requirements. The following material provides general information and focuses on good practice but is not intended to take the place of applicable construction and safety codes.