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.