Questions that cross everyone’s mind who has worked on electronics are: what does it take to feel a shock, and at what level can you get into serious trouble. The simple answer is that low levels of electricity have the possibility to cause injury, either directly or indirectly. There is some data that indicates that under certain extreme conditions the maximum reasonable safe voltage is only 10V ac.
Included in this article are physiological effects of varying magnitude of current, and a listing of symptoms. (In researching this article, I was surprised to learn that some important data was obtained by experimenting on human subjects, about 35 years ago.) Also included below are links to the US National Institutes of Health and the IEC.
While previous articles in these newsletters have dealt with defining high voltage, a glossary of related terms, and spacing considerations, it is safety that we think about all the time. Disclaimer: This article is a brief overview and is not a substitute for a comprehensive safety program. Instead, it is the result of limited, non-comprehensive research, and it contains links to websites you may want to consider when you determine for yourself what actions you need to take.
The following, table 1, is adapted from a table that appears on the Georgia State University website, among other sites. See references 1 and 2.
Table 1 Shock Physiological Effects
|
Electric Current |
Physiological Effect |
Voltage required to produce the current with assumed body resistance |
|
|
|
|
100,000 ohms |
1,000 ohms |
|
1 mA |
Threshold of feeling, tingling sensation. |
100 V |
1 V |
|
5 mA |
Accepted as maximum harmless current |
500 V |
5 V |
|
10-20 mA |
Beginning of sustained muscular contraction ("Can't let go" current.) |
1000 V |
10 V |
|
100-300 mA |
Ventricular fibrillation, fatal if continued. Respiratory function continues. |
10,000 V |
100 V |
|
6 A |
Sustained ventricular contraction followed by normal heart rhythm. (defibrillation). Temporary respiratory paralysis and possibly burns. |
600,000 V |
6000 V |
The data in table 2 can be found in many places and is “based on limited experiments performed on human subjects in 1961. These figures are not completely reliable due to the unavailability of additional data and the inherent physiological differences between people. Electricity should be considered potentially lethal at lower levels than those cited.” (See References 3, 4 and 5.)
Experimental data (reference 3) on 115 subjects indicated the following:
While the resistance of skin and shoes are generally high, the resistance of the internal human body can be as low as 500 ohms. Thus, if there is a puncture of the skin at the current path, the shock danger increases dramatically. For purposes of analysis, a value of 1000 or 1500 ohms is probably more realistic for low or medium voltage cases, involving wet or firm contact, but with skin intact (reference 3, page 9).
If there is excellent contact with the skin (such as immersing the feet in salt water), even with the skin intact, the maximum reasonable safe voltage is only 10 V ac. In addition, on very high voltage circuits, the skin can break down very rapidly, thereby causing the resistance to decrease severely (reference 3, page 8).
This data was influential in the safety portions of the U.S. National Electrical Code. Charles Dalziel, who compiled this table, and performed the experiments, is credited with patenting the ground-fault current interrupter. These devices are now commonly found in bathrooms, kitchens and outdoor electric circuits, and which contribute to our safety.
Table 2 Quantitative Effects of Electric Current on Man
(Average data. See Reference 5)
|
Effects |
Current, mA |
|||||
|
|
Direct Current |
Alternating Current |
||||
|
|
DC |
60 Hz |
10 kHz |
|||
|
|
Men |
Women |
Men |
Women |
Men |
Women |
|
Slight sensation on hand |
1 |
0.6 |
0.4 |
0.3 |
7 |
5 |
|
Perception threshold, median |
5.2 |
3.5 |
1.1 |
0.7 |
12 |
8 |
|
Shock—not painful and no loss of muscular control |
9 |
6 |
1.8 |
1.2 |
17 |
11 |
|
Painful shock—muscular control lost by 1/2% |
62 |
41 |
9 |
6 |
55 |
37 |
|
Painful shock—let-go threshold, median |
76 |
51 |
16 |
10.5 |
75 |
50 |
|
Painful and severe shock—breathing difficult, muscular control lost by 99.5% |
90 |
60 |
23 |
15 |
94 |
63 |
|
|
|
|
|
|
|
|
|
Possible ventricular fibrillation: |
|
|
|
|
|
|
|
Three-second shocks |
500 |
500 |
100 |
100 |
|
|
|
Short shocks (T in seconds) |
|
|
165/(sq rt T ) |
165/(sq rt T) |
|
|
|
High-voltage surges |
50 J |
50 J |
13.6 J |
13.6 J |
|
|
At levels where a shock does not do direct bodily harm, there are still ways to get seriously injured indirectly. As the shocked individual reacts voluntarily or involuntary (due to muscle contraction) to the shock by moving away, possibly very rapidly, he or she can fall over backwards, bang against a nearby chair or workbench, or just their hand against the chassis of the item they are working on.
Another source of information is International Electro Technical Commission document IEC 60479, Effects of Current Passing Through the Human Body.
According to the IEC, this technical report has the status of a basic safety publication in accordance with IEC Guide 104. It provides guidance on the effects of shock currents on human beings and livestock, for use in the establishment of electrical safety requirements. The report summarizes results so far achieved which are being used by TC64, Technical Committee N° 64: Electrical Installations and Protection Against Electric Shock, as a basis for fixing requirements for protection against shock. It may also serve as a guide to other IEC committees and countries having need of such information.
One section of the technical report summarizes the basic parameters for lightning and their variability insofar as they apply to human beings and livestock. The possible direct and indirect interactions of strikes with bodies of living beings are indicated. The resulting effects caused by lightning currents for the organism are described. The intention is to show the differences of effects on human beings and livestock due to lightning strokes versus those effects of electric shocks derived from electrical systems. (Reference 6)
The IEC 60479 publication is divided into four documents as follows:
IEC/TS 60479-1 (1994-09) Effects of current on human beings and livestock
Part 1: General aspects
Electrical impedance of the human body
Effects of sinusoidal alternating current in the range of 15 Hz to 100 Hz
Effects of direct current
IEC/TR 60479-2 (1987-03) Effects of current passing through the human body.
Part 2: Special aspects:
Chapter 4: Effects of alternating current with frequencies above 100 Hz
Chapter 5: Effects of special waveforms of current
Chapter 6: Effects of unidirectional single impulse currents of short duration
IEC/TS 60479-3 (1998-09) Effects of current on human beings and livestock
Part 3: Effects of currents passing through the body of livestock
IEC/TR 60479-4 (2004-07) Effects of current on human beings and livestock
Part 4: Effects of lightning strokes on human beings and livestock
For further reading, Bussmann, a division of Cooper Industries, has a handbook on electrical safety (Reference 7)
Three major ways electric current damages the body are: (1) it harms or interferes with proper functioning of the nervous system and heart; (2) it subjects the body to intense heat; and (3) it causes the muscles to contract.
According to MedlinePlus, a service of the U.S. National Library of Medicine and National Institutes of Health (Reference 8), symptoms may include:
Skin burns
Numbness, tingling
Weakness
Muscle contraction
Muscular pain
Bone fractures
Headache
Hearing impairment
Seizures
Heart arrhythmias
Cardiac arrest
Respiratory failure
Unconsciousness
According to eMedicinehealth.com, a commercially operated website, a person who has suffered an electric shock may have very little external evidence of injury or may have obvious severe burns. The person could even be in cardiac arrest. Burns are usually most severe at the points of contact with the electrical source and the ground. The hands, heels, and head are common points of contact. In addition to burns, other injuries are possible if the person has been thrown clear of the electrical source by forceful muscular contraction. Consideration should be given to the possibility of a spine injury. The person may have internal injuries especially if he or she is experiencing any shortness of breath, chest pain, or abdominal pain. (Reference 9)
Not being a doctor, I cannot determine if the following is the best advice to follow when making your own decisions on actions take. Note that this information is from website visits on January 17, 2005, and the information may have been changed or otherwise updated since then.
Instructions at MedlinePlus, a service of the U.S. National Library of Medicine and National Institutes of Health (Reference 10) include the following:
First Aid
1. If safely possible, shut off the electrical current. Unplug the cord, remove the fuse from the fuse box, or turn off the circuit breakers if possible. Often, simply turning off the appliance itself will not stop the flow of electricity.
2. Call for medical help.
3. If the current can't be turned off, use a non-conducting object, such as a broom, chair, rug, or rubber doormat to push the victim away from the source of the current. Don't use a wet or metal object. If possible, stand on something dry and non-conducting, such as a mat or folded newspapers. Do not attempt to rescue a victim near active high-voltage lines.
4. Once the victim is free from the source of electricity, check the victim's airway, breathing, and pulse. If either has stopped or seems dangerously slow or shallow, initiate first aid (CPR).
5. If the victim has a burn, remove any clothing that comes off easily, and rinse the burned area in cool running water until the pain subsides. Give first aid for burns.
6. If the victim is faint, pale, or shows other signs of shock, lay the victim down, with the head slightly lower than the trunk of the body and the legs elevated, and cover the person with a warm blanket or a coat.
7. Stay with the victim until medical help arrives.
8. Electrical injury is frequently associated with explosions or falls that can cause additional traumatic injuries, including both obvious external injuries and concealed internal injuries. Avoid moving the victim's head or neck if a spinal injury is suspected. Administer appropriate first aid as needed for other wounds or fractures.
Do Not
· DO NOT touch the victim with your bare hands while the person is still in contact with the source of electricity.
· DO NOT remove dead skin or break blisters if the victim has acquired burns.
· DO NOT apply ice, butter, ointments, medications, fluffy cotton dressings, or adhesive bandages to a burn.
· DO NOT touch the skin of someone who is being electrocuted.
· DO NOT get within 20 feet of someone who is being electrocuted by high-voltage electrical current until the power is turned off.
· DO NOT move a victim of electrical injury unless there is immediate danger.
Call immediately for emergency medical assistance if
Call for emergency medical help if the victim is unconscious, confused, has difficulty breathing, has skin or mouth burns, or was in contact with a high-voltage source.
Additional information (reference 11) can be found at eMedicinehealth.com, a commercially operated website: A person shocked by high voltage (500 volts or more) should be evaluated in the Emergency Department. It may be prudent to get pre-hospital care, usually obtained by calling 911. Following a low-voltage shock, go to the Emergency Department for the following concerns:
It doesn’t take very much voltage to cause bodily harm.
Disclaimer: This article is a brief overview and is not a substitute for a comprehensive safety program. Instead, it is the result of limited, non-comprehensive research, and it contains links to websites you may want to consider when you determine for yourself what actions you need to take
Another source of information on high voltage, along with news, articles, component information, and more is at High Voltage Info.
Contact Information
Evan Mayerhoff
Application Engineering
High Voltage Connection, Inc.
516-466-9379
evan@highvoltageconnection.com
© 2005 High Voltage Connection, Inc.