AC Equipment Grounding: How Dangerous are Electric Shocks?
This article looks at the effects of AC electric current on the human body.
An electric current passing through the human body can cause death. On many occasions, the cause of death is in the heart, which, subjected to intense and irregular activity, is exhausted and stops. Even a small amount of current that enters through the hand and exits through one or both feet could pass through the heart.
Effects of Electrical Shock
Many people think high voltage causes fatal shocks. However, numerous accidents occur by manipulating low voltage systems. The effects of a shock depend on the magnitude and duration of the current, frequency, physical attributes of the individual, gender, and trajectory through the body. The higher the voltage, the higher the current through the body, under any given set of circumstances.
Sources on the physiological effects of electrical currents are relatively abundant, but they quote broadly disagreeing figures. Table 1 shows commonly accepted values and the consequences when applying 60 Hz, AC electrical currents to the body. It shows a 500 Ω, 70 kg, adult, who is grasping a live conductor with both hands and closing the circuit by standing with both feet in the water.
Table 1 Current range and effect of 60 Hz AC on a 70 Kg, 500 Ω, adult
|Current (60 Hz)||Physiological phenomenon||Feeling or lethal|
|< 1 mA||None||Imperceptible|
|1 mA||Perception threshold|
|1-3 mA||Mild sensation. Let-go|
|3-10 mA||Painful sensation. Let-go|
|> 10 mA||Paralysis threshold of arms||"No let-go" or freezing. Cannot release handgrip; if no grip, the victim may be thrown clear (may progress to higher current and be fatal)|
|30 mA||Respiratory paralysis (asphyxiation)||Stoppage of breathing (frequently fatal)|
|75 mA||Fibrillation threshold percentile 0.5%||Heart action uncoordinated (probably fatal)|
|250 mA||Fibrillation threshold Percentile 99.5%|
|4 A||Heart paralysis threshold (no fibrillation)||
The heart stops for the duration of the current passage. For short shocks, may restart on the interruption of current (usually not fatal from heart dysfunction)
|≥ 5 A||Tissue burning||Fatal when burning vital organs|
Most data, especially the data concerning the current levels required to cause fibrillation, are extrapolated from experiments with animals. Most of these experiments are often fatal and not suitable for human beings.
The response to electrical current is approximately proportional to 1/√t. However, there is wide variability among individuals. A more massive subject requires more current for the same physiological effect.
The "no let go" or freezing figure of 10 mA causes a temporary paralysis of the extensor or flexor muscles, rendering the shock victim incapable of releasing the current source. The paralysis may also cause tensing of the muscles, pushing the victim away from the source, and maybe saving his or her life.
The respiratory paralysis at the 30 mA level may cause death, but it is also reversible if the current is removed promptly. The muscular paralysis will cease, and breathing will resume.
The 75 mA figure for ventricular fibrillation is the value that will cause that effect in approximately 0.5% of the population, and the remainder 95.5% will require contact with more significant currents. Ventricular fibrillation is a medical condition where the heart ends its blood pumping role and beats at a fast rate, with eventual brain damage and death due to insufficient oxygen.
A person with ventricular fibrillation may recover without intervention, but this event is extremely unusual. The basis for restoring the heart to regular activity is to stop it by applying a high current. Hopefully, the heart will resume its regular pumping action after disconnecting the current.
The criteria in many international standards to design grounding mats is to keep the magnitude and duration of the current applied to the human body to values below those that can cause ventricular fibrillation of the heart.
The Electrical Resistance of the Body
The electrical resistance of a human being depends on the following factors:
- Physical condition.
- Nature of the points where the current enters and leaves.
- The dry skin has a high resistance, approximately 100 kΩ at low voltage. The epidermis, which is the outermost thin layer of the skin, has high resistance because it is nonvascular, i.e., lacks the blood supply. In the range of 500 V – 1 000 V, the resistance drops to about 1 kΩ.
- The dermis is a thick layer of skin beneath the epidermis that has little resistance because it is interlaced with blood vessels, to provide nourishment and waste removal for both dermal and epidermal cells. And the blood contains mineral ions, which increase its electrical conductivity.
- A scratch in the epidermis or something else that breaks the skin will expose the dermis, and the value of resistance will drop. It is reasonable to estimate that, under this condition, an arm or a leg has a resistance of about 500 Ω. The "500 Ω man," frequently found in literature, holds a live conductor with both hands and stands with both feet in the water.
- Some researchers state that the average or reasonable resistance of human beings is from 1 000 Ω to 2 000 Ω, foot-to-foot, and 500 Ω to 1 000 Ω arm-to-foot, depending on the diverse factors involved.
- Voltage of the line or electrical device.
- The body's electrical resistance decreases as the voltage increases because the higher the voltage, the more numerous the points of the skin that are damaged, with increased access to the dermis.
The Body as a Circuit Parameter
A person can only be affected by electricity when it becomes part of the electrical circuit. That is why the best way to avoid an electric shock is to prevent contact with energized parts. But a large number of electrical devices handled daily increases the exposure to electricity and the possibility of unwanted contact.
Fig. 1 Birds are immune to electric shock as long as they are not part of the electrical circuit. Image courtesy of Pixabay
Like any other electrical parameter, there are two ways in which a person can become part of a circuit: series and parallel. When connected in series, the person is in the only path for current flow, and when connected in parallel, other channels share the current flow.
Figure 2 shows the nature of the problem. A person is touching an appliance that operates on electricity, such as a drill. The resistance Ri is the factory-installed appliance insulation, Reg represents the resistance of a conductor connected from the appliance housing to the power supply ground, and Rb is the sum of three resistances: the person's body, the contact of the hands with the appliance and the touch of the feet with the floor.
Fig. 2 Simple electrical model with the body as a circuit parameter
In the diagram above, Ri is in series with the parallel paths of Reg and Rb. When the insulation is excellent, Ri is essentially infinite, and no current will flow through Reg and Rb. But, if the insulation fails (ground fault), Ri decreases, and current can flow through Reg and Rb.
We can now analyze three circumstances when there is a ground fault in the appliance. In the first case, the device does not have the conductor that connects it to the power supply ground, which is equivalent to Reg = infinity, and all the fault current will circulate through the person. Here, the person is in series with the fault circuit. In the second case, Reg = 0 and no current will flow through the person. In the third case, Rb = infinity, and no current will flow through the person either.
In real life, Rb will have finite values, and the correct grounding must ensure that, if there is a ground fault, the fault current that passes through the body is not enough to affect it for the duration of the fault.
Reg must be low enough to carry most of the fault current, with a magnitude adequate to clear the fault in a timely fashion. A low-impedance equipment grounding conductor connected effectively to the source ground will help to attain this goal.
Rb should be kept as high as possible avoiding wet earth and simultaneous contact with metallic objects. Usually, electrical workers are required to wear insulated gloves and shoes to increase resistance. It is usual practice in substations to spread a layer of high resistivity material on the earth's surface above the ground grid. Standard materials are gravel and asphalt, and the effect is to increase the contact resistance between the soil and the feet, reducing the current through the body.
Appliance manufacturers make Ri very high using techniques like double insulation. This sort of equipment does not require an equipment grounding conductor given the unlikelihood of the user contacting energized parts. However, double insulation is not flawless, and there have been electrocutions when immersing the appliance in water.
The use of sensitive, fast ground protection is also helpful.
Current Exposure Time and Ventricular Fibrillation
The prevention of ventricular fibrillation is the objective that guides the recommendations of international standards regarding the design and implementation of grounding mats.
As indicated above, there are many published works about the effect of electric current on the human body, especially at the 50 Hz and 60 Hz frequencies that are the standards for power systems worldwide. Particularly noteworthy are the experiments conducted by C.F. Dalziel and W.R. Lee with animals (dogs, sheep, pigs, and cows) in a range of 10 kg to 80 kg. The results of these studies apply to humans. There are also findings from electrocution accidents.
Dalziel, Lee, and other researchers concluded that the amount of current that the human body can withstand in a range of 0.03 s to 3 s, is related to the energy absorbed by the body through the equation:
Sb = Ib² · ts, where:
Ib = nonfibrillating shock current in Ampere
ts = exposure time (duration) in seconds
Sb = empirical constant related to the shock energy tolerated by 99.5 % of the population = 0.0135 for 50 kg body weight, and 0.0246 for 70 kg body weight.
Then, Ib = √( Sb/ts) = 0.116 · ts-1/2 for 50 kg, and Ib = 0.157 · ts-1/2 for 70 kg
The exposure voltage V = Rb · Ib
Figure 3 shows the fibrillation threshold for an adult. It is a log-log time–current-voltage plot of the shock current (Ib) and the exposure voltage (V) vs. the exposure time (ts), for the range 0.03 s to 3 s. It assumes an arm-to-arm or arm-to-leg resistance (Rb) of 500 Ω and includes body weights of 50 kg and 70 kg.
Fig. 3 Fibrillation threshold for 70kg and 50kg adult. Voltage based on Rb = 500Ω.
Important conclusions, derived from Figure 2:
- Straight lines fit the pairs (Ib, ts) and (V, ts)
- The lower the exposure time, the higher the current tolerated
- Magnitude and duration are a function of body weight, i.e., people with higher body weight undergo the same currents for longer
- The tests are only valid for the range 0.03 s – 3.0 s
Relationship to Power Systems
Most power system voltages have a high risk of electrocution, especially in wet locations. Taking a simple household appliance like a hairdryer rated at 120V and a body resistance of 500 Ω, one calculates a current of 240 mA, which is likely to cause fibrillation — a fatal effect.
Even in cases where the current is not enough to cause fibrillation, it could cause a painful surprise, and the person could have an accident as a consequence of the involuntary reaction to the shock, such as a fall.
The electrical power circuits, as well as the devices connected to them, must be treated with extreme caution and in compliance with all applicable regulations in such a way to preserve life. The leading standard to follow for safety is the National Electrical Code (the NEC), whose purpose is "the practical safeguarding of persons and property from hazards arising from the use of electricity."
A Review of Electrical Shock and its Effects
High voltage and low voltage can cause fatalities. The effects of the current on the body depend on the magnitude, duration, frequency, physical condition, gender, and path of the current.
The most dangerous effect caused by an electric current is ventricular fibrillation. During this condition, the heart stops pumping blood. The benchmark in the design of grounding systems is the prevention of fibrillation.
The electrical resistance of a person depends on the physical condition, the nature of the contact points, and the system voltage.
To avoid an electric shock, do not form part of the electrical circuit.
Safety standards, like the NEC, protect people from the improper use of electricity.