High Voltage Safety. Dangers of Electricity. W. Maes. Department of Marine Engineering Antwerp Maritime Academy HV, 2015

Transcription

1 Dangers of Electricity W. Maes Department of Marine Engineering Antwerp Maritime Academy HV, 2015 Outline 1 The physical effect of an electric current on the human body

2 Heating Source of power Electro-magnetical Chemical Ionisation Leak currents All of these effects are normal but can become a problem if they turn to abnormal. At that moment we have to provide in a proper protection.

3 Dangers of Electricity The dangers of electricity are often not realized. The main reason is that we can barely detect them. However what we can detect are the electrical accidents and there consequence on people and their environment. Under the heading of risks, we can make a distinction between: 1 risk for people (burns, electrocution) 2 risk for their surroundings (fire and explosion) The risk to people The main risk is coming into contact with live materials. The effects, dependent upon the current strength and the voltage difference, vary from panic to death. The effect of panic should not be underestimated as it could lead to a fall or other injury. Another effect is burning.

4 Figure : The effect of panic The risk to surroundings The risk to surroundings due to an overload or short circuit, causing: an abnormal overheating of an apparatus or a circuit a fire or even an explosion all of this can result in a substantial loss off equipment end or functionality.

5 Muscle Tetanisation Figure : Involuntary contraction of muscles Muscle Tetanisation Muscle Tetanisation is the involuntary contraction of the muscles with as a result: Muscle tetanisation may not allow a detachment of the victim from the live part. Breathing difficulty breathing stops: because of the contraction of the muscles involved in breathing.

6 Effect on the heart Figure : normal heart rhythm Effect on the Heart A small current passing through the chest can result in ventricular fibrillation. If not treated immediately by defibrillation, it can be lethal as the heart muscle cells start moving independently. A shock more than 200 ma is so strong that it can literally stop the heart muscles from moving.

7 Neurological Effects Figure : neuropathy Neurological Effects An electric shock can interfere with the nervous control: This can be dangerous on the heart and lungs. Repeated electric shock that does not lead to death has been shown to be a cause for neuropathy. When the current moves near the head then loss of consciousness occur fast.

8 Burns Burns are produced by the heat developed by the current flowing through the human body. Burns can be catagorized as: Superficial burns on the surface of the skin. Internal burns can lead to organ burns. The burns from an electric shock can result in organ failure and can lead to death. A person can be blinded even permantly by the UV light emitted by an arc or arc flash. Noise The violent noise of an arc flash can cause a rupture of the tympanic membranes and temporary sensorineural hearing loss.

9 Electric Shock If one touches a source of electricity, a current will pass trough the body, taking the easiest route to the ground. This is called an electric shock. A human body consist for 2/3 out of fluid therefore people are good electric conductors. The electric current can cause the body to go into shock. This can lead to a falling blood pressure, tackicardia, and loss of consiousness. Electricity s effect on the human body Electric injury is a relative infrequent but potentialy devasting form of multi system injury with high morbidity and mortality. The severity of the injury depends on the intensity of the electric current, the pathway it follows trough the body, and the duration of the contact with the power source. Immediate death may occur either from current induced ventricular fibrillation or from respiratory arrest secondary to paralysis of the central respiratory control system or to paralysis of the respiratory muscles.

10 Electricity s effect on the human body According to Ω s law, the electric current is proportional to the voltage and inversely proportional to the resistance of the conductor. Thus exposure of different parts of the body to the same voltage will generate a different current (and therefore a different degree of damage) because resistance varies significantly between various tissues. Electricity s effect on the human body The least resistance is found in nerves, blood, mucous membranes, and muscles. The highest resistance is found in bones, fat and tendons. Skin has intermediate resistance.

11 Electricity s effect on the human body The skin is the primary resistor against the electrical current, with a resistance ranging in adults between 40 to 100KΩ, depending on its thickness (the thicker the skin the higher the resistance. The intensity of the electrical shock varies between victims of different sex and age. Even more important than the thickness of the skin is the moisture of the skin. Presence of sweat may decrease the resistance of the skin to less than 1 KΩ. Wet skin offers almost no resistance at all, thus generating the maximal intensity of current. Electricity s effect on the human body The internal resistance of the body comprises all the other tissues and is estimated between 0.5 to 1 Ω Altough bones, tendons, and fat offer the most resistance to electric current, they are not likely to be the contact points. Nerves and blood vessels, on the other hand, are the best conductors. Nerves because they designed to carry electrical currents and blood due to its high water content.

12 Electricity s effect on the human body The duration of the contact with electrical current is an important determinant of injury. Therefore it is said that an electrical shock caused by AC will produce greater injury than a shock caused by DC of the same amperage because the DC current causes a single muscle contraction that throws the victim away from the power source, minimizing the injury. These differences have practical significance only at low voltages, whereas on high voltages, both currents have a similar effect. Electricity s effect on the human body The pathway of the current trough the body determines the number of organs that are effected A vertical pathway is the most dangerous because it involves all the vital organs (central nervous system, heart, respiratory muscles, and in pregnant woman, the uterus and fetus). A horizontal pathway from hand to hand will spare the brain but can still be fatal due to involvement of the heart, respiratory muscles, or spinal cord. A pathway trough te lower part of the body may cause severe local damage but will probably be less lethal.

13 Electricity s effect on the human body Whereas electric shock from a low voltage voltage line is delivered on contact of the victim with the source, in high voltage injury, the current is carried from the source to the person trough an arc before any physical contact is made. The arc may form into or over the body of a person. Arcs can generate extremely high temperatures (up to 5000 deg C) that are usually responsible for the severe thermal injuries from high voltage. Electricity s effect on the human body Three primary factors affect the severity of the shock a person receives when he or she is a part of an electrical circuit: Amount of current flowing through the body (measured in amperes). Path of the current through the body. Length of time the body is in the circuit.

14 Electricity s effect on the human body Other factors that may affect the severity of the shock are: The voltage of the current (arc or direct contact). The presence of moisture in the environment. The phase of the heart cycle when the shock occurs. The general health of the person prior to the shock. Effects of Electrical Current on the Human Body Effects can range from a barely perceptible tingle to severe burns and immediate cardiac arrest. Although the exact injuries that result from any given amperage are not known, the following slides demonstrates a general relationship for a 60-cycle, hand-to-foot shock of one second s duration:

15 hand-to-foot shock of one second Current 1mA Perception level. Slight tingling sensation. Still dangerous under certain conditions. hand-to-foot shock of one second Current 5 ma Slight shock felt, not painful but disturbing. Average individual can let go. Strong involuntary reactions to shocks in this range may lead to injuries. If the extensor muscles are excited by the shock, the person may be thrown away from the circuit. Often, this can result in a fall from elevation that kills a victim even when electrocution does not.

16 hand-to-foot shock of one second Current 6 ma - 16mA Painful shock, begin to lose muscular control. Commonly referred to as the freezing current or "let-go" range. hand-to-foot shock of one second Current 17 ma- 99 ma Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible. Even relatively low voltages can be extremely dangerous, because the degree of injury increases with the length of time the body is in the circuit. LOW VOLTAGE DOES NOT IMPLY LOW HAZARD!

17 hand-to-foot shock of one second Current 100 ma ma Ventricular fibrillation (uneven, uncoordinated pumping of the heart.) Muscular contraction and nerve damage begins to occur. Death is likely. Note that a difference of less than 100 milliamperes exists between a current that is barely perceptible and one that can kill. High voltage electrical energy greatly reduces the body s resistance by quickly breaking down human skin. Once the skin is punctured, the lowered resistance results in massive current flow. hand-to-foot shock of one second Current of more than 2000 ma Cardiac arrest, internal organ damage, and severe burns. Death is probable.

18 HV work on board can be dangerous due to limited space. The work may be carried out in close proximity to a person(s) not familiar with HV hazards. Therefore the area must be properly cordoned off from surrounding work that may be going on and danger notices well posted. There will be large areas of earthed metal that can be easily touched, increasing the possibility of electrical shock from an HV conductor. High voltage isolation testing can be particulary hazardous when several parts of the equipment are energised for a period of time. Some equipment could be using water in its operation which can lead to an increased risk of injury. In general, water conducts electricity and reduces the resistance of the skin. The use of instruments when taking measurements of high voltages can increase the risk of injury if they are inadvertently used without the earth (protective) conductor connected. This can result in the enclosure of the instrument becoming live at high voltages.

19 High voltage equipment will store energy after disconnection. For example, on a 6.6KV switchboard a fatal charge may still be present on the equipment hours or even days later. If during maintenance an HV circuit main earth (CME) is removed from the system, it must not be worked on, as the HV cabling can recharge itself to a high voltage from induced voltages from nearby live HV cabling. Arc An arc is a discharge of electrical current across a gap. An arc fault is a high power discharge of electricity between two or more conductors. The radiation of heat in an arc is very high and it can very easily set a persons clothes on fire.

20 Arc Blast Arc blast pressure derives from two things. First, the expansion of metal in a boiling, vaporising state, and second the heating of ambient air by passage of the arc. The mixture of vaporised water and metal in air near the arc generates a rapidly expanding plasma of ionized vapor, which can lead to extensive injuries. Appendix For Further Reading For Further Reading I The Rules Lloyds register of shipping. The Rules ABS.