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The Risk of Electricity

High Voltage SafetyDangers of Electricity

W. Maes

Department of Marine EngineeringAntwerp Maritime Academy

HV, 2015

Willem Maes High Voltage Safety


Outline

1 The Risk of ElectricityEffects from electricityElectricity can killThe risk of electricity for peopleThe physical effect of an electric current on the humanbodyDangers when working on HV equipment



Effects from electricityElectricity can killThe risk of electricity for peopleThe physical effect of an electric current on the human bodyDangers when working on HV equipment

Effects from electricity

HeatingSource of powerElectro-magneticalChemicalIonisationLeak currentsAll of these effects are normal but can become a problem ifthey turn to abnormal.At that moment we have to provide in a proper protection.




Electricity can kill




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 accidentsand there consequence on people and their environment.Under the heading of risks, we can make a distinctionbetween:

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 thevoltage difference, vary from panic to death.The effect of panic should not be underestimated as itcould lead to a fall or other injury.Another effect is burning.




Figure : The effect of panicWillem Maes High Voltage Safety



The risk to surroundings

The risk to surroundings due to an overload or short circuit,causing:an abnormal overheating of an apparatus or a circuita fire or even an explosionall of this can result in a substantial loss off equipment endor functionality.




Muscle Tetanisation

Figure : Involuntary contraction of muscles




Muscle Tetanisation

Muscle Tetanisation is the involuntary contraction of themuscles with as a result:

Muscle tetanisation may not allow a detachment of thevictim from the live part.Breathing difficulty breathing stops: because of thecontraction of the muscles involved in breathing.




Effect on the heart

Figure : normal heart rhythm




Effect on the Heart

A small current passing through the chest can result inventricular fibrillation.If not treated immediately by defibrillation, it can be lethalas the heart muscle cells start moving independently.A shock more than 200 mA is so strong that it can literallystop the heart muscles from moving.




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 hasbeen shown to be a cause for neuropathy.When the current moves near the head then loss ofconsciousness occur fast.




Burns

Burns are produced by the heat developed by the currentflowing 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 failureand can lead to death.A person can be blinded even permantly by the UV lightemitted by an arc or arc flash.




Noise

The violent noise of an arc flash can cause a rupture of thetympanic membranes and temporary sensorineural hearingloss.




Electric Shock

If one touches a source of electricity, a current will passtrough 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 peopleare good electric conductors.The electric current can cause the body to go into shock.This can lead to a falling blood pressure, tackicardia, andloss of consiousness.




Electricity’s effect on the human body

Electric injury is a relative infrequent but potentialydevasting form of multi system injury with high morbidityand mortality.The severity of the injury depends on the intensity of theelectric current, the pathway it follows trough the body, andthe duration of the contact with the power source.Immediate death may occur either from current inducedventricular fibrillation or from respiratory arrest secondaryto paralysis of the central respiratory control system or toparalysis of the respiratory muscles.





According to Ω’s law, the electric current is proportional tothe voltage and inversely proportional to the resistance ofthe conductor.Thus exposure of different parts of the body to the samevoltage will generate a different current (and therefore adifferent degree of damage) because resistance variessignificantly between various tissues.





The least resistance is found in nerves, blood, mucousmembranes, and muscles.The highest resistance is found in bones, fat and tendons.Skin has intermediate resistance.





The skin is the primary resistor against the electricalcurrent, with a resistance ranging in adults between 40 to100KΩ, depending on its thickness (the thicker the skin thehigher the resistance.The intensity of the electrical shock varies between victimsof different sex and age.Even more important than the thickness of the skin is themoisture of the skin.Presence of sweat may decrease the resistance of the skinto less than 1 KΩ. Wet skin offers almost no resistance atall, thus generating the maximal intensity of current.





The internal resistance of the body comprises all the othertissues and is estimated between 0.5 to 1 Ω

Altough bones, tendons, and fat offer the most resistanceto electric current, they are not likely to be the contactpoints.Nerves and blood vessels, on the other hand, are the bestconductors. Nerves because they designed to carryelectrical currents and blood due to its high water content.





The duration of the contact with electrical current is animportant determinant of injury.Therefore it is said that an electrical shock caused by ACwill produce greater injury than a shock caused by DC ofthe same amperage because the DC current causes asingle muscle contraction that throws the victim away fromthe power source, minimizing the injury.These differences have practical significance only at lowvoltages, whereas on high voltages, both currents have asimilar effect.





The pathway of the current trough the body determines thenumber of organs that are effectedA vertical pathway is the most dangerous because itinvolves all the vital organs (central nervous system, heart,respiratory muscles, and in pregnant woman, the uterusand fetus).A horizontal pathway from hand to hand will spare thebrain 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 causesevere local damage but will probably be less lethal.





Whereas electric shock from a low voltage voltage line isdelivered on contact of the victim with the source, in highvoltage injury, the current is carried from the source to theperson trough an arc before any physical contact is made.The arc may form into or over the body of a person. Arcscan generate extremely high temperatures (up to 5000 degC) that are usually responsible for the severe thermalinjuries from high voltage.





Three primary factors affect the severity of the shock a personreceives when he or she is a part of an electrical circuit:

Amount of current flowing through the body (measured inamperes).Path of the current through the body.Length of time the body is in the circuit.





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 severeburns and immediate cardiac arrest. Although the exact injuriesthat result from any given amperage are not known, thefollowing slides demonstrates a general relationship for a60-cycle, hand-to-foot shock of one second’s duration:




hand-to-foot shock of one second

Current 1mAPerception level.Slight tingling sensation.Still dangerous under certain conditions.





Current 5 mASlight shock felt, not painful but disturbing.Average individual can let go.Strong involuntary reactions to shocks in this range maylead to injuries.If the extensor muscles are excited by the shock, theperson may be thrown away from the circuit.Often, this can result in a fall from elevation that kills avictim even when electrocution does not.





Current 6 mA - 16mAPainful shock, begin to lose muscular control.Commonly referred to as the freezing current or "let-go"range.





Current 17 mA- 99 mAExtreme pain, respiratory arrest, severe muscularcontractions.Individual cannot let go. Death is possible.Even relatively low voltages can be extremely dangerous,because the degree of injury increases with the length oftime the body is in the circuit.LOW VOLTAGE DOES NOT IMPLY LOW HAZARD!





Current 100 mA - 2000 mAVentricular fibrillation (uneven, uncoordinated pumping ofthe heart.)Muscular contraction and nerve damage begins to occur.Death is likely.Note that a difference of less than 100 milliamperes existsbetween a current that is barely perceptible and one thatcan kill.High voltage electrical energy greatly reduces the body’sresistance by quickly breaking down human skin. Once theskin is punctured, the lowered resistance results inmassive current flow.





Current of more than 2000 mACardiac arrest, internal organ damage, and severe burns.Death is probable.




HV work on board can be dangerous due to limited space.The work may be carried out in close proximity to aperson(s) not familiar with HV hazards.Therefore the area must be properly cordoned off fromsurrounding work that may be going on and danger noticeswell posted.There will be large areas of earthed metal that can beeasily touched, increasing the possibility of electrical shockfrom an HV conductor.High voltage isolation testing can be particulary hazardouswhen several parts of the equipment are energised for aperiod of time.




Dangers when working on HV equipment

Some equipment could be using water in its operationwhich can lead to an increased risk of injury. In general,water conducts electricity and reduces the resistance ofthe skin.The use of instruments when taking measurements of highvoltages can increase the risk of injury if they areinadvertently used without the earth (protective) conductorconnected. This can result in the enclosure of theinstrument becoming live at high voltages.




Dangers when working on HV equipment

High voltage equipment will store energy afterdisconnection. For example, on a 6.6KV switchboard afatal charge may still be present on the equipment hours oreven days later.If during maintenance an HV circuit main earth (CME) isremoved from the system, it must not be worked on, as theHV cabling can recharge itself to a high voltage frominduced 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 electricitybetween two or more conductors.The radiation of heat in an arc is very high and it can veryeasily set a persons clothes on fire.




Arc Blast

Arc blast pressure derives from two things. First, theexpansion of metal in a boiling, vaporising state, andsecond the heating of ambient air by passage of the arc.The mixture of vaporised water and metal in air near thearc generates a rapidly expanding plasma of ionized vapor,which can lead to extensive injuries.


Appendix For Further Reading

For Further Reading I

The RulesLloyds register of shipping.

The RulesABS.


Date post:	04-Apr-2021
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