High Voltage Hazards and Protective Equipment
2.2 Electrical Hazards Associated with High Voltage Systems
Electric shock occurs upon contact of a body part with any source of electricity that causes a sufficient current through the skin and muscles.
Arc flash is part of an arc fault, a type of electrical explosion that results from a low- impedance connection to the ground or another voltage phase in an electrical system.
The consequences of a fault are:
Electric Shock Arc flash burn Arc blast
Thermal burns
Shock Current Flow Hand-to-hand Shock Current Flow
Hand-to-foot
Heart affected in both cases Live Power Lines or
Circuit
Live Equipment
Figure 2.2 – Consequences of a Fault
High Voltage Hazards and Protective Equipment
2.2.1 Electric Shock
Human tissues, such as skin and muscle as well as blood and other body fluids electrical conductors that may be characterized based upon their conductivity. Electric potential differences applied across human tissues, or at two locations on the external skin surface generate response currents.
Electric shock is a sensation and muscular spasm caused when electric current passes through the body. Electric shock is often from hand to foot or from hand to hand as shown in Figure 2.3. The two conductors may be a hot (live) conductor and the ground or two hot (live) conductors as in two phase wires of a three-phase power distribution system.
The severity of the shock also depends upon:
The voltage applied.
The amount of current flowing through the body.
The path through the body.
The duration of flow through the body.
The extent of injury is determined by the pathway of the current through your body such as:
Current flowing through the heart causes fibrillation of the heart.
Current flowing through muscles causes contraction of the muscles.
Current flowing through the brain causes a loss of consciousness and seizures.
Chapter 2
Touch Potential Step Potential Touch Step Potential
80
15
Touch potential Step Potential Touch and Step potential Body Resistance Least body
resistance and most vital organ on the current path
Comparably higher body resistance
It is the most common shock current path but a left hand shock contact is considered more dangerous as the heart is in the current path.
Figure 2.3 – Current Paths in General and Body Resistance
Sensitivity and potential a c ea e e. A c ca e - f a current source is much more likely to be electrocuted than someone whose reaction removes them from the circuit more quickly. The victim who is exposed for only a fraction of a second is less likely to sustain an injury.
2.2.2 Effects of Electrical Shock
The effects of electric shock based on various current levels is explained in Table 2.1. In the case of females, these values could be lower by 30% to 35%.
To explain briefly, a shock current as low as 15 mA AC or 50 mA AC may be fatal. At about 100 mA (0.1 ampere), the shock is fatal if it lasts for one second or more. Obviously, the magnitude of shock current is related to the applied voltage and body resistance; however, the effects widely vary depending upon the person involved. Current from a steady DC source, in passing through the skin, will tend to cause muscular contraction at the initial contact and as contact is broken. It must be remembered that fibrillation is unlikely to occur if the current in mA is less than 116/t where t is the shock duration in seconds; thus, even though the current may be lower it may lead to this unpleasant condition if the victim is
High Voltage Hazards and Protective Equipment
Current Level Effect on Victim
1 mA Sensation that shock is occurring
5 mA* Upper limit of safe or harmless range (painful shock)
10 to 20 mA* Let-go threshold the victim cannot shake loose from the source of shock and perspires (onset of muscular contraction and could lead to sever shock)
30 to 40 mA* Sustained muscle contraction and cramping could lead to temporary lung failure too
50 to 70 mA* Extreme pain, physical exhaustion, fainting, irreversible nerve damage;
possibility of ventricular fibrillation (shocking of the heart into a useless flutter);
respiratory arrest with possible asphyxiation
100 mA* Certain ventricular fibrillation (of the heart) and death if the current passes through the body trunk
>100 mA Fibrillation, amnesia (memory loss), burns, severe electrolysis at contact sites
>5A Little likelihood of survival; could also result in severe burns.
Table 2.1 – Electric Shock Currents and Physiological Effects
Figure 2.4 – Electric Shock Injuries – Second Degree Burns after a H V Shock 2.2.3 Body Resistance with Increase in Voltage
The body resistance goes down as the voltage goes up and skin tissue breaks down. This means that the shock current is further increased at high voltage levels and is lethal.
Chapter 2
At 500 V or more, high resistance in the outer layer of the skin breaks down. This lowers e b d e a ce c e f ea . T e e a c ea e e a f c e that flows with any given voltage. Areas of skin breakdown are sometimes pinhead-sized wounds that can be easily overlooked. They are often a sign that a large amount of current could enter the body. This current can be expected to result in deep tissue injury to the muscles, nerves and other structures. This is one reason why there is often significant deep tissue injury in the way of skin burns with high-voltage injuries. The International Electro Technical Commission gives the following values for the total body impedance of a hand to hand circuit for dry skin, large contact areas, 60 Hz AC currents:
Approximate Voltage Body Resistance
24 V 6100
100 V 3200
220 V 2125
1000 V 1500
Table 2.2 – Voltage versus Body Resistance 2.2.4 Steps to Minimize the Risk of an Electrical Shock Onboard a) Electrical supply isolation
b) Insulation of the body c) Equipment grounding d) Safe work practices
Prevention
Insulate Time
Electrocution
Figure 2.5 – Causes and Prevention of Electrocution
Grounding is achieved by connecting electrical equipment and wiring systems to the earth by a wire or any other conductor.
High Voltage Hazards and Protective Equipment
The primary purpose of grounding is to reduce the risk of electric shocks when current leaks into metal parts of an appliance, power tool or other electrical devices that are not insulated.
In a properly grounded system, such leaking current (called fault current) is carried away harmlessly. Grounding is also used to prevent the accumulation of hazardous static electricity.
M
440 V Secondary Winding
125 V
0 V M
11 kV Secondary Winding
12 V
12 V
6326 V NER
5.3
M M
125 V
3175 V
250 V 6350 V
11 kV Secondary Winding 440 V Secondary
Winding
Figure 2.6 (a) – Shock Voltage When the motors are not grounded
Figure 2.6 (b) – Shock Voltage When the motors are grounded 2.2.5 First Aid in the Event of an Electric Shock
Alternating current produces a continuing spasm in the muscles through which current passes, with its change from forward to reverse flow at the rate of 50 or 60 cycles per second.
Alternating current can stimulate nerves directly. It finally results in the unfortunate victim e / e . M c f e c a e bee c ac
alternating current circuits. Serious shock results in unconsciousness or worse conditions, requiring resuscitation and medical care. Every person on board a ship must be trained and fully aware of first aid and safety procedures related to electric shock as described in the safety procedures. Posters should be displayed at high-risk areas such as the switchboard to generally portray the effects of severe electric shock and the requirement of immediate first aid for the causality. Serious shock, because of the above, can kill instantly, in so far as
Chapter 2
2.2.5.1 The Basic Procedure
Studies prove that only about 20% of victims survive if there is a delay of up to 3 minutes in rendering the right aid! The following are the basic steps to be initiated in case of an electric shock:
1. Act quickly!
2. Survey the situation 3. Develop a plan
4. A e e c c d 5. Summon help if needed
6. Administer the required First Aid 2.2.5.2 Rescue of a Victim of Electric Shock
A minimum of two competent persons are required when any high voltage work is carried out. This ensures that if one person accidentally receives a high voltage shock, the hook can be used by the other competent person to carry out a rescue operation.
With very high voltages, danger may exist even if the casualty is not actually in contact because the current may jump across the gap (arcing may occur).
In these cases, the rescue should be approached with great caution and the rescuer must keep as far as possible from any part of the electrical equipment while removing the casualty from contact with the current.
Lower the casualty to the floor taking care not to damage the head
If the casualty is conscious, make him comfortable
Should the casualty be unconscious but breathing, loosen the clothing around the neck and waist and place the casualty in the recovery position; keep a constant check on his pulse; improvise a suitable method to keep the victim warm.
Figure 2.7
Rescue Hook to be used by a Competent Person
High Voltage Hazards and Protective Equipment
When the casualty is found unconscious, but not breathing take immediate action and apply emergency resuscitation techniques that one must be aware of.
2.2.5.3 Mouth-to-Mouth Resuscitation
Lay the casualty on his or her back and check the mouth for blockages. If possible, raise
e ca a de a add f e .
Make sure the head is well back and the air-way is clear.
P c e ca a e. Ta e a dee b ea a d ea a d e e f the casualty.
B e a d f e ca a ; e c e d e a e
lungs fill with air. Repeat this until the casualty shows signs of recovery.
Place the victim in the recovery position. This will ensure that airway remains clear.
Pe f CPR e c e b ea f e c b ea a ed ee . C ec e c a .
Figure 2.8 – Mouth to Mouth Resuscitation
Place the other hand under the side of his / her head.
The back of the hand should touch the cheek.
Bend the farthest knee at a right angle.
Chapter 2
2.2.6 Arc Hazard
This hazard is beyond shock and electrocution and is a dangerous condition. It is associated with the release of energy caused by an electric arc.
An electric arc is a luminous bridge formed in a gap between two electrodes or any other two surfaces separated by a small gap and a high potential difference when the medium in between the contacts becomes highly ionized.
The interrupting current gets a low resistive path and continues to flow through this path even after the contacts are physically separated. During the flowing of current from one contact to the other, the path becomes so heated that it glows.
An electrical arc occurs whenever there is a loss of insulation between two conductive objects at sufficient voltage. Near high power electrical equipment, such as transformers, service entrance switchgear or generators, the short-circuit power available is high and consequently so is the energy associated with the electrical arc in case of a fault.
The energy released by the arc due to a fault creates a rise in the temperature and pressure in the surrounding area. This causes mechanical and thermal stress to nearby equipment and creates the potential for serious injuries in the vicinity.
An arcing fault is the flow of current through the air between phase conductors or between conductors and a neutral or ground. The energy that results from an arc fault manifests as an arc flash, arc blast or a combination of the two. The arc formation can be described in 4 phases:
1. Compression phase
The volume of the air where the arc develops is overheated due to the release of energy.
The remaining volume of air inside the cubicle heats up from convection and radiation.
Initially there are different temperatures and pressures from one zone to another;
2. Expansion phase
A hole is formed through which the superheated air begins to escape from the first instant that the internal pressure rises. The pressure reaches its maximum value and starts to decrease from the release of hot air.
High Voltage Hazards and Protective Equipment
3. Emission phase
Nearly all the superheated air is forced out by an almost constant overpressure due to continued contribution of energy by the arc,
4. Thermal phase
The temperature inside the switchgear nears that of the electrical arc after the expulsion of the air. This final phase lasts until the arc is quenched, when all the metals and the insulating materials coming into contact, undergo erosion with the production of gas, fumes and molten material.
2.2.7 Electric Arc Resulting in an Electrical Arc Blast (Explosion)
Temperatures at the arc terminals can reach up to 20,0000 C or more. The heat and intense a e f e a c ca ed e a c f a . A d e a c a ea ed a d e c d c a e a ed e eb ca a e e a e a e ed a a a c b a , c ca e a dde e ea e of large amounts of heat and light energy at the point of the arc. The main sources of this pressure wave coming from an electrical arc include:
• Heating of the air passage of the arc through it (similar to lightning).
• Expansion from melting, boiling and vapourising of the conducting metal.
Copper is known to melt at about 1085O C and expand by a factor of 67,000 times as it vapourises. This causes expulsion of near-vapourised droplets of molten metal from an arc. It also generates plasma (ionized vapour) that moves outward from the arc for distances proportional to the arc energy.
The heat, with the addition of molten metal droplets emanating from the arc can cause serious burns to personnel in the vicinity. Nowadays copper-tungsten alloy is also used for the contacts as it resists erosion due to arcing and the wear and tear is less. Tungsten alone is brittle but has a high melting point of about 3420O C.
Chapter 2
An arc flash hazard is based on:
1. Fault current 2. Arcing time 3. Distance
Figure 2.9 – Results of an Electric Arc
Pre-planning plays an important part in enhancing the safety of skilled personnel at work.
2.2.7.1 Causes of an Arc Flash a) Improper training
b) Improper work procedures or improper maintenance c) Dropped tools
d) Accidental contact with electrical systems e) Installation failure
f) Voltage testing with inappropriate equipment
g) Buildup of dust and / or corrosion on the insulating surfaces
h) Sparks produced during racking of breakers, replacement of fuses and closing of breakers in faulty lines and circuits.
i) Inattentiveness / overconfidence
High Voltage Hazards and Protective Equipment
2.2.7.2 Effect of an Arc Flash on the Human Body 1. Severe burns
2. Broken bones 3. Impaired Vision 4. Loss of hearing
5. Brain damage and / or internal injuries 6. Punctures and lacerations
7. Death
An electrical arc occurred through the air and entered his body. The current was drawn to his armpits because perspiration is highly conductive.
2.2.8 Arc Flash Regulations and Standards
The National Fire Protection Association (NFPA), Institute of Electrical and Electronics Engineers (IEEE) and Occupational Safety and Health Administration (OSHA) work together to develop regulations and standards that best protect personnel and equipment against electrical hazards, including an arc flash. Four separate industry standards focus on the prevention of arc flash incidents:
Figure 2.10 – Victims of an Electric Arc Flash
Chapter 2
Compliance with OSHA involves adherence to the following:
1. A facility must provide, and be able to demonstrate, a safety program with defined responsibilities.
2. Calculations for the degree of arc flash hazard.
3. Correct personal protective equipment (PPE) for workers.
4. Training for workers on the hazards of arc flash.
5. Appropriate tools for safe working.
2.2.8.1 Warning Labels on Equipment
The National Electric C de e e a e abe c a e e e f a protection boundary, its incident energy level and the required personal protective equipment (PPE).