Electrical Safety Flashcards
Ohms Law
The value of a current passing through a conductor is directly proportional to the potential difference between the ends of the conductor, and inversely proportional to the resistance of the conductor
Resistance
The opposition to a steady electrical current
Impedence
Is the AC (alternating Current) equivalent of Resistance
Reactance
The opposition to the flow of alternating current
Inductance
The property of a conductor by which a change in current in the conductor induces a voltage in the conductor itself
Capacitance
The property of a circuit element that permits it to store charge.
Conductors
A conductor is any material that easily allows electrons to flow. Copper and Aluminum are two great examples of good conductors that are used extensively for electric cables
Insulators
Any material that inhibits the flow of electrons. Glass and rubber are good examples of insulating materials
Semiconductors
Used in the manufacturer of diodes, transistors and integrated circuit chips. They are neither conductors nor insulators. Examples include carbon, silicon and germanium
Alternating current (AC)
Mains electricity in the UK is AC. 230v supply in the UK. Current flows back and forth
Direct Current (DC)
Can be produced by batteries. Circuit flows in one direction only
Diode
Effectively an electrical one way valve, conducts the current in a forward direction but not in reverse. Used to convert (rectify) AC into DC
Electromagnetic Radiation
EMFA arise whenever electrical energy is used. Can lead to acute effects. At low frequency can affect the nervous system. At high can raise the body temperature
High Voltage
In excess of 650 V AC
Low Voltage
AC current exceeding 50 V but not exceeding 1000 V
Circuitry - Effects of components in a series
If 2 lamps are connected in series and one breaks the circuit is broken and the 2nd lamp can’t work.
When in series the same current flows through them and the voltage is shared between them
Circuitry - Effects of components in Parrallel
If lamps are connected in parrallel and one breaks the circuit is not affected and the second lamp will still work.
When in parrallel the current is shared between them but the voltage available remains the same
Earthing Prinicples
Earth is the conductive mass of earth, whose electrical; potential at any point is conventionally taken at zero
In order to reduce the risk of serious electric shock it is important to provide a path for earth fault currents to operate the circuit protection
Dangers from Electricity - B-Safe
Burns Shock Arcing Fire Explosion
Electric Burn
Due to heating effect caused by the passage of electric current through the body tissues
Painful and slow to heal, permanent scarring is common
Electric Shock
Response to electrical current flowing through the body. Electricity can take multiple paths through the body and is difficult to predict and can be very painful or fatal
Potentially fatal effects of electric shock
Muscular contractions Respiratory failure Fibrillation of the heart Cardiac Arrest Injury from internal burns
Direct Shock
Direct contact is when a person comes into contact with a live conductor
Indirect Shock
Indirect contact is when a person coming into contact with an exposed conductive part which is not normally live, but has accidently become live due to insulation failure
Arcing
Arcing generates UV radiation which causes damage similar to sunburn.
Arcing faults occur if the energy available at a piece of electrical equipment is enough to maintain a conductive path through the air
Electrical Fires
Can occur in many ways:
Overheating of cables or overloading of conductors
Leakage of currents due to poor insulation
Overheating of flammable materials too close to electrical equipment
Ignition of flammable materials by arcing or sparking
Effects of AC on the body
More dangerous than DC. Causes muscular contractions which can lead to a person not being able to let go and can cause fibrillation of the heart muscle
Effects of DC on the body
Typically causes single convulsive contraction
Main hazards of static electricity
Creation of a spark that ignites an explosive or flammable atmosphere
Control strategies for Static Electricity
Equipotential Bonding and Earthing Humidification Antistatic additives Ionisation of the air Conductive flooring an footwear
Regulations covering Electricity
Electricity at Work Regulations 1989 (EAWR)
General duties of employer under EAWR
Electrical systems constructed so as to not give rise to danger
Electrical systems are maintained so as to not give rise to danger
All work carried out so as not to give rise to danger
Protective equipment is suitable for use, in good condition and properly used.
Definition of Duty Holder under EAWR
Employer or self employed person so far as it relates to matters within his control.
Employees also have duty to cooperate and comply with the regulations for matters which are within their control
Definition of Electrical system
An electrical system in which all electrical equipment is or may be electrically connected to a common source of energy, Includes equipment and source of energy
Definition of Electrical Equipment under EWAR
Anything used to generate, provide, transmit, transform, rectify, convert, conduct, distribute, control, store or measure electrical energy
‘Strength and Capability’ of electrical equipment
Ability of equipment to withstand the thermal, electromagnetic and electro-chemical effects of the electrical currents which pass through them
All conductors in a system should be….
Covered with insulating material and protected to prevent danger
Placed or have other suitable precautions to prevent danger
3 main approaches to protecting against electric shock in the event of system fault
System designed and constructed so uninsulated conductors cannot become live - Double Insulation
If they do become live the potential difference involved and the size and duration of current are such that no danger will arise - Earthing, Equipotential bonding, Use of safe voltages
If they do become charges their environment is such that danger won’t arise - Separated or isolated systems, Earth-free non-conducting environments
Ways to protect from excess current
Fuses and Circuit breakers
Isolation
The disconnection and separation of the electrical equipment from every source of electrical energy and disconnection and separation must be secure
Working space
Dimensions should allow people to move away from exposed live conductors without hazard and to pass one another with ease without hazard
Techniques used as protective systems
Earthing Fuses, mini-circuit breakers and residual current devices Double insulation Use of safe voltages Isolated systems Earth-free zones
Factors to determine the frequency of periodic inspection and testing of fixed installations
Commercial = Routine check annual, max period between formal inspections, 5 years or on change of occupancy
Industrial = Routine check annual, max period between formal inspections 3 years
Reason for most electrical accidents
People believe equipment is dead but it is live
It is known to be live but those involved do not have adequate training, equipment or have not taken adequate precautions
Working ‘Live’
Should be exception to rule. Only permitted if:
It is unreasonable in all the circumstances for the conductor to be dead
It is reasonable in all the circumstances for the person to be at work on or near a conductor while live
Suitable precautions (including PPE) have been taken to prevent injury
8 steps to planning, preparation and procedures for electrical work
1) Identify the electrical system and work to be done
2) Plan the work
3) Specify correct system of work
4) Select and instruct competent workers
5) Ensure correct working methods
6) Provide and ensure use of appropriate protective equipment
7) Provide information, tools and instruments and ensure workers are fully instructed
8) Arrangements for management checks and supervision
8 steps to dead working
1) Identify circuit or equipment to work on
2) Cut off supply, isolate and secure isolation
3) Retain keys. Post caution and danger notices
4) Prove circuit or equipment dead
5) Apply circuit main earth where necessary
6) Take precautions against adjacent live parts where necessary
7) Issue permit to work where necessary
8) Apply local earths
4 basic elements for working near buried underground cables/services
1) Planning the work
2) Plans
3) Cable-locating devices
4) Safe digging practices
Locating devices - 3 modes of operation
Power Mode (hum detection) Radio Mode (RF detection) Generator Mode (genny) (transmitter- receiver)
Max Voltage of overhead cables
400kV
Hierarchy of precautions for working near overhead power lines
Avoid working under or near lines
Divert lines clear of the work area
Make lines dead while the work is in progress
Work around the live lines using appropriate precautions
3 categories of work around overhead power lines
1) No scheduled work or passage of plant under the lines
2) Plant will pass under the lines
3) Work will be carried out under the lines
Hot Glove Working
Involves staff working within safety distance zones and actually handling live conductors
Hot Stick Working
Involves use of Fibreglass Reinforced Plastic constructions as clamp and switch sticks
HSE Definition of Portable Appliances
Equipment that is not part of a fixed installation, but is intended to be connected to a fixed installation, or a generator, by means of a flexible cabler and either a plug and socket, or a spur box
Levels of checks for Portable appliances
1) Checks by the user
2) Formal visual inspections by a trained person appointed to carry them out
3) Combined inspection and tests by an electrically competent person
