2.5 Electricity Flashcards
What are the 2 forms of electricity?
Electricity is generated in two forms – alternating current (AC) and direct current (DC)
What is ohms law?
The relationship is commonly stated in a mathematical formula called Ohm’s Law which states that Voltage (V) = Current (I) x Resistance (R).
What are the effects on the body from electricity?
• The current flow may block the electrical signals between the brain and the muscles.
This can stop the heart beating properly, resulting in either cardiac fibrillation or cardiac arrest.
• The operation of the lungs can be affected, resulting in the person being unable to breathe.
• Muscle spasms or convulsions can occur which can be strong enough to break bones or dislocate joints. Loss of muscle control often means the person cannot let go or escape the electric shock. The person may also perhaps fall if they are working at height or sustain some other injuries as a result of a reflex reaction.
What factors influenec the severity of electricity?
• The level of the voltage since this has a direct impact on the amount of current that can flow
• The amount of current that flows since currents as low as 50mA can be fatal if sustained for any length of time.
• The current path or route that the electricity takes through the body and in particular where this involves vital organs such as the heart, brain or lungs.
• The frequency of the supply has a significant impact on the level of harm
• The duration of the shock will obviously have a major effect on the level of harm
• The resistance of the body, which can vary due to factors such as perspiration or
otherwise wet hands
What are the effects of alternating currents on the body?
1mA Threshold of perception, i.e. you can feel the current
2mA Muscles convulse
50mA Respiration is affected, resulting in suffocation
100mA Ventricular fibrillation of the heart, which is usually fatal unless access to trained medical help is available, along with
a defibrillator unit
200mA The heart stops completely which, ironically, reduces the chance of fibrillation. Severe burns will occur at this level but
the chances of resuscitation are perhaps greater
List the common causes of electrical fires?
• Arcing – where sparks occur between uninsulated conductors, e.g. live and neutral (or earth).
• Overheating of conductors – perhaps due to excessive loading such as too many
high current appliances being plugged into an adaptor
• Short-circuits which can occur because of the breakdown of aged insulation thus
allowing two conductors to touch each other.
• Using a flexible cable wound on a reel, where the combined heating effect causes the resistance to increase and thus even more heating.
• Inappropriate (i.e. not intrinsically safe) equipment being used in a flammable or
explosive environment.
• Covering appliances, thus preventing adequate ventilation
What are the common causes of accidents in workplace equipment for electricity?
• Use of equipment in inappropriate conditions, e.g. in wet conditions
• The use of equipment that is damaged or faulty, perhaps as a result of the lack of an
inspection and maintenance regime
• Incorrect or badly wired equipment which might occur as a result of poor or
unauthorised repairs.
• Carrying out service or repairs without disconnecting the supply.
Outline some secondary effects of electrical incidents?
Failure of safety critical alarm systems might follow, as could the unintended movement of plant or equipment when a supply is removed.
Most likely, however, the reflex reactions or the muscular spasms suffered by the victim of a shock may result in the risk of falling from ladders, unintentional and sudden contact with hot or sharp surfaces, or being injured by machinery.
Name some risks associated in working with electricity?
Poorly maintained electrical equipment
One of the major causes of incidents involving electricity is the use of poorly maintained equipment.
Work near overhead power lines
Accidental contact with live overhead power
lines results in many serious injuries and deaths each year
Contact with underground power cables
As with overhead electricity supplies, striking buried cables is an all too common occurrence.
In contrast to overhead cables, the location of buried services may not be obvious
Work on mains electrical supplies
No person should
undertake work on such circuits unless they are competent to do so. Additionally, work should, so far as is reasonably practicable, only be carried out with the supply isolated
Use of electrical equipment in wet environments
Working with electricity in wet environments is potentially dangerous for the simple reason that water conducts electricity
What does the UK Work at Regulations state regarding electricity?
The UK’s Electricity at Work Regulations 1989 state in Regulation 7 that all conductors in a system which may give rise to danger shall either:
(a) be suitably covered with insulating
material and as necessary protected
so as to prevent, so far as is reasonably
practicable, danger; or
(b) have such precautions taken in respect of them (including, where appropriate, their
being suitably placed) as will prevent, so
far as is reasonably practicable, danger
Cables, busbars or other electrical conductors have to be adequately protected against accidental damage or contact. Where there is a risk of
mechanical damage to conductors then it is likely that metal enclosures will be needed.
Any insulation or protection that is used should provide appropriate protection for the
intended use and environment
Cables and trailing leads will also need to be properly secured where they are connected (e.g. at plugs) so that they cannot be accidentally pulled out.
Adequate protection should be given to trailing cables, not only to prevent trip
hazards but also to protect the cables themselves.
Likewise, overhead cables should be highlighted where there is a risk of
contact by means of plant or equipment or persons carrying long items such as ladders.
What does the term strength and capability refer to in electrical equipment?
The term ‘strength and capability’ of electrical equipment refers to the ability of the equipment to withstand any harmful effects that might be expected to occur in use or anticipated fault conditions, e.g. overheating, electromagnetism
The insulation must be able to withstand the voltages that might be present or anticipated
In order to ensure that equipment is safe even under possible fault conditions, it is
necessary to consider the types of protective devicessuch as RCDs, fuses or circuit breakers
List the various protective systems for electricity?
Fuses and MCBs (Miniature Circuit Breakers)
Fuses and MCBs generally protect against the effects of excess currents. Fuses and MCBs are therefore classified as overcurrent devices. They are designed to melt (fuse) or trip (MCB) when excessive current is flowing such as when a circuit is overloaded with too many appliances, or a short circuit occurring when two uninsulated cables touch (e.g. such as
pliers cutting through a live cable).
MCBsprotect against excess currents. They differ in being electro-mechanically operated and thus it is possible to reset them whereas a fuse would need to be
replaced.
It should be noted that neither fuses nor MCBs have much value in protecting people
Earthing
A person may unwittingly touch the metal surface and thus provide a path to
earth (low potential) and of course would risk electrocution. The risk of this happening is reduced by connecting an earth conductor to any metalwork that could become live.
It is very important to ensure that the earth conductor’s resistance is as low as possible.
This is known as the earth continuity and is one of the periodic checks that may need to be carried out on electrical appliances.
Residual Current Devices (RCDs)
RCDs have become a very common and affordable protective device over
the last couple of decades. They are indeed of immense value in protecting
people when electrical faults or hazardous conditions exist. They may be permanently installed in circuits (i.e. on a distribution board) or may be plugin devices such as ‘power-breakers’.
When an RCD
detects an imbala ce in either side of the circuit of as little as 30mA, it operates to rapidly disconnect the supply.
As they are very sensitive devices, however, it is necessary to periodically test them to ensure that are operating correctly
Double insulation
Another way, however, to prevent metal surfaces from becoming live is to create an extra layer of insulation, i.e. to ‘double insulate’ any live parts with two physical layers of insulation. This results in a level of insulation that is so strong that it is very unlikely that anyone could suffer a shock unless the equipment had suffered severe physical damage.
The principle of double insulation need not be restricted to power tools, but is also adopted in the use of rubber matting, rubber footwear or gloves, and
insulated tools during maintenance work.
Reduced and low voltage systems
Reducing the operating voltage of electrical equipment is a practical way in which the chance of somebody receiving a lethal electric current can be minimised. This can be done by means of battery operated power tools, which result in no significant electrical risk.
Isolation of supply
Switching off equipment isn’t always enough to ensure safety. Sometimes it is necessary to isolate equipment before working on it. This involves physically separating equipment (requiring a sufficient air gap, i.e. through a circuit breaker) from its power source.
Often this involves using a padlock, signage or barriers to prevent unauthorised
persons from restoring the power. This procedure is often referred to as ‘Lock-Out Tag-Out’.
Use of competent persons
Anyone working on electrical systems or equipment must be competent to do so.
For high voltage, live electrical work or testing, this will require specialist
training along with additional procedural controls such as permits-to-work.
Use of safe systems of work
Work on live electrical supplies presents obvious risks to those persons involved in it. The UK’s Electricity at Work Regulations 1989 make it clear in Regulation 14 that such work (including testing) can only be done if:
• it is unreasonable in all the
circumstances for it to be dead, and
• it is reasonable in all the circumstances for him to be at work on or near it while it is live; and
• suitable precautions (including where necessary the provision of suitable protective equipment) are taken to prevent
injury.
What controls need to be in place if live work is to be carried out?
- A live electrical permit-to-work with clear scope and details of the work to be undertaken;
- Selection of competent personnel;
- Cordoning off the area;
- Suitable PPE and tools;
- Emergency isolation and first aid procedures; and
- No lone working.
What steps should be taken when excavating?
- Consult plans where available and/or discuss with the electricity supply company to identify the cable routes.
- Where possible, arrange for isolations of the supply (this is, in practice, usually difficult to achieve).
- Check for the presence of cables using a cable avoidance tool (CAT scanner) and where these are detected, mark (with spray paint and cones) their location.
- If in doubt, don’t excavate with machinery but dig carefully with hand tools (i.e. spades).
What protection needs to be taken when working with overhead cables?
- Arrange for isolations of the supply (this is, in practice, usually difficult to achieve).
- Position goalposts to prevent inadvertent contact by vehicles or long items being carried (for 33,000 volt overhead supplies, a minimum of six metre clearance is required).
- Cordon off areas with signage and bunting.
- Where necessary, use banksmen to direct traffic movements.
- Avoid the use of metal scaffolding or ladders in the vicinity.
