c08_electrical_safety_and_electrical_at_work_regulations_1989_COMPLETE

Question 1

Electricity is energy made available by

Answer 1

the flow of electrons through a conductor

Question 2

Atoms consist of

Answer 2

a central nucleus made up of protons and neutrons, which is orbited by rings of electrons.

Question 3

movement of electrons from atom to atom is known as

Answer 3

electron flow

Question 4

Electron flow along a conductor is called

Answer 4

current

Question 5

The current or rate of electron flow (numbers of electrons passing a point per second) is measured in

Answer 5

amperes (amps) (symbol I).

Question 6

The difference in positive and negative potential between the terminals of a power supply (potential difference) is measured in

Answer 6

units of volts (symbol V). Higher voltage results in greater current flow.

Question 7

All materials, including conductors, will afford some resistance to the flow of electrons. The resistance of a conductor is measured in

Answer 7

ohms (Ω) (symbol R).

Question 8

Ohms law –

Answer 8

V = I × RI = V/RR = V/I

Question 9

Mains electricity is from an ___________________ supply

Answer 9

alternating current (AC)

Question 10

Mains electricity in the UK is a ?V supply, at a frequency of ? Hertz (Hz).

Answer 10

230V 50 Hertz (Hz).

Question 11

High voltage is defined in the HSE guidance Electricity at work: Safe working practices (HSG85) as being in excess of

Answer 11

650V AC

Question 12

The dangers (risk of personal injury) associated with electricity can be remembered by

Answer 12

the acronym B-SAFEBurns Shock Arcing Fire Explosion

Question 13

A direct contact refers to

Answer 13

a person coming into contact with a conductor which is live in normal circumstances. Protection against direct contact has been renamed basic protection

Question 14

An indirect contact refers to

Answer 14

a person coming into contact with an exposed conductive part which is not normally live, but has accidentally become live due to insulation failure or some other cause. Protection against indirect contact is now known as fault protection

Question 15

With respect to UK mains voltage electricity, outline the factors that determine the severity of the effects of an electric shock. 6 marks

Answer 15

(a) Factors include:  the voltage, the nature of the current (whether alternating or direct)  the body resistance of the individual with reference to age, gender, the amount of moisture (perspiration) on the body and the type of footwear worn  the route taken by the current through the body,  the speed of action of any protective measures  the environmental conditions, such as the floor material and the presence of water. Note: knowledge of Ohm’s Law could have been used to relate the size of current to voltage and resistance.

Question 16

Fires may be started by electricity in a number of ways. The principal mechanisms are: 4

Answer 16

(a) overheating of cables and electrical equipment due to overloading of conductors (b) leakage currents due to poor or inadequate insulation (c) overheating of flammable materials placed too close to electrical equipment which is otherwise operating normally (d) ignition of flammable materials by arcing or sparking of electrical equipment, including the scattering of hot particles from electrical equipment.

Question 17

The Electricity at Work Regulations 1989 (EAWR) require precautions to be taken against the risk of death or personal injury from electricity in work activities. The Regulations impose duties on people (referred to as duty holders) in respect of systems, electrical equipment and conductors, and in respect of work activities on or near electrical equipment. The general duties require the employer, so far as is reasonably practicable, to ensure that: 4

Answer 17

 Electrical systems are constructed so as not to give rise to danger.  Electrical systems are maintained so as not to give rise to danger.  Every work activity, including operation, use and maintenance of a system and work near a system, is carried out so as not to give rise to danger.  Protective equipment is suitable for the use for which it is provided, maintained in good condition and properly used.

Question 18

EAWR Regulation 4 (1)Systems constructed sfairp to prevent danger

Answer 18

Regulation 5Strength and capability maynot be exceeded in a waythat gives rise to danger.Regulation 6Constructed and protectedto prevent danger fromexposure to hazardousenvironments.Regulation 7Conductors to be insulated,protected or placed so asnot to give rise to danger.Regulation 8Earthing or other precautionsto prevent danger from thecharging of a non-circuitconductor.Regulation 9Integrity of referencedconductors. Should a faultoccur the impedance of theearth connector shouldremain low.Regulation 10Every joint and connectionto be mechanically andelectrically suitable for use.Regulation 11Every part of a system to beprotected against dangerfrom excess current.Regulation 12Suitable means for cuttingoff and isolating the supply.

Question 19

EAWR Regulation 4 (3)Every work activity conducted so as notto give rise to danger

Answer 19

Regulation 13Precautions to preventequipment made deadbecoming charged duringwork.Regulation 14No work on or near liveconductors unless:unreasonable to be dead,reasonable to work live andall suitable precautionstaken.Regulation 15Adequate working space,access and lighting.Regulation 16Persons to be competent toprevent danger and injur y.Adequate knowledge /experience or undersupervision.

Question 20

The term ‘strength and capability’ of electrical equipment refers to

Answer 20

the ability of the equipment to withstand the thermal, electromagnetic and electro-chemical effects of the electrical currents which might be expected to flow through them.

Question 21

There are three main approaches which can be taken to reduce the risk if a fault occurs in a system, these are…

Answer 21

The system is designed and constructed that uninsulated conductors cannot become live. If the conductors do become ‘live’, the potential difference involved, and the size and duration of the resulting electric current are such that no danger will arise. If the conductors do become charged their environment is such that danger will not arise.

Question 22

The system is designed and constructed that uninsulated conductors cannot become live. How?

Answer 22

 Double insulation

Question 23

If the conductors do become ‘live’, the potential difference involved, and the size and duration of the resulting electric current are such that no danger will arise. How? 5

Answer 23

 Earthing  Equipotential bonding  Use of safe voltages  Current limitation  Connection to a common voltage reference point on the system

Question 24

If the conductors do become charged their environment is such that danger will not arise. How? 2

Answer 24

 Separated or isolated systems  Earth-free, non-conducting environments

Question 25

A corrosive substance has a flash point of 20°C. An electrical subcontractor has been employed to design and install the electric supply to an open air production plant which is used to manufacture the corrosive substance. This plant is located next to the sea. (a) Outline the aspects of this situation which the designer of the electrical supply must address to ensure that the installed system will be compliant with the Electricity at Work Regulations 1989. 10 marks (b) Describe the types of fault that may be found in fixed electrical systems under such conditions. 10 marks

Answer 25

(a) The aspects to be addressed by the designer could have been outlined under the under a number of general headings: Mechanical damage - including that caused by impact, stress and strain, abrasion, wear, vibration and pressure. Weather impact - the system must be resistant to the impact of the weather for example the effects of rain, snow loading and UV degradation from sunlight. Environmental contaminants - including damp, dirt, corrosive leaks, salt laden air and water and steam leaks. A flammable atmosphere might also be present as a result of vapour leaks, spills and effluents. Environmental factors especially relevant to the use and maintenance of the plant that would require attention include the lighting of switch gear and instrument panels, and that provided for winter maintenance in restricted work spaces and during periods of low temperature. (b) Faults in fixed electrical systems that might be found in the conditions described include equipment unsuitable for wet or corrosive conditions together with exposed live conductors, damaged or perished insulation and damage to the protection on cable conduits. Additionally corrosion of parts such as access grills and transformer casings could facilitate the ingress of fluids resulting in short circuits. There is a danger of poor earthing and a lack of earth continuity together with damage caused to the means for isolating the supply to parts of the plant. Finally it could be found that there is inadequate excess current protection because fuses with incorrect ratings have been fitted.

Question 26

The suitability of work equipment is considered from three aspects:

Answer 26

(1) its initial integrity (2) the place where it will be used (3) the purpose for which it will be used.

Question 27

The fuse is designed to protect

Answer 27

the wiring and the appliance if a fault occurs. Fuses operate too slowly to provide protection against electric shock, and are easy to replace with incorrectly rated fuses or other metal ‘bridges’, such as nails, screws or bolts.

Question 28

RCDs for protecting people have a rated tripping current (sensitivity) of ____________________, and a non-adjustable rated operating time of _________________________ at a test current of 30 mA, and 40 ms at 150 mA.

Answer 28

not more than 30 milliamps (mA)200 milliseconds (ms)

Question 29

For EACH of the following protective devices, describe their principles of operation: (i) residual current devices (3 marks) (ii) fuses (3 marks) (iii) 110v centre tapped to earth reduced voltage systems (3 marks) Outline other design features of electrical systems intended to improve safety. (5 marks)

Answer 29

(b) (i) A residual current device is designed as a shock limiting device and not for system protection. It operates on an earth leakage fault. Any differential in the current passing through the line (neutral) and phase (live) conductors is detected, operating a switch to cut off the electrical supply to the apparatus and preventing severe electric shock. The nominal operating current is 30mA and the device should operate within 30 to 50 milliseconds of the fault being detected. (ii) A fuse is a device placed in the live side of a circuit, designed to automatically cut off the power supply to the circuit within a given time when the current flow in the circuit exceeds a given value and produces sufficient heat to melt the fuse which is designed to do so at a predetermined temperature. It prevents the overload of an electrical system and overheating of electrical wiring. However, its speed of operation is generally too slow to protect people from electric shock. (b) (iii) One of the better ways of reducing the risk from electricity is to reduce the voltage, achieved by the use of a step down transformer. A common reduction is to 110volts and a transformer used to attain the reduction is described as centre tapped to earth in that the secondary winding of the transformer is earthed to its centre thus ensuring that the maximum voltage from live to earth involved in an electric shock will be 55 volts. (c) Other design features intended to improve safety include:  the selection and colour coding of cables and placing them out of reach wherever possible  the provision of effective means of isolation to ensure the secure disconnection and separation of electrical equipment from every source of energy  the use of earthed systems and Class 1 equipment or double insulated Class 2 equipment  the use of circuit breakers and the introduction of earth free zones.

Question 30

Guidance from the Institution of Engineering and Technology (IET) recommends the following frequencies of routine checks and formal inspection and testing in commercial and industrial premises with routine installations. Type of Premises Routine check Maximum period between formal inspections

Answer 30

Commercial Annual 5 years or on change of occupancy Industrial Annual 3 years

Question 31

Maintenance work on electrical distribution panels and control circuitry commonly involves diagnostic testing and fault finding on live systems. Outline the requirements of the Electricity at Work Regulations 1989 that apply to this situation AND the practical precautions that should be in place before the work is undertaken. 10 marks

Answer 31

Regulation 14 of EAWR sets out three conditions for working on a live system, the first two of which (that it is unreasonable for it to be made dead and that it is reasonable to work on it while it is live) are clearly met in this case since much diagnostic work will require this. Therefore, the third condition (that suitable precautions are taken) requires attention and overlaps with other requirements of the Regulations, in particular Regulation 4 relating to electrical systems, work activities and protective equipment. In this respect, distribution panels should have protection rating of at least IP2X by the provision of test points that do not allow access to fingers or tools other than test probes. For example test points of 2mm in diameter. The test probes should be insulated and fused and the test meter must be checked prior to use. The area should be cordoned off by the use of barriers to prevent unauthorised access and, depending on the circumstances, insulating mats and/or gloves may be required. All live working should be subject to a permit to work system. Regulation 16 of EAWR requires the engineer to be technically competent or to be closely supervised, having regard to the nature of the work, by someone who is competent. Persons involved with the work should therefore possess recognised qualifications and have experience in the type of work being undertaken. In order to comply with Regulation 7, relating to the protection of conductors, all incoming and outgoing supplies should be suitably insulated and shrouded. Additional shrouding may be required. With regard to earthing (Regulation 8) the cabinet should be connected to the main earth and the cabinet door earth-bonded to the cabinet. The other main concern is to ensure that there is sufficient working space, suitable access and adequate lighting, as required by Regulation 15.

Question 32

Outline practical control measures that should be taken to help ensure electrical safety in relation to the provision and use of electricity on a construction site. 10 marks

Answer 32

The planning and assessment of requirements for electricity on site would be an important primary control measure to be taken. This would point to the need to use suitably robust and protected cabling for a site environment, together with appropriate equipment which as far as portable equipment is concerned, would require its connection to the system through residual current devices (RCDs) or be operated by means of a reduced low voltage system via step down transformers or be battery operated. The portable equipment should also be subjected to regular testing and maintenance (PAT). One of the problems that can arise on site is the number of contractors that might be involved and it will be necessary to coordinate and control the type and quality of equipment they bring with them. The fixed supply should be regularly tested and reassessed by a competent person and the routes of existing and temporary supplies whether overhead or underground should be conspicuously marked and precautions such as ‘goal posts’ used to avoid contact with moving equipment. Should it be found necessary for work to be carried out on the site electrical system, this should be carried out by a competent person and a safe system of work introduced, such as a permit to work particularly if live working is to be involved.

Question 33

As mentioned in Element C8.1, high voltage is generally considered to be

Answer 33

greater than 1000 V (1kV) AC or 1500 V DC.

Question 34

A safe system of work for working near buried services has four basic elements:

Answer 34

(1) Planning the work (2) Plans (3) Cable-locating devices (4) Safe digging practices.

Question 35

Outline the precautions that can be taken to minimise the risks associated with a cable strike when excavating near underground electrical cables. 20 marks

Answer 35

If excavation work is to be carried out near underground electrical cables, it is essential that the work is properly planned before it starts. This involves:  seeking advice from the relevant body such as the appropriate electricity supply company or the highway/roads authority  assessing electricity cable plans on the understanding that symbols may vary between utilities and that high and low voltage cables can be shown on separate plans  making the supply dead if this can be done or investigating the possibility of diversion  avoiding the use of mechanical excavation and utilising a permit to work system. Before the actual excavation work starts, it will be necessary to locate the presence of cables by the use of cable avoidance tools (CATs), such as Hum or radio frequency detectors, and their identification and routes clearly marked. If a cable, which is recorded on a plan, cannot be located, assistance and advice should be sought from the appropriate authority, with extreme care being taken if excavation work begins before the assistance and advice has been obtained. The excavation work should be carried out only by personnel who have been trained and are competent in safe digging practice. They would need to be aware that although no cables are indicated on plans or located by a detector, they may still be present and though cable depths are often prescribed, they should not assume that this is always the case. Additionally, though cables often have protective covers, the assumption should not be made that this is universal practice. Workers should also be aware that though a cable has been exposed, there may be further cables at a lower level, and if cables are found which are terminated with pot ended or bottle ended seals, they may still be live. When cable joints are discovered, they should be supported, should not be moved, and should not be roughly treated.Further precautions that should be taken during the excavation work include the need to look out for permanent marker posts or other visible indication of buried cables; to establish positive location by careful hand digging, exposing the cables by digging from the side and using hand held power tools only at a distance of 500 mm or more from the indicated lines of cables. If mechanical excavators have to be used in the vicinity of cables, then damage should be avoided as far as is reasonably practicable. Personnel should be kept well away from the excavator, and drivers instructed to stay in the cab if a cable is struck. If they have to leave the cab in such circumstances, they should jump clear. Once cables are exposed, any damage noted should be reported to the cable owners and no work undertaken in the vicinity. Support should be provided if more than one metre of cable is exposed, and protection, such as the use of planks or sandbags, should be provided to prevent damage from on-going work. Finally, cables should not be moved aside unless the work is supervised by the cable owners.

Question 36

The following hierarchy of precautions, from the HSE guidance Avoiding danger from overhead power lines (GS6), should be applied to work at or near overhead power lines:

Answer 36

 Avoid working under or near overhead lines.  Divert overhead lines clear of the work area.  Make lines dead while the work is in progress.  Work around the live overhead lines using appropriate precautions.

Question 37

Precautions depend on the nature of the work - there are three broad categories of work:

Answer 37

1 No scheduled work or passage of plant under the lines Use barriers to prevent close approach. 2 Plant will pass under the lines Defined passageways should be made. 3 Work will be carried out beneath the lines Further precautions are required in addition to the erection of barriers with passageways.

Question 38

Construction work is to take place in a rural area where electrical power for the site is to be gained from an existing 11kV overhead supply that cuts across the site on wooden poles. Outline control measures that should be taken to reduce risks associated with the: (a) overhead supply 8 marks (b) provision and use of electricity on site. 12 marks

Answer 38

(a) Control measures associated with work near an overhead electricity supply are covered in the HSE guidance GS6 – Avoiding danger from overhead power lines, and include starting by considering the possibility of re-routeing the lines and pointing out that, before any actions were recommended by way of protective measures, the utility supplier would need to be consulted. Other control measures include:  identification of safety distances  the use of barriers, marking tape and bunting  the use of goal posts and tunnels where access routes pass under the power lines  height restrictions on plant  the introduction of safe systems for working near to power lines  a restriction on the storage of materials within six metres of the overhead lines  the prohibition of using hand-held long metal equipment such as ladders  the erection of warning signs  the provision of training and information to the workforce on the precautionary measures that should be taken. (b) Control measures include:  planning and assessment for the development of the electricity supply by a competent person  the suitable connection and positioning of step-down transformers  the routeing, marking and protection of cables  the use of protective measures such as reduced low voltage systems, residual current devices and double insulated equipment  arrangements for the regular inspection and testing of the fixed supply including the integrity of the earthing of the system  exercising control on the equipment brought on to the site including portable items  the use of competent persons for the installation of and work on the electrical supply.

Question 39

IET Guidance recognises that different inspection and maintenance regimes are required for different types of portable electrical equipment and categorises equipment as follows:

Answer 39

S Stationary equipment, such as a refrigerator or washing machine. P Portable equipment 18kg or less intended to be moved while in operation. Examples: toaster, food mixer or kettle. IT IT equipment, such as computers, printers and monitors or telecommunications equipment. H Hand-held equipment intended to be held in the hand during normal use. Examples: hairdryer or power drill. M Movable equipment 18 kg or less in mass and not fixed. Examples: electric fire or small welding set.

Question 40

The suitability of work equipment is considered from three aspects:

Answer 40

 its initial integrity  the place where it will be used  the purpose for which it will be used.

Question 41

A system of periodic inspection and testing is necessary to demonstrate that this duty is being fulfilled. The system would typically involve the following levels of checks:

Answer 41

(1) Checks by the user. (2) Formal visual inspections by a person trained and appointed to carry them out. (3) Combined inspection and tests by an electrically competent person.

Question 42

The inspection carried out in conjunction with testing should usually include checking:

Answer 42

 the correct polarity of supply cables  correct fusing, effective termination of cables and cores  that the equipment is suitable for its environment.

Question 43

Combined inspection and testing requires an appropriate degree of competence to interpret test results. Basically, there are two levels of competence:

Answer 43

 A person who is not skilled in electrical work may routinely use a simple 'pass/fail' type of portable appliance tester.  A person with appropriate electrical skills uses a more sophisticated instrument that gives actual readings requiring interpretation.