Flashcards in 8.3 Installation, use and inspection Deck (16)
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
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.
EAWR Regulation 4 (1)Systems constructed sfairp to prevent danger
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.
EAWR Regulation 4 (3)Every work activity conducted so as notto give rise to danger
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.
The term ‘strength and capability’ of electrical equipment refers to
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.
There are three main approaches which can be taken to reduce the risk if a fault occurs in a system, these are...
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.
The system is designed and constructed that uninsulated conductors cannot become live. How?
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
Earthing Equipotential bonding Use of safe voltages Current limitation Connection to a common voltage reference point on the system
If the conductors do become charged their environment is such that danger will not arise. How? 2
Separated or isolated systems Earth-free, non-conducting environments
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
(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.
The suitability of work equipment is considered from three aspects:
(1) its initial integrity (2) the place where it will be used (3) the purpose for which it will be used.
The fuse is designed to protect
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.
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.
not more than 30 milliamps (mA)200 milliseconds (ms)
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)
(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.
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
Commercial Annual 5 years or on change of occupancy Industrial Annual 3 years
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
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.