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Some useful articles for the electrical industry.

 INTRODUCTION TO ELECTRICAL SAFETY Electricity can kill. Each year about 1000 accidents at work involving electric shock or burns are reported to the Health and Safety Executive (HSE). Around 30 of these are fatal. Most of these fatalities arise from contact with overhead or underground power cables. Even non-fatal shocks can cause severe and permanent injury. Shocks from faulty equipment may lead to falls from ladders, scaffolds or other work platforms. Those using electricity may not be the only ones at risk: poor electrical installations and faulty electrical appliances can lead to fires which may also cause death or injury to others. Most of these accidents can be avoided by careful planning and straightforward precautions. This leaflet outlines basic measures to help you control the risks from your use of electricity at work. More detailed guidance for particular industries or subjects is listed on pages 6 - 8. If in doubt about safety matters or your legal responsibilities, contact your local inspector of health and safety. The telephone number of your local HSE office will be in the phone book under Health and Safety Executive. For premises inspected by local authorities the contact point is likely to be the environmental health department at your local council. WHAT ARE THE HAZARDS? The main hazards are: contact with live parts causing shock and burns (normal mains voltage,230 volts AC, can kill); faults which could cause fires; fire or explosion where electricity could be the source of ignition in a potentially flammable or explosive atmosphere, eg in a spray paint booth. ASSESSING THE RISK Hazard means anything which can cause harm. Risk is the chance, great or small, that someone will actually be harmed by the hazard. The first stage in controlling risk is to carry out a risk assessment in order to identify what needs to be done. (This is a legal requirement for all risks at work.) When carrying out a risk assessment: identify the hazards; decide who might be harmed, and how; evaluate the risks arising from the hazards and decide whether existing precautions are adequate or more should be taken; if you have five or more employees, record any significant findings; review your assessment from time to time and revise it if necessary. The risk of injury from electricity is strongly linked to where and how it is used. The risks are greatest in harsh conditions, for example: in wet surroundings - unsuitable equipment can easily become live and can make its surroundings live; out of doors - equipment may not only become wet but may be at greater risk of damage; in cramped spaces with a lot of earthed metalwork, such as inside a tank or bin - if an electrical fault developed it could be very difficult to avoid a shock. Some items of equipment can also involve greater risk than others. Extension leads are particularly liable to damage - to their plugs and sockets, to their electrical connections, and to the cable itself. Other flexible leads, particularly those connected to equipment which is moved a great deal, can suffer from similar problems. More information on carrying out risk assessments is available in other HSE publications listed on page 6 of this leaflet. REDUCING THE RISK Once you have completed the risk assessment, you can use your findings to reduce unacceptable risks from the electrical equipment in your place of work. There are many things you can do to achieve this; here are some. Ensure that the electrical installation is safe ■ install new electrical systems to a suitable standard, eg BS 7671 Requirements for electrical installations, and then maintain them in a safe condition; ■ existing installations should also be properly maintained; ■ provide enough socket-outlets - overloading socket-outlets by using adaptors can cause fires. Provide safe and suitable equipment ■ choose equipment that is suitable for its working environment; ■ electrical risks can sometimes be eliminated by using air, hydraulic or hand- powered tools. These are especially useful in harsh conditions; ■ ensure that equipment is safe when supplied and then maintain it in a safe condition; ■ provide an accessible and clearly identified switch near each fixed machine to cut off power in an emergency; ■ for portable equipment, use socket-outlets which are close by so that equipment can be easily disconnected in an emergency; ■ the ends of flexible cables should always have the outer sheath of the cable firmly clamped to stop the wires (particularly the earth) pulling out of the terminals; ■ replace damaged sections of cable completely; ■ use proper connectors or cable couplers to join lengths of cable. Do not use strip connector blocks covered in insulating tape; ■ some types of equipment are double insulated. These are often marked with a ‘double-square’ symbol . The supply leads have only two wires - live (brown) and neutral (blue). Make sure they are properly connected if the plug is not a moulded-on type; ■ protect lightbulbs and other equipment which could easily be damaged in use. There is a risk of electric shock if they are broken; ■ electrical equipment used in flammable/explosive atmospheres should be designed to stop it from causing ignition. You may need specialist advice. Reduce the voltage One of the best ways of reducing the risk of injury when using electrical equipment is to limit the supply voltage to the lowest needed to get the job done, such as: ■ temporary lighting can be run at lower voltages, eg 12, 25, 50 or 110 volts; ■ where electrically powered tools are used, battery operated are safest; ■ portable tools are readily available which are designed to be run from a 110 volts centre-tapped-to-earth supply. Provide a safety device If equipment operating at 230 volts or higher is used, an RCD (residual current device) can provide additional safety. An RCD is a device which detects some, but not all, faults in the electrical system and rapidly switches off the supply. The best place for an RCD is built into the main switchboard or the socket-outlet, as this means that the supply cables are permanently protected. If this is not possible a plug incorporating an RCD, or a plug-in RCD adaptor, can also provide additional safety. RCDs for protecting people have a rated tripping current (sensitivity) of not more than 30 milliamps (mA). Remember: ■ an RCD is a valuable safety device, never bypass it; ■ if the RCD trips, it is a sign there is a fault. Check the system before using it again; ■ if the RCD trips frequently and no fault can be found in the system, consult the manufacturer of the RCD; ■ the RCD has a test button to check that its mechanism is free and functioning. Use this regularly. Carry out preventative maintenance All electrical equipment and installations should be maintained to prevent danger. It is strongly recommended that this includes an appropriate system of visual inspection and, where necessary, testing. By concentrating on a simple, inexpensive system of looking for visible signs of damage or faults, most of the electrical risks can be controlled. This will need to be backed up by testing as necessary. It is recommended that fixed installations are inspected and tested periodically by a competent person. The frequency of inspections and any necessary testing will depend on the type of equipment, how often it is used, and the environment in which it is used. Records of the results of inspection and testing can be useful in assessing the effectiveness of the system. More detailed guidance is available in the booklets listed on pages 6 - 8. Equipment users can help by reporting any damage or defects they find. Work safely Make sure that people who are working with electricity are competent to do the job. Even simple tasks such as wiring a plug can lead to danger - ensure that people know what they are doing before they start. Check that: ■ suspect or faulty equipment is taken out of use, labelled ‘DO NOT USE’ and kept secure until examined by a competent person; ■ where possible, tools and power socket-outlets are switched off before plugging in or unplugging; ■ equipment is switched off and/or unplugged before cleaning or making adjustments. More complicated tasks, such as equipment repairs or alterations to an electrical installation, should only be tackled by people with a knowledge of the risks and the precautions needed. You must not allow work on or near exposed live parts of equipment unless it is absolutely unavoidable and suitable precautions have been taken to prevent injury, both to the workers and to anyone else who may be in the area. Underground power cables Always assume cables will be present when digging in the street, pavement or near buildings. Use up-to-date service plans, cable avoidance tools and safe digging practice to avoid danger. Service plans should be available from regional electricity companies, local authorities, highways authorities, etc. Overhead power lines When working near overhead lines, it may be possible to have them switched off if the owners are given enough notice. If this cannot be done, consult the owners about the safe working distance from the cables. Remember that electricity can flash over from overhead lines even though plant and equipment do not touch them. Over half of the fatal electrical accidents each year are caused by contact with overhead lines. More detailed guidance on avoidance of danger from overhead electric lines is available from HSE. Electrified railways and tramways If working near electrified railways or tramways, consult the line or track operating company. Remember that some railways and tramways use electrified rails rather than overhead cables. HSE GUIDANCE ON ELECTRICAL SAFETY The following publications contain advice on the safe use of electricity for particular industries or in high risk circumstances. Risk assessment and general health and safety 5 steps to risk assessment INDG163(rev1) HSE Books 1998 (single copies free or priced packs of 10 ISBN 0 7176 1565 0) Essentials of health and safety at work HSE Books 1994 ISBN 0 7176 0716 X Maintenance of portable electrical equipment Maintaining portable and transportable electrical equipment HSG107 (Second edition) HSE Books 2004 ISBN 0 7176 2805 1 Maintaining portable electrical equipment in offices and other low-risk environments INDG236 HSE Books 1996 (single copies free or priced packs of 10 ISBN 0 7176 1272 4) Maintaining portable electrical equipment in hotels and tourist accommodation INDG237 HSE Books 1996 (single copies free or priced packs of 10 ISBN 0 7176 1273 2) General electrical guidance Avoiding danger from underground services HSG47 (Second edition) HSE Books 2000 ISBN 0 7176 1774

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Some useful articles for the electrical industry.

AC / DC cable resistance.   A.C. resistance If a conductor is carrying high alternating currents, the distribution of current is not evenly disposed throughout the cross-section of the conductor. This is due to two independent effects known as the 'skin effect' and the 'proximity effect'. If the conductor is considered to be composed of a large number of concentric circular elements, those at the centre of the conductor will be enveloped by a greater magnetic flux than those on the outside. Consequently the self-induced back e.m.f, will be greater towards the centre of the conductor, thus causing the current density to be less at the centre than at the conductor surface. This extra concentration at the surface is the skin effect and it results in an increase in the effective resistance of the conductor. The magnitude of the skin effect is influenced by the frequency, the size of the conductor, the amount of current flowing and the diameter of the conductor. The proximity effect also increases the effective resistance and is associated with the magnetic fields of two conductors which are close together. If each carries a current in the same direction, the halves of the conductors in close proximity are cut by more magnetic flux than the remote halves. Consequently, the current distribution is not even throughout the cross-section, a greater proportion being carried by the remote halves. If the currents are in opposite directions the halves in closer proximity carry the greater density of current. In both cases the overall effect results in an increase in the effective resistance of the conductor. The proximity effect decreases with increase in spacing between cables. Mathematical treatment of these effects is complicated because of the large number of possible variations. Skin and proximity effects may be ignored with small conductors carrying low currents. They become increasingly significant with larger conductors and it is often desirable for technical and economic reasons to design the conductors to minimise them. D.C. resistance Factors affecting d.c. conductor resistance in terms of material resistivity and purity are discussed elswhere The latter are associated with the fact that the prime path of the current is a helical one following the individual wires in the conductor. Hence if an attempt is made to calculate the resistance of a length of stranded conductor a factor must be applied to cater for the linear length of wire in the conductor to allow for extra length caused by the stranding effect. In a multicore cable an additional factor must be applied to allow for the additional length due to the lay of the cores. The d.c. resistance is also dependent on temperature as given by Rt --- R20[l + a20(t - 20)] where Rt : conductor resistance at t°C (Ω) R20 = conductor resistance at 20°C (Ω) a20 = temperature coefficient of resistance of the conductor material at 20°C t = conductor temperature (°C)

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Some useful articles for the electrical industry.

CORROSION CELL Corrosion always develops at the anode, where current leaves the metal and enters the electrolyte, whilst a protective effect occurs at the cathode. Thus if the whole metal surface is made sufficiently cathodic, corrosion will not occur. This is the basic principle of Cathodic Protection. In marine structures, such corrosion cells may result from the use of dissimilar metals. Usually,however, localised anodic and cathodic areas arise on the surface of the same metal through differences in the metal itself, variations in protective films or changes in the electrolyte. ie: aeration, temperature and salinity. Corrosion may be prevented by removing one or more of these corrosive elements and for marine structures, the most practicable method is to apply a protective coating, thus introducing an electrical resistance between the metal and the electrolyte. Paint in various forms normally provides the first level of protection. However, even the most efficient coatings are subject to defects during application or service, with inevitable corrosion of the exposed metal.It is therefore generally accepted that cathodic protection, in conjunction with a high performance paint system provides the most effective and economic safeguard against corrosion on larger vessels or platforms.

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Some useful articles for the electrical industry.

Earth Leakage detectors Most of the newer earth leakage monitors for 24v DC are solid state reference type, the older two lamps in series with a resistor down to the hull are known to impress a small current on the hull and can in some circumstances actually promote corrosion. The modern unit functions by measuring the current flow to earth through a potential divider and comparing it to a fixed reference using two amplifiers. If the current signal voltage falls below or rises above the reference voltage, one of two amplifiers switches from high to low, which triggers a RC timer. Once the timing period (pre-set) has elapsed, the normally energised PCB mounted relay de-energises and the voltfree contacts connected to the eleven pin socket change state. Reverse polarity protection is provided using a number of high voltage diodes. These leakage detectors are vital on the 24 - 36 volt DC circuits on a vessel as DC is much more corrosive than AC.

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Some useful articles for the electrical industry.

Galvanic isolator.   Why do I need one? Economical Corrosion prevention Protect against stray currents Peace of mind Why now? No one really bothered before! When two or more boats sit together in the water (or one boat and one jetty!) There is a tendency for a small electrical current to flow between the metal components of the two hulls. This occurs when dissimilar metals i.e. skin fittings, propellers, shafts etc are in close proximity to one another connected effectively by the water: This in itself does not create a problem as the current drawn is usually very small: The amount of current flowing is dependent on the type of metals, the area of the metals, the proximity of the hulls and finally the composition of the water, i.e. the salt content or metallic content of the water. The action of this small current flow creates a small problem! As all metals have different rates of corrosion a metal at one end of the Galvanic (corrosion) Scale will dissolve faster than one at the other end of the scale. If for example we have a brass skin fitting on one Hull and a stainless propeller shaft on the adjacent boat, the brass fitting will undoubtedly disappear before the prop-shaft! As a second example, a greater problem may exist with a large metal boat moored alongside a small cruiser with elderly skin fittings: It takes no imagination to see who wins that battle! Most boat owners are familiar with “Anodes” (or ”Sacrificial Anodes” to be technical). These are large lumps of metal usually zinc/magnesium etc, at the far end of the Galvanic Scale, clamped to the underwater hull and designed to erode away in preference to your more valuable underwater skin fittings. These anodes are an essential protection to corrosion and should be checked regularly for deterioration: Once they are gone so is your protection! In reality it may take years to have any major deterioration. Why: When we connect to shore power (mains power) we connect all our boats together via the earth (green) cable in the shore power leads. This earth cable is essential for our safety and also ensures correct operation of the shore power and vessels fuses and electrical trips. It is vital that this earth connection remains in circuit at all times (not if the vessel has its own shore power isolation transformer). Removal can be fatal! In the event of a major electrical defect, lack of proper earth connections can be lethal to not only yourselves but to your immediate neighbours. Unfortunately it becomes obvious that the earth cable now present between adjacent boats makes an excellent conductor between them, thus allowing the easy passage of electrical current twixt the vessels. This in turn increases the rate of deterioration of fittings. Thus we have the problem! We have created a giant battery! The rate of erosion is affected by several factors: The amount of salt or other minerals in the water The areas of metal involved The types of metals involved The proximity of the vessels Construction material of the jetties The temperature of the water. Condition of electrical installation on adjoining vessels. It is not unknown in extreme conditions for skin fittings to deteriorate within a few weeks through Galvanic corrosion. Although this rate of loss is rare, it is obvious there is a problem in need of redress. If your vessel is under 20 mtrs and fed from a 62 amp or less circuit breaker/rcd then you probably will not have a shore power isolation transformer and fitting back to back diodes in the earth lead is a good idea. However if you vessel is larger and almost certainly requiring a 3 phase supply you should have a shore power isolation transformer configured as delta / star with the centre tap of the star bonded to the earth rail / hull (if metal). There is no point in carrying the earth from the shore power supply as the earth loop is broken via the transformer. If you use a shore power inverter, consult with the manufacturer to determine if they have isolated the supply via a transformer before they start the inverter. In my experience as a Marine electrical engineer the best solution is an isolating transformer. Lightning or large power surges can blow the diodes and then you have no safety earth....

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