Risk Assessment Application to Electrical Rigs A guide to assist the Competent Named person who will verify the safety of rigs consisting mainly of electrical equipment. 1 Introduction Every employer shall make a suitable and sufficient assessment of the risks to the health and safety of his employees to which they are exposed whilst they are at work; and the risks to the health and safety of persons not in his employment arising out of or in connection with the conduct by him of his undertaking for the purpose of identifying the measures he needs to take to comply with the requirements and prohibitions imposed upon him by or under the relevant statutory provisions. In the case of electrical rigs, the most important statutory provision is the Electricity at Work Regulations, 1989 (summary attached). te that a regulation which omits the phrase so far as is reasonably practicable or similar, is an absolute requirement, regardless of cost. The absolute duties are those dealing with: Equipment provided for the protection of persons at work on or near electrical equipment The strength and capability of electrical equipment Earthing or other suitable precautions Integrity of referenced conductors Connections Means of protecting from excess current Means for cutting off the supply and isolation Precautions for work on equipment made dead Work on or near live conductors Working space, access and lighting Persons to be competent to prevent danger and injury Regulation 12 (means for cutting off the supply and isolation) is amplified by the Provision and Use of Work Equipment Regulations, so that the means are not adequate unless they are readily identifiable by anyone who may need to use them, and are easily accessible. 2 Hazards From Electricity The principle risks of injury are from: Electric shock Electric burn Electrical explosion or arcing Fire or explosion initiated by electrical energy Consequent injuries, such as flash burns, fractures, falls from height, etc The outcome from direct contact with electricity varies from nothing more than a jolt to death, depending on circumstances, such as the path that the electricity takes, the duration of exposure, the voltage, the current, the physical condition of the casualty and the location of the casualty. Shock can cause muscular contractions, respiratory failure, fibrillation of the heart, cardiac arrest or injury from internal burns. voltage limits are set in the Electricity at Work Regulations; a current of only a few ma can cause death, and there is no lower limit on voltage which can be said to be safe. In 1998, a welder was electrocuted by his welding set (open circuit voltage approx 80 V).
Other electrically induced injuries arise from being too close to an electrical fault. People have been killed from being too close to switchgear that has exploded under fault conditions. Being too close to a flashover, for instance on a three phase main, or from shorting the terminals of a car battery can cause very serious burns to the skin and eyes. A shorted car battery can explode, showering everyone with hot sulphuric acid. Burns can also arise from exposure to high frequency radiation, such as microwave, RF. In these cases, it is possible for the victim to be unaware that they are being harmed until significant damage has been done. Electrical equipment can give rise to explosion in atmospheres containing dust or flammable vapours or gases. For these applications, specially designed equipment is generally required. Electrical equipment is commonly the cause of fire due to fault conditions, overload or overheating. Something as simple as an electric light bulb can ignite paper. The criterion applied by the regulations is Is there DANGER? 3 Equipment Design Further details may be obtained from BS EN 60204 part 1: 1998. t all the permutations that are permissible can be presented here. Building equipment to comply with a British Standard is a powerful means of demonstrating due diligence. Equipment should be designed so that it is safe. Protection against direct contact with live parts should wherever possible be achieved by Insulation Providing barriers or enclosures around live parts Providing obstacles in the approach to live parts Placing live parts out of reach Protection against indirect contact (which is contact with parts that have become live under fault conditions) can be achieved by: Automatic disconnection of supply and equipotential bonding Use of double insulated equipment or by equivalent insulation such as reinforced or allinsulated equipment; Use of a non-conducting location Use of earth-free equipotential bonding, or Use of electrical separation. For most equipment, the solution will lie with the first option the Standards should be consulted if one of the other options is to be used. Incoming supply terminals: should be insulated, or shrouded so that they are not accessible when any access door, cover or panel is open. Alternatively, the door should be interlocked with the incoming supply to ensure that it is dead before opening. Adjusting and Setting Devices: should be segregated into a separate panel or cubicle where there are no exposed bare live conductors. Live conductors: should be either insulated and protected against mechanical damage or placed and safeguarded, for example inside an earthed metal enclosure. This is to ensure that persons do not have access to them, when they are live or energised. Where necessary to prevent danger, the
access door should be interlocked with the supply so that conductors are isolated and earthed before access is permitted. Capacitor banks: should be contained in sheeted enclosures with a power interlocked access door so that access is only permitted after they have discharged. The reliance on the bleed resistors inbuilt to capacitors for the purpose of dumping the charge is not recommended, since they are known to fail to danger. Discharge energies in excess of 50 or 60 µc (microcoulombs) should be regarded as hazardous to life. Any exposed conductive part should be discharged to 60 V or less within 5 seconds of the disconnection of the supply. If this would interfere with the functioning of the equipment, and is not feasible, a permanent, durable warning sign is mandatory. Enclosures: A minimum degree of protection of IP21 should be achieved. If the top surface is accessible, the minimum degree of protection against direct contact shall be IP4X, and live parts on the inside of doors shall be protected to at least IP1X. In general, it should not be possible to gain access to the inside of the enclosure, if it contains conductors which present a danger. Therefore enclosures are often interlocked with the supply, or are locked or require tools to remove the covers. Fuses: should be selected to be the minimum rating (but having taken into account the likelihood of them failing on, for example, the surge current on starting up a motor) Equipotential Bonding: the protective bonding circuit (as opposed to the functional earth) consists of the protective earth terminal, the conductive structural parts of the electrical equipment and the protective conductors in the equipment. It is common practice for the main protective earth to be solidly bonded to the chassis by a nut and screw not less than 4 mm, with a star blade connector to earth removeable panels. This main protective earth screw must not be used for other purposes, such as a fixing for, say, a transformer. All parts shall be constructed to withstand the worst thermal and mechanical stress that could be caused by earth fault conditions. Copper conductors shall be used, and not metal conduit or cable sheath (although they may be connected to the bonding circuit). links or fuses shall be permitted in the protective conductors, except for links to be opened only by skilled persons for certain test or measurement purposes (and the links found to the removeable panels as mentioned above). Where electrical equipment is mounted on lids, doors or cover plates, the earth shall not rely on the fastenings or hinges. Wiring: All connections, especially those of the protective bonding circuit, shall be secured against accidental loosening. Connecting two or more conductors to a terminal is permitted if the terminal is designed for the purpose (but not for protective bonding connections). Soldered connections are only permitted where terminals are provided that are suitable for soldering. Mains parts must be wired to the standard colours (brown/blue). Plugs and Sockets: The male side of the plug shall be connected to the load side of the circuit. The plug/socket combination shall be designed so that the earth connection is made first and broken last. Where more than one plug/socket combination is used, they shall be clearly identified. It is recommended that mechanical coding is used to avoid inadvertent connection of the wrong plug to a socket. Connectors normally associated with mains parts (i.e. IEC 603, BS 1363) must ONLY be used for mains purposes. High voltage connectors must be rated to the maximum voltage that could be present. Warning Signs: enclosures containing electrical equipment should have the black and yellow lightning hazard warning sign. Circuit & Functional Diagrams: There should be a circuit diagram, with a parts list. There should also be a functional diagram. These two documents will enable repair and maintenance, or
modifications to be made at a later date. Any alterations made to the wiring should be recorded. These documents should be stored with the risk assessment. Testing: when the equipment is built, the following tests are required Continuity of the protective bonding circuit visual inspection, and measure p.d. between points in the circuit and the protective earth terminal, while carrying a current of 10 A at 50 Hz.. Insulation resistance tests at 500 V dc the resistance between power circuit conductors and protective bonding circuit shall be not less than 1 MΩ. Voltage tests the equipment shall withstand a test voltage - the greater of 2x the supply or 1 kv, 50 Hz applied for 1 s. Components not rated to withstand this shall be disconnected during test. Protection against residual voltages test for compliance with discharging arrangements for capacitors. Functional tests Electromagnetic tests (requirement depends on the working environment of the equipment) 4 Integration of Equipment Into a System While the individual pieces of the system may each have been designed and built to good engineering standards, and may all pass their inspection and PAT test, the integration of these into a whole system still requires risk assessment. The points which should be covered include, at the very least, the following, where necessary to prevent danger: Add up the load that the equipment will place on the incoming supply check that it does not exceed the rating of the cable/fuse. Use a 15A cable for the incoming supply to the socket block. te that, on the whole, you should avoid feeding power to a rig from more than one source if it can be avoided, since it is possible to switch off one source in an emergency while leaving other equipment live by accident. Consider routing the incoming mains via a double pole switch and fuse at the rack. This could enable very rapid switching off. It is much easier to design a rack power distribution system at the outset than to fit it retrospectively. It is laboratory policy not to daisy-chain socket blocks. Check the route of the incoming supply line is it liable to cause a tripping hazard, or is it likely to be damaged by such things as liquid nitrogen spills, or abrasion? te that it is lab policy to keep socket blocks off the floor in laboratory areas. Check the route of the incoming supply line a second time is there a possibility of flooding giving rise to an electrocution risk? Check the integrity of earthing. If no earth is used, then the safety must be derived by, for instance, running the equipment from an isolating transformer Check the layout of the electrical equipment in the rig is it likely to lead to overheating? Check that it is obvious (e.g. by labelling) how to cut off the supply to the rig in an emergency for instance, could a first aider quickly disconnect the equipment, and know that they had operated the correct switch? Cutting off the rig may also include the shutting down of gas supplies, water, cryogens, etc. Check that the means to shut off the equipment is readily accessible at all times to anyone who might need to use it (taking into account variations in stature) Check that there is a means to isolate the equipment (as opposed to just switching it off) which can be made secure. Adequate means might include being able to pull out a plug, and
securing it so that no one accidentally plugs it back in (e.g. by cutting off the plug, or tying it to the rig and marking it with a warning notice), or being able to padlock a switch in the off position. For any apparatus associated with the rig, and fed from it: Check that there are no live exposed conductors Rigs and other conductive work spaces may have to be earthed separately where necessary to prevent danger. Such earth wiring must be mechanically sound and must not be easily removable (i.e. it should require a tool to remove it). Check the positioning of electrical items, in relation to water pipes, liquid nitrogen, etc Check that any high voltage conductors need a tool to remove them, or are interlocked with the power supply, or are of such design that the user is protected from access to the high voltage even when disconnected. This assessment should be carried out in writing (suggested form attached), and the assessment discussed with the users of the rig. Risk assessment must not be left to students to undertake, but they should be involved in the process they need to understand the basic principles of the process, and they need to understand the assumptions (if any) that led to decisions that were made. They need to understand that if they have reason to believe that the assumptions are wrong they should bring them to your attention. They should understand that they have a duty to report faults, and they should be shown how to undertake visual inspection and how to identify hazards. 5 Other hazards You need to consider other hazards, where they are relevant. Examples might include: Damp or corrosive environment Working at height Environment where cables might be damaged (e.g. workshops) Live working requirements (in exceptional circumstances) Very high voltages Equipment capable of delivering very high currents Explosive or flammable vapours or dusts The risks must be controlled to high standards, and you may need to look for additional written guidance in any or all of these circumstances. Some of them may call for the use of RCDs or the provision of emergency knock-down switches.
The Cavendish Laboratory Risk Assessment Form for the Design and Construction of Electrical Equipment Description of the item.. Intended Function/Location.. Designed by.. Protection against direct contact Are the incoming supply terminals insulated, shrouded or not accessible when any cover is removed or door opened? Are any adjusting or setting devices segregated into a separate panel or cubicle, where there are no exposed bare conductors? Are all live conductors insulated, protected against mechanical damage or placed inside enclosures, so that access is not gained to them when live or energised? Are any capacitor banks inside sheeted enclosures? Do any capacitor banks discharge to less than 60 V in less than 5 s? If no, then there must be a permanent warning sign in place Does the enclosure have the minimum degree of protection required for the application? Protection against indirect contact Is the equipment fused, with a fuse of the correct type and of the minimum current (having regard to the requirements of the apparatus)? Is the equipment protected by equipotential bonding? Is the equipotential bonding by copper conductor, of rugged construction, without links or fuses? If the equipment is not protected by equipotential bonding, record here the measures taken to protect against indirect contact: Risk Assessment for design and construction of electrical equipment Page 1 of 2 Jane Blunt, July 2000
General Measures Has the designer done a risk assessment to establish that the design does not pose danger under both the normal use, and the reasonably foreseeable misuse, or fault conditions? (attach design calculations where appropriate) Is there a circuit diagram? Is there a functional diagram? (Please store both with this document). Are plugs and sockets provided of such design as to ensure that the wrong connections are not made, if necessary to prevent danger? Are plugs and sockets suited to their purpose? Does the equipment have appropriate warning signs? Is the general construction mechanically sound? Testing Test Pass Fail/ not appropriate (give reasons below) Continuity of protective bonding circuit (record p.d. measured) Insulation resistance Voltage test: At V Items disconnected are recorded below Residual voltages Record discharging arrangements below Functional tests EMC compatibility tests (where appropriate) Space for Observations and Clarification: This equipment appears to be constructed to be fit for purpose. Assessment carried out by Date Risk Assessment for design and construction of electrical equipment Page 2 of 2 Jane Blunt, July 2000
The Cavendish Laboratory Risk Assessment Form for the Integration of Electrical Equipment into an Experimental Rig Location of rig Intended function Add up the power (or current) requirements of the individual items in the rig Is the incoming supply cable of the correct rating and fused correctly? te: recording currents to the nearest 0.5 A is sufficient, and 13 A at mains voltage is equivalent to approx 2.9 kw. Is the rig powered from a single source (preferred)? If not, is it clear which parts are powered from which source(s)? Is the incoming line placed to avoid slips, trips, damage from abrasion, liquid nitrogen, immersion of live parts in water? Has the earth connection been checked to establish its integrity? If safety is to be achieved by some other means record this below. Is the equipment placed in the rig so as to avoid overheating? Have the items of equipment been connected to distribution boards so as to avoid daisy-chaining and the use of adaptor blocks? Is it obvious how to disconnect the apparatus in an emergency (to people other than the users)? If not obvious by position, is there adequate labelling? Is it obvious how to disconnect the apparatus from all other sources of energy or associated hazards (e.g. water, compressed air, cryogens) Is the means to turn off the electrical supply in an emergency readily accessible and free from obstacles? Is there a means to isolate the equipment so that it remains dead? (could be the same as the disconnection if it is powered from a plug/socket arrangement the answer will be yes provided sufficient steps are taken to prevent inadvertent reconnection). Are all live conductors (that might constitute a danger) on any associated apparatus shrouded or insulated to prevent contact? Is any associated apparatus earthed where necessary to prevent danger? Risk Assessment for integration of electrical equipment into an experimental rig Page 1 of 2 Jane Blunt, July 2000
Continued: Are all high voltage connectors (if any) arranged so that contact cannot be made with them while live (e.g. if the connector is removed from the rig while the power supply is on.)? Have any radiation sources (e.g. RF) been assessed quantitatively, suitable shielding measures taken, and the shields tested? (enter details below) Is an RCD required? (in general for locations where water may increase the hazard, or where conductors are easily damaged, such as a workshop floor) Is an emergency red button needed? (e.g. where live electrical work is foreseeable) Have the users of the rig been shown this risk assessment, and that pertaining to the items that constitute the rig? Name the users: Have they been told of the necessity for the conditions described in this assessment (i.e. that all answers remain YES) to be maintained at all times? Have they been told what to do in an emergency? Detail below: Have they been told to report faults and get them fixed? Detail below: Have the users of the rig been warned of the prohibition on working live where there is danger? (danger being defined as a risk of injury) Comments, and any additional risks to be controlled: Risk assessment carried out by Date. Risk Assessment for integration of electrical equipment into an experimental rig Page 2 of 2 Jane Blunt, July 2000