4.1.11 Electricity at Work

1.Introduction

The Health and Safety at Work Act 1974 to which the University is subject as an employer imposes duties on the University to provide a safe working environment. As far as electrical systems are concerned, the legal requirements are contained in the Electricity at Work Regulations 1989. The precautions contained in the Safety Manual meet these legal requirements but are primarily for guidance in preventing electrical fires and injury to individuals. Nevertheless, if anyone is in any doubt concerning the safety of equipment on which they are working, they should consult their Safety Coordinator or can directly contact theHealth, Safety and Environment Unit. Reference may also be made to the local safety rules and codes held in each Department pertaining to electrical and electronic working in teaching and research laboratories.

UNDER NO CIRCUMSTANCES SHOULD A PERSON USE ELECTRICAL APPARATUS ABOUT WHICH THEY HAVE ANY DOUBTS AS TO ITS SAFETY.

2.Responsibilities

The Bursar and the Estates Department have responsibility for:

  1. The University sub-stations;
  2. Distribution facilities eg isolators, circuit breakers, fuse boxes, socket outlets etc up to the service itself;
  3. All permanent and emergency lighting;
  4. Fire alarm systems;
  5. All plant concerned with building services, both that distributed throughout local sites and that concentrated in plant rooms.

To conform to statute, only one person at a time may take charge of the 11 kV mains and this responsibility is managed by Estates. No work of any kind is performed on these mains without the knowledge and consent of that person in charge, who ensures that a rigid ‘Permit to Work’ system is enforced.
Departments are themselves responsible for the electrical arrangements and equipment fed from the sockets and for departmental equipment fed from fixed outlets.
In student residences, the Bursar is responsible for the fixed installation while the Senior Resident Tutor is responsible for electrical equipment belonging to the University. For practical reasons the University cannot assume total responsibility for the safety of students’ personal electrical equipment. Nevertheless, students should be advised that all such equipment should be in good condition and designed to a recognised specification, such as a British Standard, when it is brought onto University property. If subsequently any equipment is found not to meet these conditions, then it should be removed from use until such time as adequate repairs are effected.

4.Electrical Hazards

Injury to persons - there are several ways in which personal injury may be caused:

  1. Shock. Electric shock is the effect produced on the body, particularly its nervous system, by an electrical current passing through it, and its effect depends on the current strength which in turn depends on the voltage, the path the current takes through the body, the surface resistance of the skin (much reduced when wet) and several other factors. A voltage as low as 15 V can produce discernible shock effects and 70 V has been known to cause death. Generally speaking, however, those fatalities which occur from this cause involve normal domestic and industrial voltages of 240 V ac and above, causing currents of greater than 30 milliamps to flow through the body for longer than 40 milliseconds. The most common cause of death from shock is suffocation and accordingly it is highly desirable that those dealing with electricity should be trained in resuscitation. Minor shocks in themselves may not be serious but they can lead to serious consequences; for example, the associated muscle contraction may lead to falls from working platform or ladders.
  2. Burns. These are caused by the passage of heavy current through the body or by direct contact with an electrically heated surface. They may also be caused by the intense heat generated by arcing from a short circuit. Electrical burns are a very unpleasant form of burn and require immediate attention.
  3. Explosion. The main causes of electrically-induced explosions are listed below:
  1. In situations where flammable gases or vapours are present so that a spark could ignite an event. In such environments all electrical equipment should be flame proof.
  2. Where electric arcing takes place in a confined space causing intense local heating with a consequent bursting of the enclosure by the expansion of trapped air.
  3. Rechargeable batteries emitting hydrogen when being charged, giving rise to an explosive atmosphere. Such operations should therefore be carried out in a well ventilated area, the temperature of which should not exceed 18 degrees C.
  1. Eye injuries. These can be caused by exposure to the strong ultraviolet rays of an electric arc, where the eyes become inflamed and painful after a lapse of several hours, and there may indeed be a temporary loss of sight. Although very painful, the condition usually passes off within 24 hours. Precautions to protect the eyes must always be taken by persons working with or near electric arc welding. Also permanent injury to the eyes can arise from the energy propagated in microwave apparatus: no-one should look along the wave guide when it is in use or examine a highly directional radiator at close quarters. Precautions also need to be taken with lasers to prevent eye injury (see
  1. Body injuries from radio-frequency (rf) energy and induction heaters. The energy in microwave and rf apparatus can damage the body, especially those parts with a low blood supply. Operators should never be exposed to an energy level exceeding 10 milliwatts per sq cm. If personnel have had bones pinned with metal at any time, they must be careful not to expose themselves to these sorts of energy propagation as they may cause substantial internal damage before the operator is aware he is in danger. Similarly, induction heaters can cause rapid heating in any circuit brought within say 1m and consequently personnel using such equipment should remove any metallic objects from their person and those with pinned bones etc should avoid them altogether.
  2. Fires. A large percentage of fires are of an electrical origin, caused by one or more of the following:
  3. Sparks. A spark arises from a sudden discharge through the air between two conductors, or from one conductor to earth. The current produced is usually small so that serious fires are unlikely unless explosive gases or vapours are present, or highly flammable material is in contact with the conductor.
  4. Arcs. An arc is a much larger and brighter discharge where the current flow may be hundreds of amps. It usually arises when a circuit is broken or when a conductor melts or fractures leaving a gap across which current continues to flow. When an arc is established, the air in the vicinity becomes ionised and forms a conductor which may allow current to flow to a nearby metal framework. Any combustible material in the vicinity could therefore lead to a fire.
  5. Short circuits. A short circuit is formed when the current finds a path from the outward conductor wire to the return wire other than through the equipment to which it is connected. The current flow may be large because of the low resistance of the leads, and arcing often occurs at the contact between the conductors. Insulation may therefore be burned and set fire to adjacent flammable material. Batteries have a low internal resistance and can give rise to very large currents under short circuit conditions, causing a large arc from which molten metal may be splashed.
  6. Overloading and old wiring. Wiring must not be overloaded, otherwise it will overheat and the insulation will be damaged. This can lead to a short circuit at some point in the length of the conductor, or more likely at connection points. The insulation of wiring which has been in use for a number of years tends to become brittle and, where alterations and additions are required, the cable must always be checked by a competent electrician and replaced completely if there are indications of failure of the insulation. Installations should be protected against overloading and short circuits by fuses or circuit breakers.

4.Safety Measures

Cables. Cables must be of sufficient size to carry the current which flows through them under normal conditions and must be adequately insulated to allow handling with safety. Under fault conditions, they must be able to withstand excessive currents for the time taken for the supply to be disconnected by a fuse or circuit breaker. Those cables which provide the basic services within a building are normally housed in conduits or troughs but where apparatus is wired up from socket outlets, no such permanent protection is available and hence particular care is required. Such cable must be sufficiently robust to withstand the wear and tear of laboratory use and fully waterproof where water may come within the vicinity of the apparatus.

Fuses. These devices will open a circuit when an excessive current flows.

Circuit breakers. These are a form of switch which open automatically if the current in the controlled circuit becomes excessive. It may operate on either thermal or magnetic principle. It is essential to select the correct size of the fuse or circuit breaker for any particular circuit, especially those in series, so that the correct disconnection is made ie the faulty circuit and only the faulty circuit is isolated and not a whole subgroup of services, whilst they must allow normal operation of the equipment without random tripping.

Residual current device (RCD). These devices should be used in areas of hazard, eg where water is being used near electrical equipment as a back-up protective device. They are sensitive to earth currents and are designed to isolate the supply before the user of an equipment is subject to serious harm. (Plug-in RCD’s must be manufactured to BS7071.)

Isolation. Means must be provided of disconnecting cables or apparatus from the source of supply in an emergency or when maintenance is to be carried out and safeguards instituted to prevent the supply being remade whilst the apparatus is being worked on.
Should it be necessary for an isolating switch to be remote from the apparatus, fuses should be drawn when the switch is in the ‘off’ position or if the switch is lockable, it should be locked off. The key or fuses should then be retained by the operator directly concerned. Appropriate notices should be displayed so that all persons may be aware of the situation.

Earthing. Any conducting part of a system which could conceivably become live, say under fault conditions, and yet to be handled, eg the external metal casing of electrical apparatus, must be earthed as a legal requirement. The reasons for this are:

  1. to prevent the accessible metal parts rising to a dangerous voltage under fault conditions such as a short circuit between the live conductors and casing;
  2. to ensure that a faulty circuit is automatically disconnected from the supply by drawing sufficient current to blow the fuse or operate the circuit breaker or residual current device.

All new Class 1 equipment (ie all equipment designed with an earth) must be tested to ensure that it is in fact properly earthed before putting it into use. Bad joints, rust or paint can cause the resistance to increase to a dangerous level. Where the earth connection of a chassis is made with a nut and bolt, a shakeproof washer should be used adjacent to the chassis to ensure good contact is maintained in use.

Low voltage supplies (<1000V). Portable tools and hand inspection lamps can be a source of danger because:

  1. they are subjected to abnormal wear and tear.
  2. they are liable to be used in conditions where dampness has reduced the body’s resistance.

Where conditions are particularly dangerous, eg when working in interiors of metal enclosures or where water is continuously present, mains voltage equipment should not be used. A double-wound transformer with the secondary centre tapped to earth to give 110 V should be used, thereby ensuring that no part of the equipment will be at a voltage greater than 55 V relative to earth.
The availability of cordless power tools has had a great impact on increasing convenience and reducing the risk of injury. Operating at extra-low voltage the potential for electric shock has been dramatically reduced, although care must be taken with the charging unit. There are no power cords to tangle with, trip over or cut through, and temporary supplies are not required for electrical work. These tools are a highly recommended way of reducing the risk of injury, and consideration should be given to their purchase in place of standard 240 V tools.

Insulation. Double insulation apparatus and tools are now made which require no earthing - such equipment has the ‘double square’ logo ()and is categorised as Class 2 equipment.
‘Double Insulated’ implies the use of two layers of insulation on all live parts, each layer of insulation being adequate to insulate the conductor but together ensure an improbability of danger arising from insulation failure. This arrangement avoids the need for any external metal work to be connected to earth. Such apparatus and tools which are not earthed should be stored in a dry environment and tested to ensure that the insulation is effective.

Space. The circulation space in laboratories and workshops must be kept clear to prevent the risk of tripping and hence accidental contact with live electrical conductors.

Plug connections. In all instances the connection of equipment to the mains must be correctly made by a competent person. Should it be found that the power leads on imported equipment do not conform with the current British Standard (BS) of

brown = live,
blue = neutral
green with yellow = earth,

it is wise to replace the original powerlead with the BS pattern. Remove only the required amount of insulation so that no bare conductors are exposed when the connections are made, and remove any ‘whiskers’ which may be present. Ensure that the flex is firmly held by the cordgrip before the plug top is screwed back into position.

Extension arrangements. If there is a shortage of sockets to supply the increasing number of portable electrical appliances being used, it is permissible to feed one four-way extension block from one power point provided the block feeds only low power equipment (ie less than 500 W or 2 amps current rating). Kettles, microwaves, and heaters etc which consume much greater power must be fed from an installed socket point. Flexible cables should be run in such a way as not to present a tripping hazard. In some instances four-way extensions have overheated and caused fires due to poor connections by the fuse link. Four-way extensions are thus preferred unswitched and unfused, saved for the 13 amp fuse in its plug which provides adequate electrical protection. If a fused four-way extension becomes warm around the fuse link, immediately take out of service and replace.

5.Electrical Testing

The law requires that all electrical equipment and systems are not only designed to be safe to operate but should be maintained in as safe conditions as is reasonably practicable. To ensure the latter a testing is required at regular intervals and records kept of results.

Equipment fitted with 13 A plugs. There are four basic safety tests in addition to regular visual tests by the user:

  1. Visual inspection of mains plugs, cables and earthing arrangements. Fuses in plugs and equipment should be checked to establish that they are correctly rated. If a plug incorporates an RCD, the latter should be functionally tested.
  2. Measurement of insulation resistance between the live parts and earth, using 500 V dc.
  3. Measurement of the earth loop resistance (not applicable for double insulated equipment), using a high current, usually 25 amperes
  4. Flash test (a higher voltage insulation test) for double insulated equipment, using 3kV ac. NB: Do not subject electronic equipment (eg computers) to the flash test.

Portable Appliance Testers (PATs) are available to carry out the checks at 2, 3 and 4 quickly and reliably, and the Health, Safety and Environment Unit holds instruments which can be borrowed for the purpose should Departments have insufficient equipment to justify the purchase of their own. The testing may be carried out in-house if a member of staff is competent to do so, or the testing may be undertaken by Estates or external contractors.

Equipment permanently wired into the supply. The tests and standards are in general the same as above (except no plug is involved) but the use of PATs is not possible and Meggers and earth loop testers will be required instead. As it will be necessary to isolate supplies while the test is being carried out, the involvement or co-operation of the Estates Department will be necessary. Similarly, 415 V equipment, even if fitted with plugs, may require Estates Department involvement as again PATs cannot be used.

Fixed Installations. There are three classes of safety tests, namely: