EARTHING OF CONDCTORS FOR SAFE WORKING

33.1 General — This clause deals only with the

broad principles of the earthing of conductors for safety purposes. It is intended to cover the safety earthing of both light and heavy current equip­ ment and is generally applicable to high voltage equipment; however, in some circumstances it may, where required, be applied as an additional safety feature to low voltage equipment. Where applicable, the use of safety earths should be part of overall safe system of work, which will include isolation, locking off, permits to work or similar documents and liaison between parties in control of the supplies and in control of the work. To ensure that a safe system of work is clearly set out, a set of detailed rules and procedures will be necessary in each particular case.

33.2 Safety Earthing — When maintenance or

repair work, etc, is to be undertaken on or near to high voltage apparatus or conductors, precau­ tions in connection with safety earthing should be taken generally as indicated below. All phases or conductors of any apparatus or main to be worked on should be made dead, isolated and earthed and should remain earthed until work is completed. Due regard should be taken of changing conditions during the progress of work which may necessitate revision of earthing arrangements to ensure the continuous of safety measures, for example, if a connection is made to another source of supply, whilst work is in progress, then additional earths would be necessary as work proceeds.

Safety earthing equipment may be available as permanent equipment, such as earthing switches, as part of permanent equipment such as provision for integral earthing of a circuit breaker, or as portable earthing equipment such as portable earthing leads. All such equipment needs to recei­ ve regular maintenance and should be inspected before use.

Wherever possible, initial earthing should be carried out via a circuit-breaker of other suitable fault-rated device.

Earthing leads should, in every case, be of adequate cross-sectional area to carry with safety, during the time of operation of the protective devices, the maximum short-circuit current that may flow under fault conditions. If possible, they should either be flexible, braided or stranded bare copper conductors or aluminium conductors suita­ bly protected against corrosion and mechanical

damage. In no case, even for the earthing of light current equipment (for example, high vol- tage testing equipment), should the cross-sectional area of the earthing lead be less than 6 mm.

It has been found in some cases that a 70 mm2

copper equivalent earthing lead is the largest that can be conveniently handled. In such cases, where a larger size of lead is necessary to carry with safety, the maximum short-circuit current that can occur, it may be necessary to use a number of leads of 70 mm2 or other suitable size in parallel.

Before earthing leads are applied, it should be verified that the circuit is dead and, where appli- cable, a test by means of a suitable type of voltage indicator should be applied ( t h e indicator itself being tested immediately before and after verifi- cation) before applying earth connections.

Earthing leads should first be efficiently bolted or clamped to the permanent earthing system or to a substantial electrode of low resistance. Should no convenient permanent earth electrode be readily available, a substantial copper earth-spike driven well into the ground can be utilized to provide a quick and convenient temporary earth electrode.

Whilst such a spike is not generally adequate as a primary safety earth, it will give a degree of protection against energizing by induction.

Earthing leads should then be securely bolted or clamped to apparatus of conductors to be wor- ked on and these connections should be removed in all cases before the earthing leads are disconnec- ted from the earth electrode or earthing system.

A suitable insulated earthing pole or device should be used to apply earthing leads to appara- tus or conductors on which work is to be under- taken.

Earthing leads should be kept as short as possible and be placed in such a position that they cannot be accidently disconnected or distur- bed whilst work is in progress.

33.3 Precautions Relating to Apparatus and Cables — In the case of switchgear, phases

of the section in which the work is to be done should be short-circuited and earthed to the same earthing system. Self-contained or portable appara- tus is generally available for this purpose. Wher- ever possible, automatic tripping featnres of circuit breakers should be rendered inoperative by being disconnected from the tripping battery before the circuit-breaker is closed and the breaker operating mechanism should be locked in the closed position. With transformers, if there is any possibility of any winding becoming inadvertently live, the terminals of all windings should be earthed so that no danger from shock can occur. When the neu- tral points of several transformers are connected to a common bar, which is then earthed through a resistance of an arc suppression coil, the neutral

point of any transformer that is to be worked on should be disconnected and directly earthed as well as the phase terminals.

When liquid earthing resistors are to be worked on, particularly when they are drained for work inside, the central electrode should be shorted to the tank and not earthed remotely. This is especi- ally important where two liquid resistors are located side-by-side and one remains in commis- sion while the other is opened for maintenance.

When work is to be carried out on equipment that is capable of capacitively storing electrical energy, for example, cables and capacitors, such equipment has to be discharged to earth prior to work commencing. As, in some circumstances, charge can reappear on such apparatus without re- connecting it to a source of supply, it is important work that the equipment should remain earthed whilst is in progress. The cutting of a cable during the course of work may disconnect conductors from safety earths and precautions should be taken to prevent this happening.

33.4 Precautions Relating to Overhead Lines — After a line has been made dead, isolated,

discharged and earthed at all points of supply, a working earth should be securely attached to each phase of the line at the point or points where work is to be carried out.

The provision of a working earth entails a connection to a continuous earth wire or to a temporary earth electrode, the resistance of which need not be low. T h e application of earths to all phase conductors will, in addition to earthing the conductors, apply a short-circuit to all phases.

The connection of the earthing lead to each conductor of the overhead line should be made using a suitable mechanical clamp placed round the conductor by means of an insulated earthing pole which can also be utilized to secure the clamp tight round the line conductor. When it is requi- red to remove the working earth from the line, the mechanical clamp can be unscrewed and re- leased from the conductor by means of this rod. Even when an overhead line is earthed at each point of supply, it is necessary to place a working earth at each and every position where work is being carried out on the line on account of the danger of the line becoming energized by induc- tion from other power lines and to safeguard against the charging of the line by atmospheric disturbances. Where the work entails breaking a conductor, for example, on the jumper at a sec- tioning point, it is necessary to provide a working earth on both sides of the working point.

33.5 Saftey Earthing of Low Voltage Con- ductors — In some circumstances, it may be

necessary to apply safety earthing to low voltage conductors in order to prevent danger. Such cir- cumstances may include, for example, work on capacitors or work on bare overhead crane trolley

wires. Where the earthing of low voltage con­ ductors is adopted, then the general principles set out in 33.2, 33.3 and 33.4 should be applied and due consideration should be taken of fault current levels (which can be as high or higher than on high voltage systems), when the size of earth conductor is chosen.

34. MAINTENANCE OF EARTH ELEC­