MINES AND QUARRIES

31.1 General — Earthing requirements for

mines and quarries are based on the broad prin- ciple that exposed conductive parts of apparatus should be efficiently connected to earth or other- wise protected by other equally effective means to prevent danger resulting from a rise in poten- tial (above e a r t h ) on these conductive parts.

In some mines and certain quarries (quarries include open cast coal sites), in addition to shock risk, there are also dangers associated with the possible presence of flammable gas and explosive materials. In these cases, separate local earthing may be necessary to avoid incendive sparks caused by static electrical discharge.

31.2 Power S y s t e m Earthing — At most mines

and quarries, the incoming supply is provided by the supply authority who will instal switchgear and metering for their own purpose. It is impor- tant to clearly establish in all cases, the point at which the supply authorities' responsibilities ter- minate and where the consumer's responsibility commence.

If the supply is from a transformer ( o r generator), that is, the property of the supply authority, and is on site, a request should be made for them to facilitate connection of the consumer's earthing system to the neutral or mid- voltage point. In some cases, the supply authority will allow the use of their earth electrode for joint use, in this event the consumer may not have to provide and maintain his own earth electrode. If the supply is from a transformer

that is not the property of the supply authority, or if the consumer generates electricity privately, then the consumer should provide and maintain the earth electrodes that have the neutral or mid- voltage points bonded to them.

If the supply transformer ( o r generator) is distant from the consumer's premises, provision of an earth terminal at the premises should be requested. Where this is possible, the earth termi­ nal should be made available by means of an additional earth conductor in the supply cable or overhead line.

NOTE — The supply cable sheath and armouring may serve the purpose of this earth conductor provi­ ded that they are bonded to the supply source earth, neutral or mid-voltage point and meet the 50 percent conductivity requirement.

If the provision of such an earth terminal is impracticable, then it is imperative that the earth electrodes at the supply source and consumers' premises are maintained such that their resistance to the general mass of earth is as low as possible, for example, less than 2 Ω, and appropriate earth fault protection is provided.

In all cases, the aim should be to maintain earth electrode resistance, as low as is practicable, taking account of the site conditions, for example, soil/rock resistivity. Except, however, for the instance quoted above, the achievement of a low resistance is not so important as adequate bond­ ing of all exposed metallic parts back to the supply source neutral or mid-voltage point earth electrode.

T h e mains supply system neutral or mid- voltage points should be earthed at one point only and in the case of mines, this should be on the surface. T h e connection to earth may either be a solid connection or via an impedance to limit the prospective earth fault current and in the case of impedance earthed systems, suitable earth fault provided, that is, capable of detecting the restricted flow of fault current.

No switch or circuit-breaker or fuse should be placed in any earthing conductor, although an interlocked changeover linking device is allowed in certain cases where two or more earth electro­ des are provided. Such a device would be used to allow periodic testing of an electrode resistance to the general mass of earth.

31.3 Apparatus Earthing at Coal a n d O t h e r M i n e s — Every metallic covering of any cable

should be earthed. This may be considered as forming part of the earthing conductor except in the case of flexible trailing cables where specific earthing conductors may also be required.

Earthing conductors installed for that purpose should have a conductivity throughout ( i n c l u d ­ ing joints) of not less than half that of the conduc­ tor having the greatest current carrying capacity, to which that earth conductor is related and

should have a cross-sectional area of not less than 14 mm2, in the case of flexible cable working at

less than 125 V, the cross-section area need not be greater than 6 mm2; also a flexible cable on

the surface of the mine supplying a load less than 3 kW need not have an earth conductor larger than the power conductors.

Cables incorporating steel tape armour (unless supplementing steel w i r e ) , aluminium armour or copper sheathed (mineral insulated) cables are unsuitable for use below ground. Generally single or double, steel wire armoured cables are used. T h e use of paper-insulated lead covered cable is also discouraged from use below ground owing to the poor mechanical strength of the paper insulat­ ing material.

T h e following are released from the require­ ments to be earthed, when used solely at the surface of the mine:

a) any lamp holder, that is, efficiently protec­ ted by a covering which is insulated or earthed and made of fire resisting material; b) any hand held tool that is double insula­

ted;

c) any portable apparatus working at less than 50 V dc or 30 V ac; and

d) any other non-portable apparatus working at less than 250 V dc or 125 V ac.

In the case of electrical circuits used for con­ trol, interlocking and indicating instruments, the regulations allow one pole of the auxiliary trans­ former secondary winding serving these circuits to be connected to earth as an alternative to mid¬ point earthing.

Where mobile apparatus containing its own source of electricity, for example, mobile genera­ tor sets and diesel-electric vehicles/cranes, is used on the surface, then an exception is required from the present regulations if the requirement to earth these to the main earth electrode is impracticable. However, the bonding together of all exposed metallic parts is required.

New regulations are proposed which, it is hoped, will eliminate this anomally by calling for all parts of such apparatus to be securely bonded together to prevent danger and relex the require­ ment to connect the structure to the main earth system.

Below ground, where self-contained mobile apparatus is used, for example, battery locomo­ tives, these should be operated as totally insulated systems ( t o avoid sparks between metal parts of the a p p a r a t u s ) . Warning systems should be provided to give an indication of leakage to frame.

At places below ground, where flammable gas may occur in quantity to indicate danger (usually deemed to be places where 0·25 percent

flammable gas could be present in the general body of a i r ) , then limitation of the maximum prospective earth fault current is called for on power systems working at voltages between 250 and 1 200 V ( t h e range of voltage normally used for coal winding machinery served by flexible trailing cables). In these cases, the maximum prospective earth fault current should be limited (normally by impedance earthing) to 16 A at voltages between 250 and 650 V and to 2 A at voltages between 650 and 1 200 V. In either case, the switchgear contorlling the circuit should be able to detect and cut-off the supply of electricity with less than one-third of the maximum prospec- tive earth fault current flowing.

NOTE — The ratio between maximum prospective earth fault current and protection settings is known as the 'tripping ratio'. In practice it has been found that in order to take account of voltage depressions occurr¬ ing when a short circuit coincides with an earth fault the tripping ratio should be set to at least 5 : 1. Multi¬ point earthing of a power circuit (sometimes referred to as an 'insulated' or 'free neutral system') is allowed at any place in a mine, including places where flam- mable gas may occur, provided that a transformer is used which has a means to cut off the supply and prevent danger should a breakdown occur between the primary and secondary windings. In these systems the maximum prospective earth fault current does not usually exceed 2 A and switchgear is set to trip at less than one-fifth of this value.

Signalling and telephone circuits may be con- nected to earth where safety is enhanced and the method of connection is approved by the concer- ned authority for that type of apparatus.

31.4 Apparatus Earthing at Miscellaneous M i n e s a n d Quarries — Every earthing conduc-

tor should have an equivalent cross-sectional area of not less than 14 mm2 except this requirement

does not apply to an earthing conductor, that is:

a) the metallic covering of a cable, which should have conductance not less than half that of the largest current carrying capacity conductor in that cable;

b) one of the conductors in a multi-core flexi- ble cable used to supply portable appa- ratus, in which case the earth conductor has to be equal in cross-sectional area to that of the largest current carrying conduc- tor; and

c) a part of an overhead line on the surface which should have a cross-sectional area of not less than 12 mm2.

Every cable at a miscellaneous mine or quarry operating at voltages exceeding 250 V dc or 125 V ac, other than flexible cables and those not required to be covered by insulating material, should be protected throughout by a suitable metallic covering that has to be earthed. Metallic covering is defined in the regulations and it should be noted that this does not include any

metals other t h a n iron or steel, therefore cables with armourings or metallic cover made of soft metals such as aluminium and copper ( M I C C cable) cannot be used on these premises where the voltages exceed 250 V dc or 125 V ac.

Where a cable is provided with a lead sheath, in addition to the required 'metallic' covering, the conductance of the lead sheath may be taken as contributing to that of the metallic covering. For such installations, plumbed joints have to be used where the lead sheath is jointed or termi- nated.

Where flexible cable is used to supply portable apparatus at voltages exceeding 250 V dc or 125 V ac, such cable should be protected by one of the following:

a) A metallic covering (flexible wire armour- i n g ) that encloses all the conductors and having a conductance of not less than half that of the largest current carrying conduc- tor, or where this is impracticable, having a conductance not less than that of a 14 mm2 cross-sectional area copper conductor.

b) A screen of wires to enclose all the conduc- tors (collectively screened type cable) having a conductance not less than that of a 14 mm2 cross-sectional area copper con-

ductor.

c) A screen of wires arranged to individually enclose each conductor (individually screened type c a b l e ) , other than the earth conductor. Cables of this construction for use in quarries have to be approved by HSE. For miscellaneous mines, the screens should each have a conductance of not less than that of 6 mm2 cross-sectional area

copper conductor.

Where flexible cables are used with portable apparatus at quarries and the size of the conductor is such as to make the use of one multicore cable impracticable, single core cables of such construction and bonded in such a manner as HSE may approve, may be used.

32. STREET LIGHTING AND OTHER