SWITCHBOARDS
. 269. Composition of switchboards. Switchboard panels and/or frames shall consist wholly of durable, non-ignitable, non-hygroscopic materials which, unless all live parts of apparatus mounted on the panels or connected thereto are adequately insulated therefrom, shall also be of insulating material of permanently high electric strength and insulation resistance.
270. Location. Every switchboard shall be placed in a position suitable for the safe and effective control of the circuits and apparatus connected thereto and supplied therefrom. Unless special protection against moisture is provided, switchboards shall be placed only in dry, well-ventilated places.
271. Connection oi switches. Every main switch and every switch required under these regulations to control a circuit outgoing from a switch- board shall be so connected at such board that no corresponding circuit cut-outs are alive when the switch is in the off position,
272. Protection from exposed live parts. Unless placed in a switch room or compartment specially arranged for the purpose, every switch- board with exposed live conductors shall be enclosed in a protective case.
DIVISION XI
EARTHING
273. Earthing conditions. No earthing conductor shall be used to carry the normal cmrent of any circuit.
274. Earthing of equipment. All metallic sheaths, coverings, handles, joint boxes, switchgear frames, instrument covers, switch and fuse covers, and boxes and all lamp holders unless efficiently protected by an insulating covering made of fire resisting material, and the frames and bedplates of generators, transformers and motors if the pressure exceeds extra low pressure, shall be protected to guard against dangers of shock or undue rise of potential by being efficiently connected to an effective earthing system at the surface of the mine, or by being connected to an approved
underground earth, or by being protected in such a manner as will ensure at all times the isolation of any defective portion of an installation through the operation of a circuit breaker, or cut-out.
275. Minimum size of earthing conductor. Earthing conductors, excepting overhead lines and metallic coverings of a cable or a flexible metallic covering comprised in a flexible cable, shall have a cross-sectional area of not less than 0 · 022 of a square inch, when used underground and in an open-cut.
276. Conductivity of earthing conductor. All the conductors of an earthing system underground and in an open-cut, shall have a conduc- tivity at all parts and at all joints at least equal to 50 per cent. of the largest conductor used solely to supply the apparatus a part of which it is desired to earth: Providep that no .conductor of an earthing system shaH have a cross-section less than that stated in Regulation 275.
277. Metallic sheath as earthing conductor. Where cables are provided with a metallic covering, such metallic covering may be used as a means of connection to the earthing system: Provided that where a covering is not equal to 50 per cent. of the conductivity of the largest conductors enclosed by the said metallic covering, it shall be augmented by sufficient additional earthing conductors so that the combined conductivity shall be as specified and cross bonding between the metallic covering and the additional earthing conductors shall be provide<J at intervals not exceeding 250 yards.
278. Joints. All joints in and connections to earthing condw;tors shall be made by soldering or by clamping and shall be mechanically secure and electrically efficient. No automatic circuit opening device shall be introduced into an earthing conductor.
279. Connections. All connections of the earthing conductor to the installation and to the earthing system itself shall be in an accessible position, and shall not be exposed to possible mechanical injury.
280. Bonding of earth screens. Where the conductors of trailing cables are provided with individual or collective screens the screens used for earthing shall be bonded at or near each end of the cable.
281. Earth conductors in low current flexible cables, etc. If the cross-sectional area of the largest live conductor of a flexible cable or cord used with a portable appliance is smaller than 0 · 022 square inches, the cross-sectional area of the earthing conductor in the flexible cable or cord shall not be less than, but need not be greater than, that of such live conductor.
282. Earth electrodes. Earth electrodes provided for the purpose of these regulations shall be constructed and maintained so that the resistance between the electrodes and the general mass of earth does not exceed 2 ohms.
283. Circuits to be connected with earth. Electrical systems and circuits shall be connected with earth as follows:-
(a) In a system used for traction and where the running rails are used as a conductor the pole that is connected to th'e running rails shall be earthed;
(b) In the secondary circuit of a current or voltage transformer used for-
(i) Indicating or other instruments; or
Regulations rr. 284-286 97 (ii) Accessories of switch gear (including fixed lights associated
therewith)
the pole that is connected to the metallic structures of the apparatus shall be earthed;
(c) In the secondary .::ircuit of a voltage transformer used for (i) Remote control; or
(ii) Electrical interlock circuits where an earthing conductor is used to complete the circuit
the pole that is connected to the earthing conductor shall be earthed;
(d) In an alternating current system where the voltage exceeds the limits of low pressure except as provided in paragraphs (b) and (c) of this regulation the neutral point shall be earthed;
(e) The neutral point of an insulated direct current system may be connected with earth.
284. Impedance permissible in earthing circuit. It shall be permissible to introduce impedance into the connection between the system and earth subject to compliance with the requirements of these regulations as to the automatic protection of all circuits comprised in the system.
285. Underground circuits to be earthed at surface. Except as otherwise provided in these regulations, each system underground shall be earthed at one point only which shall be on the surface.
DIVISION XII .
HIGH AND EXTRA HIGH PRESSURE INSTALLATIONS
286. Limitation of voltage. (1) Electrical energy shall not be transmitted or used at a voltage exceeding 11,000 volts underground or in an open-cut except With the permission of the Chief Inspector and subject to such conditions as he may require.
(2) Electrical energy shall not be used in any motor having a continuous rating-
(a) Of less than 250 horse-power, at a voltage exceeding 3,300 volts; or
(b) Of less than 50 horse-power, at a voltage exceeding 65 0 volts.
( 3) Excepting transformers comprised in and auxiliary to switch gear, electrical energy shall not be used in any transformer having a continuous rating-
(a) Of less than 4 kilo-volt amperes at a voltage exceeding 3,300 volts; or
(b) Of less than 2 kilo-volt amperes at a voltage exceeding 650 volts:
Provided that in transformers of the compound-filled type installed in permanent positions the corresponding limits shall be-
(c) Not less than 0·75 kilo-volt amperes at a voltage exceeding 3,300 volts; and
(d) Not less than 0 · 3 kilo-volt amperes at a voltage exceeding 650 volts.
287. ~eutral point star winding. The neutral point of the star -vinding of each distinct polyphase system used for high-pressure or extra high-pressure supply may be connected with earth, or may be insulated.
If connected with e3rth through a resistance, that resistance :,hall be surliciently low to ensure that the fuses or automatic circuit breakers in the circuit shall act.
If the neutral point is not connected with earth, a separate electrostatic voltmeter shall be connected permanently between each distinct system and earth; and if the reading of the voltmeter shows that the insulation of any of the circuits of a system is faulty, immediate steps shall be taken to restore insulation.