Earthing Rules
- The following provisions should be made for earthing for system voltage greater than 125 V and medium voltage.
- Neutral conductor of three phase, four wire system and middle conductor of two phases, three wire systems shall be earthed by two separate and distinct connections with minimum two earthing electrodes in order to bring earth resistance to satisfactory low value.
- The earth electrodes may be interconnected in order to reduce earth resistance. It may be also earthed with distribution system or service lines.
- The external conductor of concentric cable should be earthed by two separate and distinct earth connections.
- The connection of earth may be included with link in order to temporary interruption for testing purpose.
- There should not be connected any impedance, cut out or circuit breaker in the alternating earthing system and current passes through earthing system is recorded by supplier. The alternating current system which is connected to earth may be electrically interconnected.
- The frame of every stationary motor, portable motor, metallic parts of all transformers, generator and any controlling or regulating apparatus shall be earthed by the owner by two separate and distinct connections with earth.
- All metal castings / metal covering which protecting any electric line / apparatus shall be connected with earth. When the supply is at low voltage this rule shall not apply to isolated wall tubes / brackets, switches, ceiling fans / other fittings. However this rule is applicable to portable and transportable apparatus, unless provided with earth terminal and class II apparatus / appliances.
- When the supply voltage is at low voltage and when installation is new, all three pin shall be permanently and efficiently earthed.
Earthing in DC System
- The middle conductor of three wire DC supply system is earthed at the generating station only.
- The current in the middle conductor is continuously recorded by ammeter and if current exceed 1/10th of maximum supply current, immediate steps should be taken.
- When middle conductor of three wire DC supply system is earthed by means of circuit breaker with a resistance connected in parallel, the resistance should not exceed 10 ohm and immediate steps should be taken to improve insulation of the system while opening of the circuit breaker.
- The circuit breaker should be closed as soon as possible. The resistance should be used only for protection of ammeter in the case of earth on the system and until such earth connection are removed.
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