1. HV systems
HV system neutrals shall be earthed at each source of supply. For grid infeed system voltages above 36 kV, the neutral point of transformers should be solidly earthed, unless otherwise required by the public utility.Transformer feeders to HV switch boards with a system voltage not exceeding 36 kV shall be resistance earthed. The rating of each resistor shall be such as to limit the earth fault current to a magnitude approximately equal to the rated full load current of the supply equipment (generator or transformer). When multiple generators are operated in parallel, only one earthing resistor may be put in line to avoid circulating current due to zero sequence triplen harmonics. If a generator with an earthing resistor trips in such a system, an alarm should be generated for alerting the operators to switch on the earthing resistor of another generator manually. The earthing resistor shall be rated to withstand the earth fault current for a duration of not less than 10 s.
3. UPS systems
AC UPS systems shall have their neutrals solidly earthed. This applies equally to single and three phase systems. The inverter neutral shall be connected to the neutral of the bypass mains neutral, which is solidly earthed. If isolating transformers are required in the bypass supply, then inverter neutrals and transformer secondary neutrals shall be solidly earthed to the instrument ‘clean earth’ system. DC systems supplying instrumentation loads and switchgear control and protection loads shall be earthed through a high resistance earth fault monitoring circuit with a sensitivity of 5 mA. DC supplies for telephone systems shall be solidly earthed at the positive pole in line with normal telecommunication practice.