Interconnecting Transformer/DG Neutral Earthing with Main Earth Grid

Hi,

I have following questions regarding earthing system.

1. Should we interconnect transformer/DG neutral earthing with main earth grid in project premises or not?

The inter connection is to be done below ground level.

Above ground level separate neutral earthing strips are provided to neutrals & electronic items.

Please note/consider following

1. 3150KVA 11KV /0.433KV - DYn11 TR - solidly earthed neutral

2. 1500KVA DG with solidly earthed neutral

3. TN-S earthing system is implimented

4. All the equipment body, machinery, lightning protection, electronics earthing is connected to main earth grid.

5. 11 KV system earthing is separate grid.

6. System Resistance is <1 Ohm

Following are the reference from various standards

IS 3043, page 12, Figure 3 shows neutral earthing connected with body earthing,

IEC 61000 confirms connection of power, electronics, lightning protection earthing together.

IE Rule 61 (Now CEA Safety Regulations) says Neutral conductor of a 3 phase, 4 wire system and the middle conductor of a 2 phase, 3-wire system shall be earthed by not less than two separate and distinct connections with a minimum of two different earth electrodes or such large

number as may be necessary to bring the earth resistance to a satisfactory value both at the generating station and at the sub-station. The earth electrodes so provided, may be inter-connected to reduce earth resistance.

IE Rule 61 IMPORTANT [[[[[ It may also be earthed at one or more points along the distribution system or service line in addition to any connection with earth which may be at the consumer' s premises.]]]]]

Above rule is applicable to LV distribution system as well. So, if From electricity board transfomer earthing is provided and that earthing is connected with earthing pit/bus in consumer premises than it is as good as connecting TR neutral with main earth grid in single project premises.

Cl. 9.10.12.2 of IEEE 1100, "the use of any separate, isolated, insulated, dedicated, clean, quite, signal, computer, electronic or other such improper form of earth grounding electrodes for use as a point of connection of the EGC (Equipment Grounding Conductor) is not recommended".

Cl. 5.5.4.5 of IEEE 142, 'The NEC requires that all equipment served from an electrical source be grounded or bonded to the grounding point of that source.

BUT THE PROBLEM IS - It is general practice to keep neutral earth pits separate from body earth pits and not to connect these neutral earth pits with main earth grid (below ground).

I am not sure why this practice is followed as it may have implications.

For example In TN-S system when there is earth fault in any equipment current returns via body > earthing conductor > earthing grid > neutral. The heavy current passing during this trips the circuit breaker. Also, the potential of the equipment body with reference to ground (touch potential) stays within limit (say less than 50 V).

Now if there is no connection between main earth grid and transformer neutral than return current has to pass through earth. If the earth resistivity is higher, resistance of return path will increase, which may give higher touch potential. Also, connecting equipment body with neutral is the fundamental requirement of TNS system which also won't be fulfilled.

Please provide your suggestions/inputs/justifications/logic (with examples/calculations if possible).

Thank you in advance.

Jatin