The ‘Assumed System’ – The analysis of the problem has been carried out for the network wherein the neutral is earthed at distribution transformer end and along the distribution system (at LT feeder pillars A, B as shown below). It is connected to earth at two separate and distinct points. The neutral of a main-line cable is earthed at both ends via LT feeder pillars (A & B), while neutral of a service cable is earthed at one end (via LT feeder pillar B) and not at meter cabin end with due consideration of safety issues. Earthing of neutral (which is normally a current carrying part) and consumer earth wire (which is normally a non-current carrying part) may get electrically connected due to space constraints at consumer end. In real life, due to lack of inspection and poor maintenance, the earthing may get open and likely to remain unnoticed. In case of a phase to neutral cable fault, there is a likelihood of the consumer earthing and thereby consumer house-hold equipments get charged to a voltage unsafe to human life. Hence, it is always preferable not to earth the neutral at consumer (meter cabin) end.

The Single Line diagram of the ‘Assumed System’ is as –

[pic]

I.E. Rule 1956, section 61(a) requires that “Neutral conductor of a 3 phase, 4 wire system shall be earthed by not less than two separate and distinct connections with a minimum of two different earth both at the generating station and at the sub-station (Neutral Earthing at distribution centre). The earth electrodes so provided, may be inter-connected to reduce earth resistance. It may also be earthed at one or more points along the distribution system (LT feeder pillars A, B) or service line in addition to any connection with earth which may be at the consumer’s premises”. Please note, earthing of the neutral conductor at consumer end is a “may be” condition and not a “shall be” condition. Thus the ‘Assumed System’ complies with all the requirements of section 61 of I.E. Rules 1956*. Even if the neutral is earthed at meter cabin end, as explained in the article, it does not stop the occurrence of event rather it just postpones the event.

As per IEC code, the assumed system is of TN-C-S type (shown below) which is followed in some of the foreign countries. [pic] L1, L2, L3 represent the three phases, N represents neutral and PE is protective earth. Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, and separated in the service head. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as multiple earthed neutral (MEN).

When the Neutral is solidly earthed the voltage of the neutral point (at load end) with respect to Earth would likely to remain low (0.5 volts as mentioned by you) and would not raise to a considerable value even the supply side neutral fails. We had used a potential transformer in the circuit and whenever the current flowing from neutral point to earth exceeds a pre-determined value, the static circuit gets charged with a voltage higher enough to operate. The circuit will then actuate the trip signal to MCCB. We can also use a voltage relay, a CT or a low rated MCB (lowest available rating is 500 mA) for sensing the current. Accordingly, rest of the circuit can be modified.

Cost Calculation for Module developed for protection against Over Voltage due to failure of neutral

One module is required per meter cabin (service end). A module consists of MCCB, Shunt coils, static circuit and a cabinet. The prices of different modules mentioned below have been derived as per the retail prices in Mumbai

|Sr. No. |Retail Price of a|Retail Price|Whole-sale Price |Labour, supervision & Other |Final Price |Assumption Consumers per |Cost per consumer |
| |Module | |(30-40% Discount) |Costs (30% on Whole sale price)| |meter cabin | |
|1 |100 Amp Module |7000 |4900-4200 |1470-1260 |6370-5460 |20 Residential |318-273 |
|2 |250 Amp Module |15000 |10500-9000 |3150-2700 |13650-11700 |20 Residential |682-585 |
| | | | | | |OR |OR |
| | | | | | |10 Commercial |1365-1170 |
|3 |400 Amp Module |26000 |18200-15600 |5460-4860 |23660-20280 |40 Residential |592-507 |
| | | | | | |OR |OR |
| | | | | | |10 Commercial |2366-2028 |

The above table shows that average One Time cost for a residential consumer is about Rs. 500/-. The cost will further reduce as main cut-outs or switch fuse units will not be required as MCCB will take care of over-current protection. Additional features like remote tripping in case of emergency like fire, single phase prevention can be added easily as per the requirements of the consumers. The operations permitted by manufacturers on a MCCB are large enough to consider the life of the module more than twenty five years.

*I.E Rule 1956, Section 61

61. Connection with earth. – (1) The following provisions shall apply to the connection with earth of systems at low voltage in cases where the voltage normally exceeds 125 volts and of systems at medium voltage:-

(a) 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. 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.

(b) In the case of a system comprising electric supply lines having concentric cables, the external conductor of such cables shall be earthed by two separate and distinct connections with earth.