Neutral and body earthing of Transformer and DG Part -1
Earthing at Industrial and Commercial Premises
(Two for neutral, Two for body)
Industrial and commercial premises the electrical inspectors after conducting inspection used to make the following comment (or comments with similar meaning) with a reference to regulation 41 of CEA regulation “Measures relating to safety and electric supply 2010“.
-
Transformer bodies shall be double earthed with (SIZE) GI flats and they shall be connected to two separate and independent earth electrodes.
-
Transformer Neutrals shall be double earthed with (SIZE) copper flats and they shall be connected to two separate and independent earth electrodes.
- The lightning arrester shall be independently earthed with (SIZE, copper / GI) flats and they shall be brought down by insulators.
Note: The electrical inspectors rarely refer the above claims with regulation 48 (Connection with earth for apparatus exceeding 650V).
Users are compelled to do as per these comments (by the inspector).
The first part of this blog is to find out the basics of the recommendation by electrical inspector w.r.t regulations and code of practices. This regulation is one of the most mis interpreted regulation and is one of the reasons for electrical accidents in the country.
Analysis of the regulation Part 1
sub regulation 1, 12 and 13 are referred in this part
From the 2010 regulation
i. 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.
Definitions
Sub regulation 1 is applicable to the neutral conductor, 12 about frame of DG and transformer and 13 about neutral of a transformer.
-
Neutral conductor shall be connected to two separate earth electrodes (in soil).
-
Frame of DG and TR shall be earthed by two separate and distinct connections with earth.
-
Neutral of DG and TR shall be earthed by connecting to earthing system (2 connections).
From the regulation and the definitions it is clear that body require two connections with earth and not to earth electrode and Neutral require connection with earthing system and not to earth electrode.
(Note: earthed, connected with earth and earthing system doesn’t mean earth electrode in soil, but by some means to earth)
Background of the regulation (41 – 1, 12 & 13): These regulations are continuation from IE rule 1956 and E rule 1937.
From 61 (a) of IE rule 1956 (neutral conductor): (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.
the owner by two separate and distinct connections with earth.
From of 61 (4) IE rule 1956: All earthing system shall – (a) consist of equipotential bonding conductors capable of carrying the prospective earth fault current and a group of pipe/rod/plate electrodes for dissipating the current to the general mass of earth without exceeding the allowable temperature limits as per relevant Indian Standards in order to maintain all non-current carrying metal works reasonably at earth potential and to avoid dangerous contact potentials being developed on such metal works;
From 1937 electricity rule (rule 58):
58. Connection with earth of a multi-wire system. In every distributing system in which there is a neutral conductor, where the pressure between the neutral conductor and an outer or phase conductor exceeds 125 volts, the neutral conductor shall be connected with earth by two separate and distinct connections from the neutral bus-bar and in accordance with the following provisions, namely:
From the IE rule 1956, it is evident that the neutral conductor is connected to earth electrode, body to be “earthed” by two connections and neutral to be earthed to equipotential bonding conductor with two connections. The E. rule (1937) recommend connecting the neutral conductor at one point only in the bus bar.
Safety regulation (2010) 14, 15 and 16, (1956 IE rule 31, 32 & 33) (E. rule 1937 40 & 51) the recommended either TN-S & TN-C-S system by the energy supplier. How ever the wording used still follow the systems in 1937, as a result the definitions of some nomenclatures used in 2010 regulations do not exist and not practiced in the modern world.
In a TN-C-S system the PEN conductor must be earthed to earth electrode at distribution, as an open circuit PEN can induce full phase voltage on exposed metallic parts connected to the system downstream of the place of open circuit in PEN. In order to avoid such shock from the full phase voltage, the IS3043 clause 4 recommend using PME system at distribution. (Similar to PME system used in UK, multiple earthed neutral (MEN) in Australia and New Zealand and, multi-grounded neutral (MGN) in North America,)
Note: the energy supply companies violate regulations 14 to 16 in India and provide a system earthing of TT, with earthed neutral at distribution which is a nonstandard system earthing.
Coming back to the comments of electrical inspectors asking for each body and neutral of a source (such as DG/TR) to “two separate connections to earth electrode” some time to “copper strip and copper plate for neutral” or to “GI strip and GI pipes for body” does seems to be from the regulation or the IS code of practices.
Following the recommendation of the electrical inspector (two connection to earth electrode) have the following disadvantages.
-
Use of nonstandard system earthing by the industrial and commercial consumer.
-
Nonoperation of protective device during fault which may lead to accidents such as electrocution and fire (fire due to short circuit).
-
Exceeds the power frequency with stand voltages of 3 kV (as stipulated in regulation 91 of Technical Standards for Construction of Electrical Plants and Electric Lines 2010).
-
Unreliable and non standard practices are followed for the electronic system affecting the reliability of electronics.
Investigations in electricity related accidents shows that the recommendations by the electrical inspector (two separate connections to earth electrodes) are carried out in majority of the installations and is the reason for non-operation of protective device during earth fault, resulting in the accident..
(note: Some engineers claim the recommendation of inspector as TT, which is untrue as most industries interconnect these earth electrodes under the soil (similar to an inefficient TN-S system). Protective equipotential bonding is not common in India, and the power frequency stress voltages are not considered)
This recommendation by the electrical inspector is not only violation of the safety regulation, the code of practices and standards, but are the reason for electrical accidents such as electrocution and fire due to short circuit.
Solution
IS3043: Code of practice of earthing fig 38 to 41 explains the correct method of earthing in Low Voltage DG. The below figure is 38 from IS3043, shows earthing of body and neural in a TN-C-S system with DG and manual change over. This is the correct method to be followed.
-
TN-S or TN-C-S to be adopted in an industrial or commercial premise with own transformer and DG
-
Body and Neutral to be connected to MET (or equipotential bonding conductor).
