Earthing resistance as per IE rule

Why not try to read IE 1956 if that is what it says then your safety engineer is right.

I could not find the relevant clause.

The IE Rule is available on Net from different gov sites, like this one.

Ask your safety engineer to quote the IE Rule Number, which specifies that the earth resistance should be less than 1 Ohm. As far as I know, nowhere in the 143-odd IE rules, anywhere, has it been specified a minimum or maximum value for earth resistance. IE Rules discuss earthing in Rule Nos. 64 & 67 and both Rules do not specify any absolute value for earth resistance. It just says that the earth resistance should be "low enough". But, how much is low enough?

All the same, let me share a secret too with you. Even though the IE Rules do not specify a maximum permissible Earth Resistance Value, the Indian National Electric Code, 1985, does specify a maximum permissible value for equipment earthing - it says that the equipment earthing resistance should be less than 5 Ohms. Your installation is also bound by the National Electric Code.

Now, it is your choice to share this information with yhour safety engineer.

The functional requirement of earthing in electrical installation is the safety of the electrical system and the proper timely operation of the protective devices. The actual value of the earth resistance is insignificant

"It just says that the earth resistance should be "low enough". But, how much is low enough?"

IE rules Specifies:-

"Limit Earth resistance sufficiently low to permit adequate fault current for operation of protective device in time to and to reduce Neutral shifting"

IEEE80 states

" The goals are

a) To provide means to dissipate electric currents in to the earth without exceeding any operating and equipment limits

b) To assure that a person in vicinity of grounded facilities is not exposed to the danger of critical electrical shock"

"14.1 Usual requirements

A good grounding system provides a low resistance to remote earth in order to minimise GPR . For most transmission and other large substations , the ground resistance is usually about 1 Ω or less. In smaller distribution substations ,the usually acceptable range is from 1 Ω to 5 Ω depending on the local conditions"

The NEC does not use the resistance of earth to provide any safety means at all. Safety is provided be the rules of bonding of metal equipment that is likely to be energized during a fault. I wonder what improvement in safety 1 ohm provides over 1.4 - 3.5 ohms?

Hi Guest,

The safety engineers / Electrical Inspectors insist on specific value of earth resistance due to relevant guide lines provided to them for inspection and they do not insist on any arbitrary values. In India I know certain large Government Organisations who have their own safety rules and prescribed mandatory requirements .(An earth resistance of less than 1 Ω is given as mandatory requirement in their standing instructions.) Depending up on the site conditions the Inspector can approve deviations also and may not stick to rule book, but it is the duty of the construction Engineer to convince the Inspector of the technical reason why the resistance cannot be improved beyond certain value at the specific site due to earth conditions .So try to improve the earth value or convince the safety officer.

Dear friend,

it is not mentioned anywhere in IE1956 to maintain earth value below 1 ohm. (as per IE1956 :2000). The values which u mentioned are good enough to ensure the easy flow of leakgae and short circuit current.

Please ask your engineer to mention the cluse of IE 1956. There are 148 pages in IE rule 1956 . as well ask him to provide the revision year of the code.

Below 5 ohm is good enough.....ask him to study the soil conditon of your site.

Regards,

Vir

The necessity of having a certain value for earthing resistance depends on the shortcircuit current.

The contact voltage increases as the short current increases, for a given earthing resistance.

The maximum contact voltage should be arround 50 V if not having other means of electrical protection against electrocution. If there are other means for protection (eg:gloves, boots, insulating barriers etc) the voltage could be higher, like 100-200 V.

There may be cases where you can't lower the earthing resistence and the contact voltage is too high, case in which protection measures should be taken.

In an intrinsically safe system the earth ground must be less than 1 ohm. Ground wire must be 12 GA minimum and should be connected to the star point of your main incoming power supply. If your system is IS (Intrinsically Safe) than you must follow this rule in the USA (ref NEC 500 - 505). In an ATEX or IECEX application the same rule would apply I just can't remember the exact rule. I would consult with your safety engineer to determine which rule to follow.

He is in India and the rules he is referring to are the Indian Electricity Rules, 1956, which are applicable to all electrical installtions in India.

After reviewing the IE rule 1956 refer to section 61.d.ii which states "

Where the middle conductor is earthed by means of a circuit breaker with a

resistance connected in parallel, the resistance shall not exceed 10 ohms and on

the opening of the circuit breaker, immediate steps shall be taken to improve the

insulation of the system, and the circuit-breaker shall be reclosed as soon as

possible." The applications I have done for India in the past followed NEC codes and rules. Granted these were US companies doing a job in India but, they went by NEC rules 9 out 10 times the other time they followed IECEX rules for an LNG sub-station. From what I have found that in IS systems less than 1 ohm rule must be followed in non-IS systems less than 10 ohms for the ground is usually ok.

IE Rule 61 (d) itself is for Direct Current - 3-Wire System. One cannot quote it for AC Systems.

I don't think it is talking about the resistivity of the earth being less than 1 ohm but your ground grid should be less that one ohm.

Quote VI Abraham "

1. The functional requirement of earthing in electrical installation is the safety of the electrical system and the proper timely operation of the protective devices. The actual value of the earth resistance is insignificant

2. A good grounding system provides a low resistance to remote earth in order to minimise GPR . For most transmission and other large substations , the ground resistance is usually about 1 Ω or less. In smaller distribution substations ,the usually acceptable range is from 1 Ω to 5 Ω depending on the local conditions"

VI

This sounds like a contradiction to me.

1.If the function of earthing is safety, then the resistance of earth is very significant.

2. This may be ok for substations but what about inside plant wiring. Suppose you have a 440/277 volt 1000 amp main breaker. If the the is a 1 ohm earth resistance you only get 277 amps in the fault. The NEC does not allow earth to be the main fault return path. Metal object that might get energized must be bonded with a proper sized conductor to provide a low impedance path the the source of the fault. Earth is never considered a an adequate return path for the fault.

Hi wareagle,

The statement I made "The actual value of earth resistance is not insignificant"( it was only with respect to the earth resistance value given in the original thread where the Electrical Inspector did not agree with the value due to clause in IE Rule) , I did not mean correct earth value is not insignificant ,the value of earth is to be decided actually considering the safety of the system by timely operation of the protective relay, as stated by Indian Electricity Rules. I have only quoted from Indian Electricity rules and IEEE clause and I do not think that it is contradictory.

Regards

Let me make it very clear! The question is:" whether the IE Rules stipulate a certain maximum permissible value of earth resistance?"

The following IE Rules mention something about the "value" of earth resistance:

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.]

(4) All earthing systems shall: -

(b) limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting

66 Metal sheathed electric supply lines – Precautions against excess

leakage: -

(1) The following provisions shall apply to electric supply lines (other

than overhead lines) for use at high or extra-high voltage:

(b) The resistance of the earth connection with metallic sheath shall be

kept low enough to permit the controlling circuit breaker or cutout to

operate in the event of any failure of insulation between the metallic

sheath and the conductors

67 Connection with earth: - [(1) All non-current carrying metal parts

associated with HV/EHV installation shall be effectively earthed to a

grounding system or mat which will -

(c) maintain the resistance of earth connection to such a value as to

make operation of the protective device effective.

As can be seen from the above, none of the above rules do mention a

specific value to be maintained for the earth resistance. They just mention

"a satisfactory value", "sufficiently low", "low enough" and "such a

value..." No absolute value is specified anywhere.

So, a straight answer to the question raised is that the IE Rules do not

specify any value – whatsoever – for the earth resistance.

So is the case with the IS, the IEC & the IEEE on Earthing. If need be, I shall

quote from them too.

All other answers regarding the advantages of having 1 Ohm earth

resistance are all "wish lists".

Generally,it is very significant that the earth provides a resistance low enough to activate any earth fault tripping device.If you carry out tests at different areas and you are able to get a resistance value of up to 2.0 ohm,then you hv achieved something good.

From practical experience on earthing protective measures,2.0 ohms has been a good resistance value for fast operation of earthing device.You may also treat the soil and reduce the earth resistance to acceptable value,using charcoal and salt.I Believe that 1.4-3.5 ohmic value is good,but use the appropriate trip device for this value.

Patrick Whowha

VI Abraham

Does the Indian electric code allow earth(dirt or sand) to be used as a return path for a fault with the hopes that there will be enough current to trip a breaker? As I said here the NEC does not allow earth to be used as the only return path. The resistance is too high.

No! IS 3043 - 1987, the Indian Standard Code of Practice for Earthing - in its Clause 0.3 says that "As a matter of fact, the earth now rarely serves as a part of the return circuit but is mainly being used for fixing the voltage of system neutrals".

So, even in India, we do not allow the earth to carry current. We use a sepearte conductor for the purpose as in a TN System - may be TN-C or TN-S.

If my poor failing memory serves me correctly, we were required to have an earth ground of 3 ohms or less checked with a vibraground for comm-electronics stuff, e.g. radars, TACANs ILS, etc. I can't see where one or two ohms would make much diff at a single facility? Unless you are housing livestock, then the building earth ground and electrical system neutral should be separate due to livestock sensitivity of emf which, believe it or not, can affect milk and egg production.

Get the safety dude to get you a definitive reading, or contact the code inspector/enforcer.

ya.... for grid it should be 0.5 Ohm.... for Single earth pit it should be 2 ohms.......