Relation of Earth Conductor Length With Fault Capacity

British Standard 7671, preferably before someone gets hurt/killed, please.

The fault current won't pass down the strip to the earth nest, it will go pass through the N→E link at the transformer. (Assuming one is fitted).

"my query is does earth strip length affect fault current capacity."

Yes it does. All electrical conductors have an impedance-per-foot/meter factor that must be taken into consideration when designing a ground/earth grid for a substation. If the conductor is not sized properly it can be damaged and/or burned into by excessively high fault current.

"Does transformer kept at ground level and at 7th floor will have same strip size for neutral and body"

I am not clear on what you are asking. What is "body"? Are you talking about the transformer frame and enclosure? Or?

You must refer to and adhere to the applicable local code and design requirements for the application.

Sir, If by calculation transformer frame earth and neutral earth strip size at ground level coming to 300 sq.mm ( transformer installed near Earth pit at 3 meters) , then same strip size can be used for same capacity transformer which has installed at 50 meters from earth pits.

The impedance of earth conductor and earth rod should be minimum

Sure, and its CSA and cost should be maximum.

Not having knowledge of or a copy of the applicable electrical code for your area I cannot accurately answer the question.

However, my advice is:

The minimum conductor size would be 300 sq. mm however you must factor in the impedance presented to the circuit by the length, type of metal, ambient temperatures, and installation method of the conductors.

1) Determine/calculate the maximum earth/ground fault current available in the circuit.

2) Consult the applicable conductor amperage table to identify the correct size of conductor/bus required to safely handle the maximum available fault current.

3) Consult the applicable conductor size and type properties table to identify "impedance-per-meter" values.

3) Multiply the "impedance-per-meter" value by the total length of the conductor/bus/strip to determine total impedance presented to the circuit.

4) Based on the results of the above steps apply OHMS Law to determine if the impedance presented by the conductor length will cause enough of a voltage drop to limit the amount of current flow through the conductor during a fault condition.

5) If the impedance presented by the length of the conductor is high enough to negatively affect the fault current capacity of the conductor to the degree the system voltage does not meet equipment protection requirements then you must increase the conductor by one (1) size larger to ensure there will be no damage to the conductor during fault conditions.

6) Note: You must be sure to identify the required conductor insulation by identifying the installation method and all site conditions that affect the current carrying capacity of the installed conductors. (Temperature, humidity, Installed in free-air, enclosed bus duct, conduit, ?)

If this does not make sense then I strongly recommend you hire an outside consultant to perform the calculations and provide their recommendations.

Length matters.

In your calculations and estimates "length" should appear frequently as a significant result influencing variable. If not then you are probably not on the right track or are using the wrong software tool or staring at a table on the wrong page in the standards.

A ground fault on the primary (MV?) side has different consequences to a ground fault on the secondary LV side.

Then there's lightning and structure and quiet earths as well.

That's a start for you.

The LV side of the transformer should be firmly earthed local to the transformer. Any earth fault will take the route of lowest resistance.

If you have the correct bonding in place for circuits fed from the 7^th floor transformer almost no current will pass through the down conductor. It will go direct to source and not meander up and down the building.

How about the MV to the transformer?

An earth fault on the primary needs to be considered too.

Echo what you said about LV faults.

I agree with PWSlack: for low-voltage see BS7671 clause 54. However, for medium voltage BS7430 it would be suitable more.

NEC allow only copper and aluminum for electrode-or equipment- grounding conductor. No length limit is recommended. The cross-section corresponds to live conductor cross-section. See, for instance art.250-66.

See also The Indian CODE OF PRACTICE FOR EARTHING IS-30043/2006? SECTION 2 CONNECTIONS TO EARTH.

A suitable earth conductor attached to the building structure should be at the 1st floor and the ground grid outside the building. A suitable conductor from the ground of the transformer attached to the building steel at the 7th floor will be more than sufficient.

Could be a concrete structure....

Also, a steel building frame is not the place to connect a PE.

Any grounding of a steel structure is not for the convenience of reticulating a convenient electrical path to ground up and through the building. The frame grounding is for protecting the frame and those who may come in contact with it.

PE must be connected to a ground point that was designed and installed for that purpose.