Fault Duration For Ground Conductor Sizing

British Standard 7671.

Sorry PWSlack. I was looking for some concrete answer. Please do not give any reference to any standard. However, you may elaborate what BS 7671 specifies. I do not have access to BS.

Everyone has access to British Standards, as they are available for purchase from the British Standards Institute's outlets on-line.

At this moment, the forum does not know the country for which the installation is intended. Each country has its own electrical standards, which encompass the varying supply parameters and installation practices prevailing in each. The applicable standard is always the starting point. Without it, one is relying on opinions from anonymous postings in an international forum without the wherewithall to evaluate them, which exposes one to the possibility of interminable wrath if an installation hurts, maims or kills someone. Don't go there.

According to Bs7671,for sizing earth conductor in the adiabatic equation K^2S^2=I^2t

t is normally considered as 1sec.

To answer your last question first, IEEE 80-2000 is the principal international standard for grounding system design. Some additional time spent familiarizing yourself with more of the body of the standard should answer your questions.

The first reference you bring up relates to the selection of ground grid conductor size based on thermal damage to the conductors caused by fault current in the grid. The tables in 11.3.2.2 give you information on how much current can be carried by different size conductors for how long before they fuse / melt. The times you quote (3 and 1 sec.) are not referring at this point to safety of a person or proposed fault clearing times, but how long the grid can withstand the damaging effects of fault current before the conductors burn up.

Therefore "smaller" and "larger" substations in that section don't have an absolute definition - they're just relative descriptors. It's up to the designer to determine whether you have enough copper in the ground for the fault currents, the soil conditions, and the protection schemes to keep your system physically intact and your people safe.

Section 5 gives details on the effects of various levels of current in the human body, and concludes by recommending that maximum design fault clearing time be limited to 0.5 sec. or less. Most people designing ground grids work based on that limit. Using 0.5s and modifying the design as needed to achieve acceptable step and touch potentials and ground potential rise is usually possible, but may require more copper in the ground or a larger (in area) ground grid than you originally planned for. This is part of the "art & science" of ground grid design.

You would do well to do another careful read-through of IEEE 80 and make sure you grasp all the parts together.

You are right: the fault clearing time of 3 sec it is only for conductor sizing. See IEEE-80/2000 :

CH.11.3.3 Additional conductor sizing factors

"a) Relay malfunctions can result in fault duration in excess of primary clearing times. The backup clearing time is usually adequate for sizing the conductor. For smaller substations, this may approach 3 s or longer. However, because large substations usually have complex or redundant protection schemes, the fault will generally be cleared in 1 s or less."

It is still not a compulsory requirement but a "usual" consideration.

That depends on protection fault clearing time-mainly for backup protection.

For instance: if the first step could be 6 cycles[100 msec] the backup could be 1 sec.

However, there is not a recommendation to calculate Etouch or Estep for more then first step taken into consideration that statistically it occurs very seldom [almost never]: a man could be in danger of touch- or step- and in the same time the first protection would fail.

See: 16.2.2 Fault duration (tf) and shock duration (ts)

"Typical values for tf and ts range from 0.25 s to 1.0 s."

The British Standard is not connected to IEEE-80, I agree. By the way, BS7430 Code of practice for Earthing [ch.19-20] it is more suitable for Substation Grounding, I am afraid.

PW Slacks answer was correct.

Unfortunately too many people want someone else to do their work, buy standards, etc.

So gentlemen please first of all buy the proper and applicable standards.

Next if you are not an engineer or designer, then hire a specialist.

Otherwise the answers they will give you can be costly and could even cause damages to people and installations.

I think then there is no need of existence of CR4 at the first place. I thought this and similar type of forum are for sharing knowledge. No professional and sensible engineer will design as per the feedback from this forum unless and until he is sure about the source and reference.

Sorry PW Slack and vargaalex this is not the right place for you.

What is it for? Is it for relay setting or for ground conductor sizing or for earth mat design?

It is for ground conductor sizing and earth mat design. Please go thru the question properly.