I have a ground loop around a small rectangular building, ground rods at four corners of the loop. Lightning protection provided on roof. Presently, the down conductors from the air terminals connect to the ground loop, and the ground bus in the panel connects to this same ground loop. NEC and NFPA says
that the lightning ground system and the power ground system need to be seperate systems, but connected together.But, if these two systems need to be bonded together anyway, why can they not be the same system (why can't I have one grounding system that both the power grounding and the lightning protection tie in to)?because i blv bonding makes them one system i.e parallel paths to earth

Yes, but i am sure the NEC states that if both systems are within 6ft of each other they must be bonded to prevent flash-over.

You could only have a parallel path if the general electrical system is bonded to the same ring at two different points.

RGDS

The answer to your question: From NFPA 780
A. There are several types of lightning grounding, used separately or in combination, as selected by the installation contractor. The lightning grounding electrode is a dedicated part of the lightning protection system. No other ground shall be used as a lightning ground (see section 4.13.1.3).
B. Other (non-lightning) systems requiring grounding shall be integrated (bonded or connected) to the lightning ground to form a common ground potential (see section 4.14). This is to protect electrical equipment from voltage rise mismatches where separate grounds are employed. Under lightning attachment conditions, the lightning ground will function independently of any other grounds. Should electrical system grounds not be connected to the lightning ground, the overall lightning protection system will still function as designed.

As examples of the above, there are many instances of functional lightning grounding on structures that have no electrical systems, such as railroad tracks, military munitions storage depots ect..., and high-voltage power line towers.

Halim Bensmaia, I understand the point your are making however, it is very important that you quote information correctly. Please see quote below.

"4.13.1.3 Electrical system and telecommunication grounding electrodes shall not be used in lieu of lightning ground electrodes; this provision shall not prohibit the required bonding together of grounding electrodes of different systems."

Please clarify you comment below
"This is to protect electrical equipment from voltage rise mismatches where separate grounds are employed."

Norman ,
There are several checking you need to make for grounding and among the checking you need to make sure that the lightning surge current will not create enormous voltages between the ground rod and the building's grounding electrode system. We know this voltage as transferred earth potential. Transferred earth potentials often create damage to other equipment. Therefore you need to be make sure you that grounded electrodes are effectively bonded.

OK i get you, this is known as FLASH-OVER, hence my earlier point. anything within 6ft or 2.5m of the lightning protective system, must be bonded to the said system.

i still dont get the point that after bonding how does the fault current discriminate that which way it should go down to earth..i dont feel any difference electrically in bonding and making a ring for one system

Remember that the ring system is 3 - 6ft in the soil, your main reference for the general system should be accessible for maintenance and so on. Now how the be if there is no reference point for your power panels?