why 4 ohm grounding

[m3new]

here in uk we use 1.2metre rods for earthing (grounding),railway signalling equipment location cases .its an IT system ie.no return earth conductor, only means of earthing is local rods (max of 4 OHMS earth resistance).Our railway authoraties will not except chem rods ,just finished a 12 mile re-signalling scheme and installed 4500 RODS and aprox 25000 metres of g/y earth cable .MY ? WHY 4 OHM( EQUIPMENT SUPPLIED AT 650V AC THEN STEP DOWN TO 110V AC).

ANY HELP WOULD BE APPRECIATED

 

[mc5w]

U.S> National Electrical Code requires and equipment grounding conductor from the source of an ungrounded system to each service switch.

On grounded systems the grounded conductor must run to each service switch even if not used to carry load.

The earth is NOT considered an effective path for ground faults here in the U.S.

Your 4 ohm specification could be to control lightning damage. A grounded conductor that travels a considerable distance is not an effective lightning protection conductor.