TNS Systems and BS7671

<...WhaT is TNS system as per BS7671....>

http://en.wikipedia.org/wiki/Earthing_system

Simply put,TNS system is a method of distribution of Power,we also have other methods which are clearly explained in various electrical Technology books.A very good example is Electrical Technology by B.L.Theraja.Check Transmission and Distribution of power.Also check yr BS regulations and see what it says for BS7671.

For short cct calculation,use Is=In x K (short cct current=rated current of yr device x factor K),where k is factor dependent on the size of cable applied in the installation.We have k factor of 2.5 or3.5 etc,but u must understand yr circuit,read yr electrical books and regulations proper as to enable u get d right rating and setting of yr short cct device as well as what factor is to be used.

For yr earth resistivity,u can simply apply the formular V=IR.You should know yr voltage value and the value of yr earth fault current,then divide yr voltage value by the current value and you get yr resistance.

But i think u should be concerned about the earth fault current,so as to get a suitable earth fault trip device or ELCB.Bcos earth resistance can be between 1 to 5 ohm.Simply dig dip into the earth at various points and find a resistance between this ohmic values.

Patrick Whowha

The first letter 'T' implies that the System (Neutral) is 'solidly' connected to earth at the source. (T stands for "TERRA" - the French word for "Earth"). The second letter "N" implies that all the exposed and the extraneous conductive parts are connected to earth through the system neutral only which itself is earthed at source ("N' for Neutral). And, 'S' implies that the exposed and extraneous condcutive parts are so connected to the earthed neutral using a seperate conductor ("S" for Seperate Conductor).

I wouldn't suggest a short-cut formula to fault level calculation, as it is a detailed subject and many parameters need to be considered.

Also, the Earth Resistivity is "measured" and it cannot be calculated.