I work in oil refinery, Can any body tell me about the earthing resistance. The tranformers, motors and every electrical equipment is earthed for protection purposes. But I just wana ask that what is the standard value(according to NEC or IEC or equivalent) for earthing resistance. The standard use here is 10 ohm but we don't know which standard body recommends this value.



Regards
Nasir.

Nasir,

Earthing resistances are used to lessen the earth fault current on a fault. I don't know if there is a standard value for these units but I have fitted them on transformers at 3.3kV and on generator star points.
The rule I use (and seems to be generally applied) is to rate the resistor to limit fault current equal to the current rating of the device (remembering that the voltage is neutral to ground (i.e. dont forget the root 3). The idea is to reduce equipment damage on an earth fault. You ned to select a rating for the resistor that will allow it to pass current for enough time to operate the protection (they are usually rated for 3s or 10s for a particular temperature rise). They get very hot if a fault occurs so place them somewhere where nobody might accidentally touch them.
I have never added these units at LV but at 3.3kV and above. I think that the current can be limited to greater extent if necessary for generators and motors which are not very earth fault tolerant. Clearly the fault must be such that the protection can recognise it and on a machine or transformer there is normally some sort of unit protection like restricted earth fault to allow a more sensitive setting.
Often when fitted to a distribution transormer star point in addition to the restricted earth fault protection a standby earth fault protection is also fitted which is a single element overcurrent relay designed as back up protection.
I've attached a file that might be useful

Regards

Matt

As per IEEE 80 and NFPA, earthing resistance should be less than 5 ohm for LV and LT systems, and should be less than 1 ohm for HV and EHV systems. during calculations also we have to consider step and touch voltages for HV systems. the standard that mentions is IEEE 80. Please make a note of the same.