Determination of fault current, expected to flow through the primary winding of a C.T. at every point of installation of the C.T., may not always be possible. The system fault level is then considered as the basis and the fault current is usually determined and specified.

It has already been pointed out earlier that the rated primary current of a C.T. and the rated short time current for which the C.T. can be designed are closely related. It has also been suggested earlier that an over current factor of 150 for 1 sec. may be economically obtainable. In view of this it is suggested that following procedure for determining the fault current and the rated primary current be followed:

  • Determine the fault-current (IF) from the considerations of the system fault MVA.
  • The load which is proposed to be fed through the C.T. is determined (IL).
  • The rated primary current (IP) shall then be either IF/150 or IL whichever is higher with the rated short time current time current of 150 IP for 1 second.
  • If it is observed that IP is too large and required settings of relays are not available , the use of suitable Interposing C.Ts. may be considered. It may be noted that the Interposing C.T. shall further load the main C.T. and that while determining the output requirement of I.C.Ts., the lead burden need not be considered as the I.C.T. is expected to be located close to the relays/instruments.
  • If the fault current IF is less than 150 IP, the short time current may be specified as, IF with duratiuon t such that ?t = 150IP/IF. If durations less than 1 second are likely to be acceptable, this same relationship may be used to determine t in cases when IF is greater than 150 IP.