Knee-Point Voltage

To roughly estimate the required Ek=2(If/N)r, where If=Switchgear Fault current rms(normally 20kA), N=turns ratio (Ip/Is), r=rough estimate of CT secondary resistance (10A=0.1ohms, 5A=1ohms, 1A=10ohms)+connection conductor loop resitance= 0.006ohms/m.

For an 11KV switchgear CT the Ek can be based on a rough rule of thumb of 0.4v/Ts & at N of 1/350(10A sec)=140v, 1/700(5A)=280v, 1/3500(1A)=1400v.

See IEC60044-1, 14.4 for CT Ek etc.

Small correction: the limiting condition is Ek≥2(If/N)r; other thing is as above.

thanks.