Ipn

Rated primary current (nominal primary current) of the CT

Isn

Rated secondary current (nominal secondary current) of the CT

Ipsc

Rated primary (symmetrical) short-circuit current

Kssc

Rated symmetrical short-circuit current factor:

Iref

Reference current of IDMT protection element

Rbn

Rated resistive burden (secondary connected) of the CT

Pbn

Equivalent power over the rated resistive burden of the CT for rated secondary current:

Rb

Actual resistive burden (secondary connected) of the CT

Pb

Equivalent power over the actual resistive burden of the CT for rated secondary current:

Rct

Secondary winding resistance of the CT

Pct

Equivalent power over the secondary winding resistance of the CT for secondary rated current:

Vsal

Secondary accuracy limiting voltage (e.m.f.) of the CT

Vk

Rated knee point voltage (e.m.f.) of the CT

nn

Rated accuracy limit factor of the CT

nb

Actual accuracy limit factor of the CT:

Rl

One-way lead resistance from CT to relay

Rrel

Resistive burden of relay’s CT input

Tp

Primary time constant (primary system time constant)

ω

(System) angular frequency

Xp/Rp 

Primary impedance ratio (system impedance ratio):

Kd

Dimensioning factor for the CT

Kemp

Relay specific, empirically determined dimensioning factor for the CT