For protective CTs, the ability to accurately reproduce the primary current during a fault—which often includes a decaying DC component—is vital. IEC 61869-2 details the requirements for TP (Transient Protection) classes (TPX, TPY, TPZ). These classes dictate how the transformer core behaves under the magnetic saturation caused by the asymmetrical fault current, ensuring relays do not maloperate.
: Used for precision measurement and billing (revenue-grade). iec 61869-2 current transformers
IEC 61869-2 introduces several technical clarifications and rigorous testing regimes compared to its predecessor. For protective CTs, the ability to accurately reproduce
Classes like 0.2S and 0.5S maintain their accuracy across a wider range (20% to 120% of rated current), whereas non-"S" classes typically only guarantee accuracy from 100% to 120%. IEC 61869-2:2012 For protective CTs