What is a Class PX Current Transformer?

A Class PX CT is a low-leakage reactance protective current transformer without a remanent flux limit, where:

• Knowledge of the excitation characteristic
• Secondary winding resistance (Rct)
• Secondary burden resistance (Rb)
• And turns ratio
is sufficient to accurately assess its performance in combination with a protective relay system.

🔹 Why is Class PX Important?

• Custom Relay Coordination:
Unlike standardized classes like 5P or 10P (which use fixed error limits), Class PX allows precise modeling of the CT + relay system using known parameters.
• Stability & Sensitivity:
It's ideal for high-impedance differential protection schemes, where even a small CT error could cause unwanted tripping.
• Flexibility in Design:
The designer can simulate CT behaviour under fault conditions using excitation curves and system resistance values—no fixed error class constraints.

🔹 Use Cases:

• Transformer Differential Protection (87T)
• Busbar Differential Protection (87B)
• Line Protection with High-Impedance Relays
• Applications requiring transient stability and low saturation

🔹 Note: The rated knee point e.m.f. appears in the specifications of class PX protective current transformers

• Ek = Kx X (Rct + Rb) X Isr

where,
• rated knee point e.m.f. (Ek);
• rated resistive burden is Rb;
• dimensioning factor is Kx;
• CT secondary winding resistance is Rct.

🔹 For class PX, the turns ratio error shall not exceed ±0,25 %.