Protective relays typically operate on standardized secondary voltages of 110–120 V for voltage circuits and 1–5 A for current circuits, scaled down from high system voltages and currents using PTs and CTs.

Voltage Levels

Protective relays are designed to monitor system voltages safely. High-voltage transmission lines and generators produce voltages far above what relays can handle directly. Potential transformers (PTs) or voltage transformers (VTs) step down these voltages to standard secondary levels, commonly 110 V or 120 V AC for AC systems, which are suitable for relay operation and measurement devices . In low-voltage systems, relays may operate directly at system voltage levels, typically 24 V, 48 V, or 110 V DC, depending on the control and protection scheme.

Current Levels

Similarly, current transformers (CTs) reduce high line currents to manageable secondary currents for relays. The most common secondary current ratings are 1 A or 5 A, regardless of the primary current magnitude, which can range from tens to thousands of amperes in transmission and distribution systems . The CT ratio is selected based on the maximum expected fault current and the relay's rated input, ensuring accurate operation during normal and fault conditions.

Relay Power Requirements

Relays require a power source to operate their internal logic, trip coils, and signal processing units. Electromechanical relays often use the same secondary voltage from PTs and CTs, while modern microprocessor-based relays may require auxiliary DC supplies, typically 48 V or 110 V DC, to power the electronics and communication interfaces .

Practical Considerations

• Overcurrent relays are set based on the expected short-circuit currents in the system, with CTs providing the scaled current input .
• Voltage relays monitor overvoltage or undervoltage conditions using PTs to provide a safe, standardized input .
• Motor protective relays may include time-delay elements to prevent tripping during startup currents, which can be several times the rated motor current .
• Relay settings and coordination are typically calculated using system studies to ensure proper tripping at both maximum and minimum fault currents . In summary, common secondary voltages for relay protection are 110–120 V AC or 24–110 V DC, and common secondary currents are 1 A or 5 A, with CTs and PTs scaling the primary system values to these levels for safe and reliable relay operation. Proper selection and coordination of these voltages and currents are essential for accurate protection and minimal disruption during faults.