Selection of Secondary Cables for Relay Protection

Secondary cables for relay protection must be selected based on current rating, voltage drop, insulation, and reliability to ensure accurate and safe operation of protective relays.Key Considerations for Cable Selection1. Current Carrying Capacity: The secondary cable must safely carry the maximum secondary current from the CTs or PTs to the relay without overheating. Typically, the cable is sized to handle 125% of the relay secondary current to account for inrush or transient conditions ( ). 2. Voltage Drop: Excessive voltage drop can affect relay accuracy. The voltage drop along the secondary cable should be kept below 1–2% of the rated secondary voltage to ensure proper relay operation ( ). 3. Insulation and Voltage Rating: Cables must have insulation rated for the system voltage and environmental conditions. For medium voltage systems, PVC or XLPE insulated multicore cables are commonly used, with color coding for phase identification and polarity ( ). 4. Cable Type and Shielding: Multicore cables are preferred for relays to reduce wiring complexity. Shielded cables may be used in environments with high electromagnetic interference to prevent false tripping or maloperation ( ). 5. Relay Coordination and Selectivity: The cable selection should support the relay's speed and sensitivity, ensuring that the relay discriminates correctly between fault and normal conditions. Short, direct runs with minimal splices improve reliability ( ). 6. Mechanical Protection and Routing: Cables should be routed to avoid mechanical damage, moisture, and heat sources. Conduits or cable trays are recommended, and proper termination with ferrules or lugs ensures secure connections ( ).Practical Steps for SizingDetermine Relay Secondary Current: Identify the rated secondary current of the CT or PT feeding the relay.Calculate Maximum Expected Current: Include safety margin (typically 125–150% of rated secondary current).Select Cable Cross-Section: Use standard tables for current carrying capacity, considering insulation type, ambient temperature, and installation method.Check Voltage Drop: Ensure voltage drop along the cable does not exceed relay tolerance.Verify Compliance: Follow relevant standards such as IEEE C37.90, IEC 60287, and local electrical codes ( ).SummarySelecting secondary cables for relay protection involves balancing electrical, mechanical, and environmental factors to ensure reliable relay operation. Proper sizing prevents overheating, maintains voltage integrity, and ensures accurate fault detection. Using multicore, insulated, and properly routed cables with correct cross-section and voltage rating is essential for safe and effective protection system performance ( ).