The three strictly prohibited mistakes in relay protection are incorrect relay configuration, improper CT/VT ratio settings, and poor coordination of protection devices.

1. Incorrect Relay Configuration

A common and critical mistake is misconfiguring relay parameters, which can cause the relay to operate outside its intended protection logic. Even if the hardware is functional, incorrect settings may lead to unreliable fault detection, unnecessary shutdowns, or equipment damage. This often occurs when settings are copied from previous projects without validation against the current system, or when temporary changes are not properly reverted after maintenance or testing .

2. Improper CT/VT Ratio Settings

Current transformer (CT) and voltage transformer (VT) ratio errors are another major issue. If CT or VT ratios are incorrectly programmed, the relay receives inaccurate measurements, making the protection logic unreliable. This can result in false trips, failure to isolate faults, or overlapping operations of multiple breakers, which may compromise system safety and reliability .

3. Poor Coordination of Protection Devices

Failure to coordinate relays and circuit breakers is a critical mistake. Protection coordination ensures that the device closest to the fault operates first, minimizing the impact on the rest of the system. Poor coordination can lead to unselective tripping, causing unnecessary outages, widespread system instability, or even cascading failures in high-voltage networks . This is particularly dangerous in complex systems with multiple generators, feeders, and distributed loads.

Additional Considerations

• Always update relay settings after system expansions, load changes, or equipment upgrades.
• Conduct regular testing and commissioning to verify relay performance under actual operating conditions .
• Avoid using relays beyond their rated capacity or in unsuitable environmental conditions, as this can reduce performance and increase the risk of insulation failure or fire . By strictly avoiding these three mistakes—incorrect configuration, CT/VT ratio errors, and poor coordination—engineers can ensure reliable, fast, and safe operation of protective relays in electrical systems.