Protective Relays and Instrument Transformers
Protective Relays and Instrument Transformers tests whether the technician can verify relay operation and the CT/PT interface that supplies it. Relays are tested by secondary injection; CTs and PTs are checked for ratio, polarity, burden, and wiring. The exam emphasizes safe CT handling and correct primary-to-secondary scaling.
Core rules
- Never open-circuit an energized CT secondary. Short the secondary with a proper shorting terminal before removing burdens.
- Verify CT ratio and polarity. A 600:5 CT should output 5 A at 600 A primary, ignoring ratio error and burden.
- Check PT ratio and polarity; never short a PT secondary.
- Use secondary injection to test pickup, dropout, time delay, and target operation. Confirm the relay settings match the issued setting file.
- Account for test-set burden and wiring when results are close to tolerance.
Worked example
A 600:5 CT feeds an overcurrent relay set for 5 A pickup. Secondary injection raises current until the relay picks up at 4.8 A. Primary pickup is 4.8 A times 120 = 576 A. The technician compares 576 A with the intended 600 A pickup and the relay tolerance. A 5 A secondary pickup would equal 600 A primary. The reasoning is ratio scaling: primary amperes equal secondary amperes multiplied by the CT ratio.
Traps
Opening a CT secondary under load; shorting a PT; forgetting polarity; testing a relay without confirming the settings file; and ignoring test-set burden when pickup is marginal.