CT Ratio Mismatch Discovered During Primary Injection
On a 13.8 kV feeder, the phase overcurrent relay is fed by a 600:5 multi-ratio CT. The settings engineer calculated pickup and time dial using a 600:5 ratio. During commissioning, a primary injection test is performed at 300 A. A calibrated clamp-on ammeter confirms 300 A on the primary, and the relay meter displays 3.02 A secondary. The expected secondary current is 2.50 A. The CT nameplate lists taps for 600:5, 500:5, and 400:5. The CT secondary circuit has a test switch and a shorting terminal block. The technician wants to adjust the relay pickup from 5.0 A to 6.0 A to compensate, then finish the test. The feeder is energized from the remote substation, and the relay is currently in service. The test switch is in the test position, but the CT secondary is not shorted.
1. Calculate the effective CT ratio and explain what the discrepancy means for protection.
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The effective ratio is primary current divided by secondary current: 300 A / 3.02 A ≈ 99.3, which is essentially 100:1. A 600:5 CT has a 120:1 ratio. A 500:5 CT has a 100:1 ratio. The measured result therefore indicates that the CT is effectively on the 500:5 tap, or that the secondary circuit is otherwise producing a 100:1 ratio.
The relay is seeing about 21% more secondary current than the settings engineer intended: 3.02 A versus 2.50 A. That shifts the primary pickup. A relay pickup of 5.0 A secondary would operate at about 500 A primary instead of the intended 600 A. Time-current coordination with downstream and upstream devices will also shift, so the feeder may trip earlier than designed.
Adjusting the relay pickup to 6.0 A to compensate is not acceptable. It hides the ratio error and leaves the time dial and coordination incorrect.
2. What corrective actions and CT safety precautions are required?
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First verify the CT tap setting, nameplate ratio, secondary wiring, test switch position, and shorting terminal condition. Confirm that the CT is not accidentally left on the 500:5 tap and that the drawings and relay settings match the installed ratio.
Because the feeder is energized and the relay is in service, treat the CT secondary as live. Never open a CT secondary circuit under load. Before changing taps or test links, de-energize and isolate as required, or use the shorting terminal block to short the CT secondary first. The test switch being in the test position does not by itself prove the CT is shorted.
Correct the tap or the settings to match the intended 600:5 ratio. Update the drawings and setting sheets, then repeat the primary injection test at multiple points to confirm pickup and timing. Only then return the relay to service.