Is the "Extremely Inverse" Curve of Your SIPROTEC 5 in ETAP Really the Relay's Own Curve?
Can the same curve name mean different trip times? That's what I found while reviewing a coordination study in ETAP: modeling a Siemens SIPROTEC 5 relay with the "ANSI Extremely Inverse" curve, the curve came out much higher than expected. Comparing the times the software produced against the manufacturer's own manual, they didn't match — instead, they matched the previous generation, SIPROTEC 4.
To confirm whether the issue was in the software or in my setting, I repeated the test in Relay Tripping Curves PRO2 (RTC2), explicitly selecting each relay model. The difference turned out to be real and documented: SIPROTEC 4 and SIPROTEC 5 use two different constants for the same "Extremely Inverse" curve — one is, literally, the other divided by 5. With the same setting, that translates into trip times up to 5 times slower or faster, depending on which model the software is applying internally.
In this article I walk through how the discrepancy was detected, the exact formula for each generation (with its source: Siemens' manuals and the IEEE C37.112 standard), the numeric comparison between both, and why this detail can be the difference between correct protection coordination and a non-selective trip during a real fault.
📄 Download the full article as a PDF, with screenshots and formulas for each case:
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