IEC 60255-151 inverse-time characteristics

Overcurrent Relay Secondary Injection Report Format, Free PDF

A commissioning form for phase overcurrent relays, with a plot sheet and a reference page that gives the expected operating time for any TMS.

  • Free
  • No sign-up
  • A4, 3 pages
  • Formulas and limits included
Preview of the Overcurrent Relay Secondary Injection Report form
Download the PDF formA4, 3 pages

Print it at 100 % on A4. Page 1 is the form you fill in. The next page is the worksheet and reference.

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What is inside

  • Settings, CT ratio and the working to turn primary settings into secondary injection currents
  • Pickup, drop-off, six timing points and the high-set element, each with expected, measured and result columns
  • Tolerance boxes for you to fill from the relay datasheet. The form assumes none
  • Base operating times at TMS = 1 for the standard, very, extremely and long-time inverse curves, so expected time = base × TMS
  • An eight-item checklist, a sign-off block and a log-log plot sheet for expected against measured times

How it was checked

Base times are calculated from t = k / ((I / Is)^α - 1) and the formula was checked against independent calculations. The constants are from IEC 60255-151, so confirm your relay uses the same characteristic.

Related

Common questions

How do I calculate the expected operating time of an IDMT relay?

Use t = TMS × k / ((I / Is)^α − 1), with k and α from the curve. The reference page lists base times at TMS = 1 for standard, very, extremely and long-time inverse curves, so you multiply by your TMS.

Does the form include relay tolerances?

No. Pickup, timing and reset tolerances differ by relay, so the form leaves boxes for you to fill from the relay datasheet instead of assuming a value.

Using the form

  • Limits and constants come from the standards named above, as commonly quoted. Check them against the edition you work to, and use the stricter of those and your specification.
  • Where a value is a common guide rather than a limit from a standard, the form says so.
  • This is a calculation aid for qualified people. It does not replace the standard, the instrument manual or your site safety procedure.

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