Percent Error and Metering Specifications


Percent Error and Metering Specifications

We skipped a step when we performed the acceptance tests.  We forgot to figure out whether the results were within tolerance.  This video will show you how to:

  • Find and understand the relay specifications
  • Calculate percent error
  • Evaluate the test results


Click “Mark Complete” below after watching the video so you can keep track of your progress.

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Hi Chris,

Where did you get the test sets error percentage from?

Chris Werstiuk (Administrator) October 14, 2016 at 10:57 am

All the test-set manufacturers have a specification sheet listing all the guaranteed specs. They usually beat the specs in most cases. Some allow you to download the specs from the website, some make you ask for the specifications.

Joshua Liuga Suiramo February 8, 2021 at 11:53 am


Appreciate an example pointing to % error from SEL relay say 311L or 751A panel ?


Chris Werstiuk (Administrator) February 10, 2021 at 9:16 am

Thanks for the question, but I’m not sure what you are asking for.

All SEl-3xx relays are basically the same and the SEL-7xx are very similar.

Joshua Liuga Suiramo February 12, 2021 at 9:51 am

Hi Chris,

Thanks a lot you have answered the question in your lecture. Much appreciated.


Hi Chris, In the last part of the video, you mentioned that current & voltage is the foundation of all other calculations. Base on the other calculations that included real power, apparent power & references for determining direction, would you agree that the frequency & phase shift accuracies of the voltages & currents are important too.

Chris Werstiuk (Administrator) September 9, 2021 at 2:05 pm

Yes. The frequency and phase angles will have similar accuracy specifications that will be spelled out by the test-set and relay manufacturers in their literature. Remember that test-set output waveforms are constructed and will be much cleaner than anything out in the power system, so the relay will have a much easier time figuring out the phase angle and frequency with test values.

Some relay testers run into problems with high-speed frequency (<5 cycles) elements because the relay can spend additional time when the frequency jumps are too high. Therefore, be careful to look for maximum frequency deviation, frequency recalibration time, and/or frequency element pickup delays when reviewing specifications.

If there is a voltage input, the relay probably uses voltage for frequency.

If you are testing a phase angle, you typically leave the voltages alone because they are usually the reference, and then you vary the current angles.

Thank you for your reply Chris. Very useful indeed. For the sake of knowledge, would you agree that the reason the manufacturers chooses voltage as the frequency reference is because the voltage signal is much cleaner as far as the fundamental /positive sequence component is concerned during steady state operation as well as fault condition? Do you reckon that the current signal is much “dirtier” as far as harmonics, DC offset/decay etc? If the voltage is usually chosen, what happens when there’s transient disturbances? I am not familiar with U/O frequency protection element.

Chris Werstiuk (Administrator) September 10, 2021 at 8:09 am

Voltage is used because the system can be energized and online with no current. There should always be some voltage except for the most extreme faults.

All digital relays apply filters to clean up the waveform before they perform analysis, which is usually on the output of the nominal frequency filter. Line relays typically have a setting to filter out ccvt noise that occurs during switching and fault events.

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