Performing a defensible MV/HV Protection Relay Setting Calculator inspection requires more than writing down a number from a display. Auditors, insurers, and national regulatory bodies under IEC 60255 / IEEE C37 require calibrated instrument traceability, correct lead placement, environmental / temperature compensation, and verification under representative operating conditions.

1. Required Calibrated Test Instruments

Before going to site, register your instrument's brand, model, serial number, and valid calibration expiry date inside OhmCert's Equipment Register. For Module #04 (Protection Relay Settings), use:

  • Primary Instrument Set: 3-Phase Secondary Injection Relay Test Set (e.g., Omicron CMC 356, Megger Sverker 900, Doble F6150) + Primary Injection Kit for CT Polarity & Ratio
  • Traceability Requirement: Valid ISO/IEC 17025 calibration certificate (within 12 months).

2. Step-by-Step Field Connection & Measurement Procedure

  1. Isolate the circuit breaker HV/MV truck to Test/Disconnected position and open CT shorting/isolating test blocks before connecting secondary injection leads.
  2. Download the active relay firmware and setting group (Ip pickup, TMS / Time Dial, IEC/IEEE IDMT curve type, and 50/50N high-set thresholds) and compare against the approved coordination study.
  3. Inject current at 0.95x Ip (must NOT pick up), 1.05x Ip (must pick up), and at 2x, 5x, and 10x Ip to record exact operating times against the theoretical IEC 60255-151 curve.
  4. Verify actual trip-coil mechanical opening and ANSI 74 Trip Circuit Supervision before restoring CT links.

3. Governing Equations Evaluated During the Test

As you enter your field readings into OhmCert Module #04 (Protection Relay Settings), the engine evaluates these normative formulas in real time:

t_IEC: t = TMS x [ k / ((If / Is)^alpha - 1) ] — IEC 60255-151 inverse-time operating equation for fault multiple M = If / Is > 1.0.

t_IEEE: t = (TD / 7) x [ A / (M^p - 1) + B ] — IEEE C37.112 operating equation where TD/7 maps to TMS = 0..1.0.

Delta_t: Delta_t = t_upstream - t_downstream >= 0.25 s (0.30 s target) — Coordination Time Interval (CTI) accounting for CB opening time (50-80 ms), relay overshoot, and CT errors.

4. The 3 Most Common Field Measurement Mistakes

  • Leaving CT secondary shorting links open when disconnecting test leads on an energized primary feeder (dangerous open-CT high voltage).
  • Mixing IEC Time Multiplier Setting (TMS 0.05-1.0) equations with IEEE Time Dial (TD) constants.
  • Ignoring upstream-to-downstream Grading Margin (< 0.25-0.30 s causes simultaneous nuisance tripping of the main incomer).

5. Mandatory Site Verification Checklist (13 Normative Points)

  • Relay model, serial number, firmware revision, and active setting group verified against approved protection coordination study [IEC 60255]
  • Current Transformer (CT) ratio, accuracy limit factor (5P10 / 5P20), knee-point voltage, and star-point polarity proven by injection
  • Voltage Transformer (VT) ratio, secondary MCB/fuse supervision, and phase rotation verified
  • CT shorting links and test block isolation verified safe prior to secondary injection kit connection
  • Phase overcurrent pickup (51 Ip) and instantaneous/high-set (50 I>>) trip thresholds verified within +/-2.5% tolerance
  • IDMT curve timing (IEC SI / VI / EI or IEEE MI / VI / EI) tested at 2x, 5x, and 10x Ip against theoretical curve [IEC 60255-151]
  • Earth fault / residual overcurrent (50N/51N or 51G) pickup and time delay proven across full grading range
  • Upstream-to-downstream selectivity Grading Margin (>= 0.25-0.30 s for numerical relays) verified at maximum fault level

→ Open Module #04: MV/HV Protection Relay Setting Calculator Calculator in OhmCert