Understanding Current Sag Residuals
Current sag residual analysis helps you study a short drop in load current. It is useful during motor starts, feeder faults, breaker trips, and weak supply events. The calculator compares a reference current with the lowest measured current during the event. It then shows the sag depth, residual percentage, recovery percentage, duration cycles, and residual vector current.
Why The Result Matters
A current sag is not always harmful. Some loads naturally dip during control transitions. The concern starts when the residual current is too low, the recovery current is weak, or the phase currents become badly unbalanced. These signs can point to loose conductors, undersized cables, incorrect protection settings, transformer stress, or abnormal motor behavior. A single number rarely explains the whole event, so the tool presents several checks together.
Phase Residual Check
Three phase systems need special attention. Balanced phase currents should cancel as vectors when they are separated by about one hundred twenty degrees. If the vector sum is high, residual or neutral current can appear. That value may indicate leakage, phase angle error, CT wiring issues, harmonics, or a real ground return path. The calculator uses the entered phase angles to estimate this residual current.
Duration And Recovery
Event duration matters as much as sag depth. A deep sag for one cycle may be acceptable for some equipment. A smaller sag lasting many cycles can still disturb drives, relays, and controls. The recovery result shows whether current returns near the reference value after the event. This makes the report useful for maintenance logs and power quality reviews.
Practical Use
Use measured RMS values from a power recorder, relay event record, or meter log. Keep units consistent. Enter CT ratio only when the readings are secondary values and must be scaled to primary current. Review the pass or warning flags, then compare them with site standards. The calculator does not replace protection studies, but it gives a clear first review. It also exports results for notes, reports, and repeatable troubleshooting.
Record Keeping Tip
Save each event with date, feeder name, load state, and instrument range. These details help compare later events and prevent false conclusions from mixed operating conditions. Use the same meter range.