EL CID (Electromagnetic Core Imperfection Detection) testing is one of the most powerful diagnostic tools available for assessing generator stator core condition. It provides a non-destructive assessment of core lamination insulation integrity without requiring a full rewind — making it one of the highest-value tests that can be included in a major generator inspection.
Despite its value, EL CID testing is frequently misunderstood, misinterpreted, or omitted from outage scopes. This article explains what the test measures, how to interpret results, and what action thresholds actually mean for the machine's condition and remaining life.
EL CID testing measures the electromagnetic response of the stator core to an applied excitation signal. The test identifies areas where the insulation between core laminations has degraded, allowing circulating currents to flow between laminations. These circulating currents cause localized heating that can damage the core and, ultimately, the stator winding.
The test produces a plot of the electromagnetic response around the circumference of the core, identifying the location and relative severity of any insulation defects. The primary output is the QUAD signal, which represents the reactive component of the electromagnetic response and is the primary indicator of lamination insulation degradation.
EL CID results are typically interpreted against the CIGRE threshold of 100 mA QUAD signal. Results below this threshold are generally considered acceptable; results above it warrant further investigation and may indicate the need for core repair or rewind.
However, the 100 mA threshold is a guideline, not an absolute criterion. The interpretation of EL CID results requires engineering judgment that considers the location of the defect, the trend relative to previous tests, the machine's operating history, and the severity of the signal relative to the threshold.
The action threshold for EL CID results is not a binary pass/fail criterion. Results above the threshold indicate that further investigation is warranted — not necessarily that the machine requires immediate rewind. The appropriate response depends on the severity and location of the defect, the trend relative to previous tests, and the machine's operating schedule.
EL CID testing should be standard on every major generator inspection for three reasons: it provides the only reliable non-destructive assessment of core lamination insulation integrity; it is relatively low-cost compared to the cost of a core failure; and it provides a baseline for trend analysis that informs future maintenance decisions.
Our engineers are available to discuss your specific situation — no sales process, no obligation.
Discuss a Project