By Daniel Foster – Condition Monitoring Services Consultant

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In condition monitoring projects, measurement accuracy often depends less on the sensor itself and more on how it is installed. A recent customer case involving the Bently Nevada 163947-00-05-02-84-90 non-contact current sensor illustrates this point clearly.

The sensor was fully functional. The data, however, was not.

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Background

• Site type: combined-cycle power plant

• Objective: trending motor current for early fault detection

• Monitoring scope: multiple identical motors with comparable loads

• Issue reported: inconsistent current values between units

From a monitoring perspective, the variation exceeded acceptable tolerance.

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Data Anomalies Observed

• One motor consistently showed lower RMS current

• Load and process parameters were confirmed identical

• No alarms, no dropouts, no saturation

• Trends were stable—but wrong

This is often more dangerous than obvious failure.

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On-Site Verification

Physical inspection revealed subtle but critical differences:

• Sensor-to-conductor distance varied by several millimeters

• One sensor was slightly angled relative to the current path

• Cable bundles caused uneven magnetic field distribution

These deviations were within what many installers would consider “acceptable.”

They were not.

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Root Cause Analysis

The 163947-00-05-02-84-90 measures current indirectly via magnetic field intensity. Small geometric changes result in:

• Reduced magnetic coupling

• Systematic under-reading of actual current

• False conclusions when comparing machines

The sensor output remained stable, which masked the underlying measurement error.

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Corrective Measures

• Reinstalled sensors using a fixed alignment template

• Standardized distance from conductor across all units

• Separated power cables to reduce field distortion

• Re-baselined current trends after mechanical correction

Once geometry was corrected, all motors showed consistent current behavior.

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Why This Matters

• Condition monitoring relies on relative accuracy

• Stable but inaccurate data leads to incorrect maintenance decisions

• Installation tolerances must be treated as measurement parameters

• Non-contact does not mean non-sensitive

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Professional Assessment

The Bently Nevada 163947-00-05-02-84-90 performed exactly within specification. The fault lay entirely in installation practice.

In monitoring systems, precision begins with mechanics, not software.

— Daniel