Electrical Testing Calculator

How to use the electrical testing calculator

Use this page when the immediate job is screening and comparison, not when you need a complete acceptance procedure for every asset class.

1. Choose the testing mode that matches the question

• Insulation Resistance Screen corrects a megger reading to one reference temperature and, for rotating machines only, compares it with a common IEEE 43 style (kV + 1) MOhm rule-of-thumb.
• Ground Fall-of-Potential Screen gives the 62% starting location for the potential probe and compares the measured resistance with your project target.
• Contact Resistance Comparison compares a micro-ohm reading with a prior or sister-pole baseline from the same equipment family.
• Tan Delta Trend Comparison compares the current tan-delta or power-factor reading with a prior baseline so you can see whether insulation losses are drifting upward.

2. Use the insulation mode for normalization, not universal pass-fail

• Enter the measured insulation resistance, the temperature logged with the reading, and the rated voltage.
• The calculator applies a simple field-style temperature correction so the reading can be compared at one reference temperature.
• The (kV + 1) MOhm rule-of-thumb is shown only for motors and generators. The page does not invent a universal minimum for transformers, switchgear, busway, or cable systems.

3. Use the ground mode to set up a three-point field check

• Enter the measured ground resistance, your project or screening target, and the distance to the remote current probe.
• The page returns the 62% starting point for the potential probe.
• You still need to sweep the probe around the flat portion of the curve before final acceptance.
• The familiar 25-ohm screen belongs only to the NEC single-rod supplemental-electrode rule. It is not a universal facility-grounding maximum.

4. Use baseline comparisons for contact resistance and tan delta

• Contact-resistance readings make sense when compared with the same device, the same test current, and similar poles or phases.
• Tan-delta or power-factor readings also need the same asset, voltage class, test set, and environment before the comparison means anything.
• This page shows trend movement and percent change so you can decide whether a full follow-up review is needed.

5. Know what this page does not do

• It does not replace manufacturer withstand limits, cable acceptance tables, or a full NETA ATS or MTS procedure.
• It does not determine safe work boundaries, PPE, or energized-work authorization. Those decisions still belong to your NFPA 70E process.
• It does not create one universal micro-ohm or tan-delta limit for every device family.

6. Use related tools when the task becomes more specific

• Use the Insulation Resistance Calculator for a more focused megger correction workflow.
• Use the Grounding Resistance Calculator when the work shifts to electrode geometry and soil-resistivity screening.
• Use the Cable Testing Calculator when the job becomes cable-specific rather than general testing workflow.
• Use the Arc Flash Calculator separately for energized-work safety studies.