Megohm Test Trend Calculator

A megohm test trend calculator compares two insulation resistance readings taken under comparable conditions and expresses the change as a measurable deterioration or improvement trend. The primary result is the change between the Initial resistance (MOhm) and Latest resistance (MOhm), supported by percent change, daily change, drop percent, and the remaining margin against an entered review limit.

Insulation-resistance trend review is used in preventive-maintenance records for conductors, feeders, branch circuits, motors, transformers, and other electrical equipment where insulation condition is monitored over time. A single megohm reading can be useful, but a declining series of readings may identify a developing moisture, contamination, thermal-aging, mechanical-damage, or insulation-degradation concern that requires field evaluation.

The calculator does not determine whether insulation is acceptable for service, whether equipment must be repaired, or whether a circuit complies with the NEC. It performs the entered trend arithmetic only.

Insulation Resistance Trend

An insulation resistance test applies a DC test voltage and measures resistance between conductors, or between a conductor and equipment grounding path. Results are commonly recorded in megohms, abbreviated MOhm.

For trend comparison, readings should be collected with the same equipment condition and as nearly the same test conditions as practical. A lower latest reading produces a negative resistance change and a positive drop percent. A higher latest reading produces a positive resistance change and a drop percent of zero.

The calculator uses these fields:

Field	Electrical use
Initial resistance (MOhm)	Earlier insulation resistance reading used as the comparison baseline
Latest resistance (MOhm)	Most recent insulation resistance reading
Days between readings (days)	Elapsed time used to calculate daily change
Allowed drop reference (%)	Drop percentage established by the maintenance review process

The entered Allowed drop reference (%) is not an insulation-resistance acceptance criterion. It is a review threshold selected for the specific maintenance program, equipment history, manufacturer guidance, or engineering process.

Calculation Results

The worksheet produces six results from the two entered readings and elapsed days.

Result	Meaning
Change	Direct megohm difference between the latest and initial readings
Percent change	Change stated as a percentage of the initial reading
Daily change	Average megohm change per day across the entered interval
Drop percent	Percentage reduction from the initial reading, limited to zero when resistance increased
Drop margin	Remaining percentage points between the entered drop reference and calculated drop percent
Trend comparison	Whether the calculated drop percent is within the entered drop reference

A negative Change indicates the latest resistance is lower than the initial resistance. A negative Percent change also indicates a decline. The Drop percent presents that same decline as a positive percentage, which makes it easier to compare against the allowed drop reference.

If the latest resistance is greater than the initial resistance, the calculator reports a positive change and a positive percent change, while drop percent remains 0%. This avoids treating an increase in measured resistance as a negative drop.

Megohm Trend Formula

Let:

• R_i = Initial resistance
• R_l = Latest resistance
• D = Days between readings
• A = Allowed drop reference

\(\displaystyle \text{change} = R_l - R_i\)

\(\displaystyle \text{percent change} = \frac{\text{change}}{R_i} \times 100\)

\(\displaystyle \text{daily change} = \frac{\text{change}}{D}\)

\(\displaystyle \text{drop margin} = A - \text{drop percent}\)

The daily change is an average over the selected time period, not a prediction of future insulation deterioration. Insulation condition can change nonlinearly after water intrusion, overheating, contamination, equipment damage, or changes in storage and operating conditions.

Calculation Example

Assume a maintenance record contains the following readings:

Input	Entered value
Initial resistance (MOhm)	1000 MOhm
Latest resistance (MOhm)	800 MOhm
Days between readings (days)	30 days
Allowed drop reference (%)	25%

The resistance change is:

\(\displaystyle 800\text{ MOhm} - 1000\text{ MOhm} = -200\text{ MOhm}\)

The percent change is:

\(\displaystyle \frac{-200}{1000} \times 100 = -20\%\)

The average daily change is:

\(\displaystyle \frac{-200\text{ MOhm}}{30\text{ days}} = -6.6667\text{ MOhm/day}\)

The drop percent is:

\(\displaystyle \frac{1000 - 800}{1000} \times 100 = 20\%\)

The drop margin is:

\(\displaystyle 25\% - 20\% = 5\%\)

The worksheet result is:

Result	Value
Change	-200 MOhm
Percent change	-20%
Daily change	-6.6667 MOhm/day
Drop percent	20%
Drop margin	5%
Trend comparison	Within entered drop reference

The reading has fallen by 200 MOhm over 30 days, representing a 20% reduction from the original 1000 MOhm measurement. Because the entered allowed drop reference is 25%, the calculated trend remains within that entered reference by 5 percentage points.

Field Verification

Megohm trend arithmetic is meaningful only when test conditions are comparable. Temperature, humidity, surface contamination, equipment condition, test voltage, test duration, connections, and whether the circuit was isolated can materially affect insulation-resistance measurements.

Document the test setup with the readings, including the test instrument, applied test voltage, test duration, conductor or equipment identification, ambient conditions when available, and whether loads, electronics, surge protective devices, variable-frequency drives, control components, or parallel paths were disconnected or isolated.

A trend result should be reviewed alongside the actual electrical system condition. For a feeder or branch circuit, that may include visual inspection, conductor termination condition, raceway moisture exposure, evidence of overheating, load history, voltage-drop concerns, and recent damage or alterations. For motors and equipment, maintenance personnel may also evaluate winding condition, contamination, moisture, connections, and applicable manufacturer procedures.

The calculator does not apply temperature correction, normalize readings for humidity, select test voltage, establish maintenance intervals, or make an energization or repair decision. Those actions remain subject to equipment instructions, site procedures, qualified-person judgment, and any requirements enforced by the AHJ.