Insulation Test Trend Calculator

An insulation test trend calculator compares two entered insulation resistance readings over a measured time interval. It produces the Resistance ratio, Change, Percent change, Change rate, and Trend comparison needed to document whether the entered megohm readings increased, remained level, or decreased during that interval.

The calculation is useful when reviewing timed insulation-resistance readings from conductors, feeder cables, branch-circuit wiring, motor windings, transformers, or other electrical equipment after a megohmmeter test. A rising resistance value can indicate that the insulation measurement improved during the entered interval; a falling value identifies a decreasing reading that requires investigation in the broader test record.

This calculation evaluates the two readings provided. It does not establish insulation acceptability, equipment condition, or code compliance.

Test Readings and Elapsed Time

Enter the readings exactly as recorded from the insulation resistance test.

Input	Electrical meaning
Starting resistance	The insulation resistance reading at the beginning of the comparison interval, entered in MOhm
Ending resistance	The insulation resistance reading at the end of the comparison interval, entered in MOhm
Elapsed time	The number of minutes between the two recorded readings

For a valid interval calculation, Starting resistance must be greater than zero and Elapsed time must be greater than zero. Both resistance values must be expressed in the same unit, normally megohms (MOhm), before comparing them.

A repeated measurement interval should be documented consistently. Changes in test connections, test voltage, conductor temperature, equipment temperature, humidity, surface condition, or parallel connected equipment can affect insulation resistance readings independently of the insulation system being evaluated.

Resistance Trend Results

The calculator returns five values.

Output	Meaning
Resistance ratio	Ending resistance divided by starting resistance
Change	Numerical difference between the ending and starting readings
Percent change	Percentage increase or decrease relative to the starting reading
Change rate	Resistance change per minute across the entered interval
Trend comparison	Indicates whether the entered readings are non-decreasing or decreasing

A Resistance ratio above 1.0 means the ending resistance is higher than the starting resistance. A ratio of 1.0 means the readings are equal. A ratio below 1.0 means the ending resistance is lower.

Trend comparison describes the direction of the entered readings only. “Non decreasing entered readings” means the ending value equals or exceeds the starting value. It is not a pass/fail determination for a motor, cable, transformer, branch circuit, feeder, raceway installation, or electrical distribution system.

Calculation Method

The calculator uses interval arithmetic:

\(\displaystyle \text{Resistance ratio} = \frac{\text{Ending resistance}}{\text{Starting resistance}}\)

\(\displaystyle \text{Change} = \text{Ending resistance} - \text{Starting resistance}\)

\(\displaystyle \text{Percent change} = \frac{\text{Change}}{\text{Starting resistance}} \times 100\)

\(\displaystyle \text{Change rate} = \frac{\text{Change}}{\text{Elapsed time}}\)

Resistance is reported in MOhm, while Change rate is reported in MOhm/min.

Calculation Example

A timed insulation test produces the following readings:

Field	Entered value
Starting resistance	100 MOhm
Ending resistance	250 MOhm
Elapsed time	10 min

The results are:

\(\displaystyle \text{Resistance ratio} = \frac{250 \text{MOhm}}{100 \text{MOhm}} = 2.5\)

\(\displaystyle \text{Change} = 250 \text{MOhm} - 100 \text{MOhm} = 150 \text{MOhm}\)

\(\displaystyle \text{Percent change} = \frac{150}{100} \times 100 = 150\%\)

\(\displaystyle \text{Change rate} = \frac{150 \text{MOhm}}{10 \text{min}} = 15 \text{MOhm/min}\)

Result	Value
Resistance ratio	2.5
Change	150 MOhm
Percent change	150%
Change rate	15 MOhm/min
Trend comparison	Non decreasing entered readings

The entered resistance increased from 100 MOhm to 250 MOhm over 10 minutes. The ending value is 2.5 times the starting value, with an average increase of 15 MOhm per minute across that interval.

Field Interpretation Limits

An insulation resistance trend is not interchangeable with a polarization index (PI), dielectric absorption ratio (DAR), or an insulation-resistance acceptance criterion. The calculator does not calculate PI or DAR, and it does not evaluate the test voltage, test duration requirements, equipment rating, winding type, conductor insulation type, temperature correction, or manufacturer acceptance limits.

The result also does not determine conductor ampacity, AWG or kcmil sizing, adjustment factor, correction factor, terminal rating, voltage drop, raceway fill, overcurrent protection, or branch-circuit and feeder compliance. Those installation decisions require the applicable electrical design data, equipment instructions, and AHJ-enforced code requirements.

Before using the trend in a maintenance or commissioning decision, verify that the test setup remained comparable throughout the interval. Record the test instrument, applied test voltage, test duration, equipment isolation condition, conductor or winding temperature, ambient conditions, and any discharge procedure required for the equipment.