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Summary: Method to Determine the 1°F Coefficient Factor for Insulation

1. Sample Preparation
- Three representative samples, preferably 14 AWG wires with a 0.045-inch insulation wall, should be obtained.
- The samples must be long enough to ensure insulation resistance values fall within the calibrated range of the measuring instrument at the lowest water bath temperature.

2. Immersion in Water Bath
- The samples are immersed in a water bath with heating, cooling, and circulation facilities.
- Sample ends should extend at least 2 ft. (0.609 m) above the water surface and be prepared to minimize leakage.
- The samples are left in the water at room temperature for 16 hours before adjusting the bath temperature to 10°C or transferring them to a 10°C bath.

3. Resistance Measurement
- Measure the conductor resistance at regular intervals until it remains constant for at least 5 minutes. This ensures the insulation has reached the bath temperature.
- Insulation resistance is then measured according to section 2.3.

4. Temperature Variation Testing
- The samples are sequentially exposed to water temperatures of 10°C, 16°C, 22°C, 28°C, and 35°C, and then the sequence is reversed back to 10°C.
- Insulation resistance readings are taken at each temperature after equilibrium is reached.

5. Averaging and Plotting
- Two sets of readings taken at the same temperature are averaged.
- The readings, including the one at 35°C, are plotted on semi-log paper with temperature on the linear axis.
- The insulation resistance at 15.6°C (60°F) is read from the plot.

6. Calculating the 1°F Coefficient
- The 1°F coefficient is calculated by dividing the insulation resistance at 15.6°C (60°F) by the insulation resistance at 16.1°C (61°F).

7. Using the Resistivity Coefficient (CIR)
- The resistivity coefficient (CIR), rounded to two decimal places, is used to find the corresponding factor in Table 2-2.
- This factor converts insulation resistance measured at any temperature to the equivalent value at 15.6°C (60°F).

Comprehensive Guide on Converting Insulation Resistance to Insulation Resistance Constant

1. Introduction
In the electrical industry, insulation resistance is a crucial parameter for determining the quality and reliability of cable insulation. However, insulation resistance varies with temperature, making it necessary to convert measured values to a standardized reference temperature (usually 15.6°C or 60°F). This guide explains how to convert insulation resistance (IR) to the insulation resistance constant (IRK) using appropriate formulas, correction factors, and measured diameters.

2. Formula for Converting Insulation Resistance to Insulation Resistance Constant

The insulation resistance constant (IRK) is calculated using the following equation:


Where:
- IRK = Insulation resistance constant in megohms per 1000 ft.
- IR = Measured insulation resistance in megohms per 1000 ft. at 15.6°C (60°F)
- TCF = Temperature correction factor for converting insulation resistance to 15.6°C (60°F) (obtained from Table 2-2)
- D = Diameter over the insulation in inches
- d = Diameter over the conductor stress control layer (or over the conductor, if no stress control layer is present) in inches

3. Temperature Correction Factor (TCF)

Insulation resistance is highly sensitive to temperature changes. As the temperature increases, insulation resistance decreases due to increased molecular activity within the insulating material. The **Temperature Correction Factor (TCF)** is used to normalize insulation resistance values to a standard temperature of **15.6°C (60°F)**.

Table 2-2 provides the TCF values for different temperatures and 1°F coefficients (CIR). CIR is determined by dividing the insulation resistance at 15.6°C by the insulation resistance at 16.1°C. Once the CIR is calculated, the appropriate column in Table 2-2 is used to find the corresponding TCF for the measured temperature.

4. Table 2-2: Temperature Correction Factors (TCF)

Temperature (°F)	Temperature (°C)	0.99	1.01	1.02	1.03	1.04	1.05	1.06	1.07	1.08	1.09	1.10	1.11	1.12
40	4.4	1.22	0.82	0.67	0.56	0.46	0.38	0.31	0.26	0.21	0.18	0.15	0.12	0.10
41	5.0	1.21	0.83	0.69	0.57	0.47	0.40	0.32	0.27	0.23	0.19	0.16	0.14	0.11

...
[/table]

Note: The table is partially shown above. The full table includes TCF values for a wide range of temperatures and CIR coefficients.

5. Example Calculation

5.1 Given Data:
- Measured insulation resistance (IR) = 500 megohms per 1000 ft.
- Temperature = 21.1°C (70°F)
- Diameter over insulation (D) = 1.5 inches
- Diameter over conductor stress control layer (d) = 0.8 inches
- CIR = 1.04

5.2 Finding TCF:
From Table 2-2, for a temperature of 21.1°C and CIR of 1.04, the corresponding TCF is **1.34**.

Explanation of Temperature Correction Factor (TCF) Formula

The temperature correction factor (**TCF**) is calculated using the following formula:


Where:
- **CIR**: Resistivity coefficient, determined according to section 2.3.3, representing the ratio of insulation resistance at 15.6°C (60°F) to that at 16.1°C (61°F).
- **t**: Cable temperature in degrees Fahrenheit (°F) during the insulation resistance measurement.


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Purpose of the Formula
This formula is used to determine the temperature correction factor (**TCF**) required to convert the measured insulation resistance at any given temperature to the equivalent resistance at the reference temperature of **15.6°C (60°F)**. Since insulation resistance varies with temperature, applying the TCF ensures accurate standardization, enabling consistent comparison of insulation properties across different temperatures.

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Application Example
1. **Determine CIR:** Calculate the resistivity coefficient (CIR) by dividing the insulation resistance at 15.6°C by the insulation resistance at 16.1°C.
2. **Identify Cable Temperature:** Measure the cable temperature (t) during the test in degrees Fahrenheit.
3. **Apply the Formula:** Use the given CIR value and temperature (t) in the formula to find the TCF.
4. **Convert Insulation Resistance:** Multiply the measured insulation resistance by the TCF to obtain the equivalent insulation resistance at 15.6°C (60°F).

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This correction method ensures that insulation resistance values are consistent regardless of the environmental conditions during testing, making it


5.3 Applying the Formula:

IRK = (500 × 1.34 × 1.5) / (log10(1.5 / 0.8))

First, calculate the logarithm term:

log10(1.5 / 0.8) = log10(1.875) = 0.273

Now, apply the values:

IRK = (500 × 1.34 × 1.5) / 0.273 = (1005) / 0.273 = 3682 megohms per 1000 ft.

6. Summary of Steps
1. Measure the insulation resistance (IR) at the given temperature.
2. Determine the CIR by dividing the insulation resistance at 15.6°C by that at 16.1°C.
3. Use the CIR to find the corresponding TCF from Table 2-2.
4. Apply the formula to calculate the insulation resistance constant (IRK).

7. Conclusion
Converting insulation resistance to an insulation resistance constant (IRK) provides a standardized way to evaluate the insulating properties of cables under different temperature conditions. By using the provided formula and correction factors from Table 2-2, engineers can ensure accurate and reliable assessments of cable insulation performance.

Note: The formulas and example calculations are based on industry standards. Actual values may vary depending on specific cable designs and operating conditions.

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