Chemical measurement techniques determine cable condition by measuring a chemical property of the cable insulation and then correlating the results with a known measure of electrical performance (U. S. NRC, 2001). In chem­ical cable testing, a small piece (a few milligrams) of cable insulation or jacket material is shaved off for chemical analysis in a laboratory using one of the following techniques:

• oxidation induction time/temperature (OIT/OITP) test;

• Fourier transform infrared (FTIR) spectroscopy;

• gel content or gel fraction tests;

• density tests.

Chemical tests are not considered in-situ since they require a small sam­ple, but the sample is so small that they are sometimes considered non-destructive.

The polymers used in cable insulation respond to radiation and thermal degradation through oxidation processes. The greater the radiation or ther­mal ageing conditions imposed on the cable insulation, the more antioxidants (manufactured into the insulation to slow degradation) in the polymer are consumed. Differential scanning calorimetry (DSC) instruments can be used to measure the rate of oxidation induction time (OIT) and oxidation induction temperature (OITP) in polymers. The OIT and OITP values correlate with the degree of cable insulation degradation. OIT — a measure of the remaining anti­oxidant in the insulation polymer — decreases with age (U. S. NRC, 2001).

The DSC instruments measure the difference in heat flow between a polymer sample oxidizing under heat and an identical empty sample pan (acting as a control) also being heated. The levels of antioxidant in the sam­ple will determine how long it takes for the heated polymer sample to begin oxidizing. The complete depletion of antioxidants would typically simulate polymer degradation after a 20-year operational life. A sample that takes a long time to begin oxidizing has substantial antioxidant levels, and therefore minimal degradation (U. S. NRC, 2001).

Fourier transform infrared (FTIR) spectroscopy, a laboratory technique for studying the molecular structure of materials,, can identify operating conditions where heat may cause cable insulation to become brittle or crack. FTIR involves applying infrared radiation to a small piece of cable insulation using a spectroscope. The spectroscope measures the ability of the material to absorb or transmit the radiation. When the chemical bonds in the sample absorb the radiation they begin to vibrate at specific wave­lengths. The FTIR technique compares the actual measured maximum vibrations of these chemical bonds to their known maximum vibrations to ascertain to what extent the bonds have already oxidized or degraded over time. The more the cable surface has been exposed to heat over time, the more likely the measured vibrations of the sample chemical bonds will dif­fer from the original values.

FTIR is extremely accurate — measuring to one tenth of a degree Fahrenheit — and enables relatively easy, trendable, non-destructive degra­dation monitoring. However, it requires expensive equipment and a small sample, which may be difficult to obtain from remote sections of a cable circuit (U. S. NRC, 2001; 2010a; 2010b).