Environmental Engineering Reference
In-Depth Information
distance to a fault in a cable by coupling a repetitive DC pulse to the shield
and allowing the pulse to travel the length of it. The time delay between
the DC pulse and when the signal is received can be measured by simul-
taneously monitoring the cable signal path (Hashemian, 2010). These time
delays make it possible to identify the point at which the electromagnetic
interference (EMI) couples into the cable system. This reveals the location
of degraded connectors or cable shields because such interference usually
couples at cable connections or terminations that tend to degrade through
ageing or damage (IAEA, 2011).
6.3.3 Chemical measurements
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 chem-
ical 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 sam-
ple, 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 ther-
mal 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 anti-
oxidant in the insulation polymer - decreases with age (U.S. NRC, 2001).
The DSC instruments measure the difference in heat fl ow 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 sam-
ple 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
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