nominal waveform cannot be observed, as it is masked by the L/R
time constant of current growth.
According to our experience, the selection time is usually very
short, especially in high power transformer, when it is around 30
ms. With this selection time, the following table gives the limits for
the X/R parameter.
X/R L/R WAVEFORM
2 0,5 GOOD
5 2 LIMIT
20 5 MASKED
However, this does not mean that the LTC cannot be tested: it
means that the resistors values cannot be measured. If, during
the test, the LTC has a fault (open circuit), then the R of L/R
becomes infinite, and the resistance waveform clearly shows the
problem.
The following pictures show some recorded test results.
1. Selection time about 30 ms; L/R time constant about 30 ms;
the resistance cannot be measured; LTC is OK.
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