Unitronics electric UM2B, Руководство пользователя

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3.- UM2B EQUIPMENT
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2.4.- ETPRA. Data interpretation
It follows interesting details if you have available the Insulation Resistance
Measurement Software (ETPRA).
From the exam of the circuit of figure 2-1, it states that to discharge Cg, you only
need to short-circuit the dielectric terminals, but to discharge Cpx, it will be required a time
proportional to the time constant
Rpx*Cpx . This means to say that if a transformer has not
been phase-grounded the required time to discharge that Cpx, it will exist some residual
charge that make hard to compare consecutive test of Insulation Resistance performed in the
dielectric.
The software developed for UM2B unit accounts for this effect and let you perform a
discharge period previous to the test, that will make consecutive test easy to exactly
compare. Anyway, in some case it could be appreciated slight differences between the
insulation measurement performed with the Recovery Voltage software and the one
performed with the Insulation Resistance software (ETPRA). This is due to that the
Recovery Voltage measurement software uses one of the charging cycles longer than 10
minutes to measure insulation. In this software the discharge conditions of that Cpx have
been restricted enough not to make the test last too long and give accurate results. This limit
could offer slight measurement differences between both applications.
It is therefore recommendable to begin the test with the unit UM2B after having
shorted the machine to test. If it had been made previous test, that short should last at least
around four times the charge time of the last test.