Eaton SEFELEC 1000-M, User Manual

Page: 192 / 209

User manual SEFELEC 5x Series 191 v1.06
19.5.1. Selection of the test voltage
The dielectric strength tests intending to verify that materials or equipments comply with stand-
ard requirements, it is important to refer to these standards to select the voltage.
In case of no indication regarding the test voltage, a common rule is to apply the following formu-
la:
Utest = 2 x Un 1,000 volts
Most of the standards specify the type of the test voltage: AC (50-60 Hz) or DC. A common rule
is to test the specimen with a test voltage of the same type that the voltage which will be apply
during the final use. However, certain among of technical difficulties exist which force to depart
from this common rule.
19.5.1.1. DIELECTRIC STRENGTH TESTS WITH AC VOLTAGE
BENEFITS:
•
The specimen is stressed with the both voltage polarities.
•
The specimen being not charged, there is no need for a discharge system
DISADVANTAGES:
•
Most of the tested specimens having a certain amount of capacitance, the HV source have to
supply the leakage current as well as the reactive current, this involves an over-sized genera-
tor with an increase in prices, in weight and a decreasing of the operator safety who is ex-
posed to higher currents.
The reactive current can be evaluated with the following formula:
Impedance = Voltage / Current (Ohm’s law: Z = U/I)
for capacitances: Z = 1 / Cw with w = 2*Pi*F
reactive current: Ir = U*C*2*Pi*F
i.e.
U = 3,000 volts C = 1 nF (~ 10 meters of shielded cable)
Ir = 3,000*1E-9*2*3.14*50 = 0.942 My
•
Require to adjust the permanent leakage current threshold (IMAX) regarding the capacitance
of each specimen.
•
When testing a specimen which will be used with a DC voltage, the AC voltage test can result
in a decreasing of the life time because in particular of the heating and the CORONA effect.
Under the effect of an electrical field, the orientation of molecules is done with
friction which
will occur with an AC voltage at each cycle (every 20 or 16 mS). Consequently the AC test is
more severe than the DC voltage test.