14
3.3.4. wItH A VoLtAGE rAMP
This test is based on the principle that an ideal insulation produces the same resistance whatever the test voltage applied.
Any negative variation of the insulation resistance therefore means that the insulation is defective: the resistance of defective
insulation decreases as the test voltage increases. This phenomenon is barely observable with low test voltages. At least 2,500V
must therefore be applied.
Since the application of the voltage is gradual, it causes no premature ageing or deterioration of the device tested. Unlike the
increase in steps, the gradual increase of the current means that the capacitive current is constant. A variation of the current
therefore directly represents a variation of the insulation resistance.
Evaluating the result:
a negative slope of the resistance versus test voltage curve exceeding 500ppm/V generally indicates the presence of
mildew or other deterioration.
a larger negative slope, or a sudden drop, indicates the presence of localized physical damage (arcing, perforation of
the insulation, etc.).
Set the switch to U-RAMP.
The following screen appears:
RAMP FUNCTION
1
Input voltage
Frequency
Input current
Date 2011.05.24
-0.1 V DC
0.2 Hz
55.7 nA
Time 15:31
Min. 50 V
Max. 100 V
BURN
Test Run Time 00:00:30
MR
U-STEP
U-RAMP
U-VAR.
U-FIXED
OFF
SET-UP
U-RAMP
Use the
keys to select a preset test voltage ramp:
Ramp function 1: 50 to 100V
Ramp function 2: 100 to 500V
Ramp function 3: 1000 to 2000V
The voltages at the beginning and end of the ramp can be programmed with the CONFIG key (see §4.3). The duration of the test
is the sum of the three durations specified: the duration of the initial plateau, the duration of the ramp, and the duration of the
final plateau.
3.3.5. wItH A StEPPEd VoLtAGE
The stepped voltage has ten plateaus. The duration of each of the voltage plateaus is identical. At the end of each plateau, the
capacitive current is normally zero and only the measurement current remains.
Unlike the ramp test, the step test stresses the insulation and can cause a breakdown. A sudden increase of the current (or a sudden
decrease of the insulation resistance) means that a breakdown point is near. It is then possible to discontinue the measurement
by hand (by pressing the START/STOP button) or automatically (E-BRK or Break at I-Limit type of test; see §4.3.1).
A drop of 25% or more between the insulation resistance of the first plateau and that of the second plateau is a sign of deterioration
of the insulation.
