Using A Voltage Ramp - AEMC 6550 User Manual

4.4.4 Using 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 2500V 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).
The test with a voltage ramp is ideally suited for testing semiconductors (diodes, transistors, and thyristors).
Take care in this case to choose a non-destructive type of test: Break at I-limit (see § 2.4.3) and a maximum
output current less than or equal to 1mA.
Set the switch to the V-RAMP position. The following screen appears:
SET-UP
V RAMP
V VAR
V FIXED
30
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ADJUSTABLE VOLTAGE
Input voltage
Frequency
Input current
Date 2011.05.24
MR
Step
Voltage
OFF
www.
BURN
750 V
0.1 V AC
0.2 Hz
11.56 pA
Time 15:31
RAMP FUNCTION
Min. 50 V
Test Run Time 00:00:30
Input voltage
Frequency
Input current
Date 2011.05.24
ShopAEMC
.ca
BURN
1
Max. 100 V
-0.1 V DC
0.2 Hz
55.7 nA
Time 15:31
Megohmmeter Models 6550/6555
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