Capacitance display
At the end of a test the capacitance of the circuit under test is calculated
and displayed. After a test the display shows the last measurement made
until the V
s
, V
t
, or
TEST
buttons are pressed.
Time constant:
At the end of a test the time constant of the circuit under test is calculated
and displayed. After a test the display shows the last measurement made
until the V
s
, V
t
, or
TEST
buttons are pressed.
Measurements above 100 G
Ω
Measurements up to 100 G
Ω
can be made without any special precautions,
assuming that the test leads are reasonably clean and dry. The guard lead
can be used to remove the effects of surface leakage if necessary. When
measuring resistances above 100 G
Ω
, the test leads should not be allowed
to touch each other, or any other object since this will introduce leakage
paths. Sharp points at the test lead connections should also be avoided
since this will encourage corona discharge.
The output is isolated, and so will float relative to ground such that the
positive terminal is at plus half of the test voltage, and the negative terminal
is at minus half of the test voltage with respect to ground. Leakages
therefore occur between the positive terminal and ground, between the
negative terminal and ground, and directly between the positive and
negative terminals. These leakages have a significant effect and can occur
through the air itself.
If the guard lead is connected to ground, then since the negative terminal
is at the same voltage as the guard terminal, the leakage into the negative
terminal will be considerably reduced. This will improve accuracy because
the current flowing into the negative terminal is measured by the
instrument and used to calculate resistance. This technique is only
permissible if the item under test is isolated from ground. “Isolated” in
this context means insulated by a resistance of at least 5 M
Ω
for the
positive terminal, or at least 10 k
Ω
for the negative terminal.
Conversely, if the positive terminal is grounded, then the negative terminal
will be at a voltage equal to the test voltage relative to ground, which will
result in an increase in leakage current, and worsening of measurement
accuracy.
When making measurements above 100 G
Ω
therefore, the user should
ground the Guard Lead where possible, otherwise parallel leakage paths
may occur.
Alternatively, screened leads are available as an optional accessory from
Megger. The lead to the negative terminal is fully screened. The screen is
plugged into the Guard terminal, diverting any stray leakage currents. This
considerably improves measurements made with a floating output, where
the leads might touch each other or anything other than the test piece.
9
+ Test V
2
- Test V
2
Ground
+
-
+ Test V
0V
Ground
Guard
+
-
