Guarded Resistance Measurements; Resistance Measurements With External Voltage Source - Keithley 6512 Instruction Manual

Applications
Figure 4-5
Insulation resistance measurement (unguarded)

4.6.2 Guarded resistance measurements

For resistance measurements above 10
er than three feet, guarded measurements are recommended,
as shown in Figure 4-7. In this case, the rear panel V, Ω
GUARD switch is used to apply a guard signal to the inner
shield of the connecting triax cable. The guard is carried
through to the inner shield of the test fixture. The inner shield
must be insulated from the outer shield, which is a safety
shield. Incidentally, a shielded fixture is recommended for
both unguarded and guarded resistance measurements above
Ω if stable readings are to be expected (in the unguarded
7
10
mode, the shield should be connected to input low).
4-6
TRIAX CABLE
INPUT
OFF ON
VΩ, GUARD
6512 SET TO OHMS, VΩ, GUARD OFF
TEST FIXTURE
HI
I
R X
LO
Ω , or for cables long-
8
SHIELDED TEST FIXTURE
A. CONNECTIONS
6512 PREAMP
R
I
V
HI
LO
GND
B. EQUIVALENT CIRCUIT
Since the inner shield carries the guard signal, the COM ter-
minal acts as circuit LO.
4.6.3 V/I resistance measurements with external
voltage source
With the constant-current method discussed above, the Mod-
el 6512 can make measurements as high as 200G Ω . Howev-
er, the insulation resistance of such materials as polyethylene
may lie above this range. By using the Model 6512 along
with an external voltage source to make resistance measure-
ments in the constant-voltage mode, measurement range can
be extended up to 10
constant-voltage method will be faster.
HI
I
R X
LO
INSULATION
RESISTANCE
TO A/D
V
R X =
I
Ω . Also, for a given resistance, the
16