Keithley 6512, Instruction Manual

Page: 85 / 156

Applications
4-6
Figure 4-5
Insulation resistance measurement (unguarded)
4.6.2 Guarded resistance measurements
For resistance measurements above 10
8
Ω
, or for cables long-
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
10
7
Ω
if stable readings are to be expected (in the unguarded
mode, the shield should be connected to input low).
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
16
Ω
. Also, for a given resistance, the
constant-voltage method will be faster.
INPUT
ON
TRIAX CABLE
LO
SHIELDED TEST FIXTURE
INSULATION
RESISTANCE
RX
6512 SET TO OHMS, V
Ω
, GUARD OFF
A. CONNECTIONS
HI
I
TO A/D
6512 PREAMP
R
V
I
I
HI
LO
RX
TEST FIXTURE
B. EQUIVALENT CIRCUIT
RX =
V
I
V
Ω
, GUARD
OFF
HI
LO
GND