Section 3: Connections
Model 6517B Electrometer User's Manual
3-8
6517B-900-01 Rev. B August 2022
Guard shield
The GUARD option enables or disables guard for voltage measurements. For current, resistance, and
charge measurements, guard is always disabled. When guard is disabled, the inner shell (shield) of
the triaxial connector and cable is connected to meter input LO.
When guard is enabled, the inner shell (shield) of the triaxial connector and cable is connected to
guard, which follows the potential of meter input HI. Guarding greatly reduces leakage current in a
high-impedance test circuit. Leakage resistance exists in the input cable between conductor paths
and in the test fixture at connectors and insulators. The concept of guarding is to surround the input
high node or DUT with a guard shield that is at the same potential. Current cannot flow through a
leakage resistance that has a 0 V drop across it. A typical connection for the guard shield is shown in
the following figure. A safety shield is also used because guarded measurements can place
hazardous voltages on the guard shield (see
(on page 3-9)).
Figure 18: Guard shield
1
Device or circuit under test
2
Metal safety shield
3
Metal guard shield
4
Connect to 6517B guard using a triaxial cable
Safety earth ground
