Floating Circuits - Keithley 6517B Reference Manual

Model 6517B Electrometer Reference Manual
Use guard for:

Guarded voltage measurement

Guarded, floating current measurements
For voltage measurements, guarding should be used when the test circuit impedance is 1 G or
when long input cables are used. Guard is enabled from the Configure Voltage menu structure.
When enabled, the guard potential is placed on the inner shield of the triaxial input cable.
For current measurements, guarding should be used when the test circuit impedance 1 .
Significant leakage could occur across a DUT through insulators and corrupt the measurement. Input
LO (inner shield of the input triaxial cable) is used as the guard.
For floating current measurements, a unique guard technique is used in a high impedance test circuit
where significant leakage current may exist between the ammeter input and test circuit common.
Safety shield
A safety shield is required whenever a hazardous voltage is present on the noise shield or guard
shield, or when a test circuit is floated above earth ground at a hazardous voltage level. A shock
hazard exists at a voltage level equal to or greater than 30 V
appear on the noise/guard shield when performing floating measurements or guarded measurements.
Typical connections for the safety shield are shown in the following figure. The metal safety shield
must completely surround the noise or guard shield, and must be connected to safety earth ground
using #18 AWG or larger wire.

Floating circuits

Many measurements are performed above earth ground and, in some test situations, can result in
safety concerns.
6517B-901-01 Rev. C / August 2015
. Hazardous voltages up to 500 V may
rms
Figure 20: Safety shield
Section 3: Connections
3-9