Front Panel Operation
2-13
CAUTION
Do not make floating measurements
with electrometer LO connected to chas-
sis ground. If the rear panel ground link
is installed between COMMON and
chassis ground, remove it before floating
the instrument.
Guard shield —
Guarding is used to greatly reduce 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 0V drop across it. The
generic connection for the guard shield is shown in Figure 2-
12, which also summarizes the measurements that guard is
used for. Notice that a safety shield is also used since guard-
ed measurements can place hazardous voltages on the guard
shield (see Safety Shield).
For voltage measurements, guarding should be used when
the test circuit impedance is
≥
1G
Ω
or when long input cables
are used. Guard is enabled from the Configure Voltage menu
structure (see paragraph 2.5.2). When enabled, the guard po-
tential is placed on the inner shield of the triax input cable.
Figure 2-21 in paragraph 2.5.1 shows detailed connections
for guarded voltage measurements. See paragraph 2.5.3
(Guarding) for more information on guard.
For current measurements, guarding should be used when
the test circuit impedance
≥
1G
Ω
. Significant leakage could
occur across a DUT through insulators and corrupt the mea-
surement. Input LO (inner shield of the input triax cable) is
used as the guard. Paragraph 2.6.3 (Guarding) explains how
guarding affects high impedance current measurements and
is shown in Figure 2-30.
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. This unique guard technique for float-
ing current measurements is explained in paragraph 2.6.3
(Guarding) and is shown in Figure 2-31.
Safety shield —
A safety shield is required whenever a haz-
ardous voltage is present on the noise shield or guard shield,
or when a test circuit is floated above earth ground at a haz-
ardous voltage level (see paragraph 2.4.5). A shock hazard
exists at a voltage level equal to or greater than 30V rms.
Hazardous voltages up to 500V may appear on the noise/
guard shield when performing floating measurements or
guarded measurements.
The generic connections for the safety shield are shown in
Figure 2-13. The metal safety shield must completely sur-
round the noise or guard shield, and must be connected to
safety earth ground using #18 AWG or larger wire.
2.4.5 Floating circuits
Many measurements are performed above earth ground and,
in some test situations, can result in safety concerns. Figure
2-14 shows two examples where the Model 6517A floats at
a hazardous voltage level. In Figure 2-14A, a shock hazard
(100V) exists between meter input LO and chassis ground. If
meter input LO is connected to a noise shield, then the shock
hazard will also be present on that shield. In Figure 2-14B, a
shock hazard (200V) exists between the meter input (HI and
LO) and chassis ground. If meter input LO is connected to a
noise or guard shield, then the shock hazard will also be
present on that shield.
Use Guard for:
1) Guarded voltage measurement
2) Guarded, floating current measurements
Connect to 6517A Guard
(via triax cable)
Device or
Circuit
Under
Test
Metal Safety Shield
Metal Guard Shield
Safety
Earth
Ground
Figure 2-12
Guard shield
Use safety shielding whenever
≥
30V is present on the guard or
noise shield. Guarded measure-
ments and floating measurements
can place hazardous voltages on
the guard/noise shield.
Connect to 6517A chassis
ground (via triax cable)
Device or
Circuit
Under
Test
Noise or Guard Shield
Safety
Earth
Ground
Metal
Safety
Shield
* Connect the safety shield to safety earth ground
using #18 AWG wire or larger.
Figure 2-13
Safety shield
