Fluke 5520A - External Guard

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To avoid ground loops and noise you must have
only one earth ground-to-LO terminal connection
in the system. Usually you make all signal ground
connections at the UUT and verify the Z
annunciator is off. Generally, Z is on only for ac
and dc volts where the UUT is isolated from earth
ground. There must, however, be a safety ground
for the 5520A. See “Connecting to Line Power” in
Chapter 2. When enabled by the sourced output, a
softkey LOs appears, which allows you to tie or
open an internal connection between the NORMAL
LO terminal and AUX LO terminal. When tied and
Z is on, then both LO terminals are tied to
chassis ground.
External Guard
The guard is an electrical shield, isolated from the
chassis, that protects the analog circuitry. The
guard provides a low-impedance path for common-
mode noise and ground loop currents. There is
normally an internal connection between the guard
and the NORMAL LO terminal. By pressing the
B key, you break this internal connection, which
allows you to connect a lead from the GUARD
terminal to earth ground on another instrument in
an interconnected system. Use this external guard
connection whenever you are testing a UUT that
has a grounded LO terminal. Remember to always
maintain only one earth ground tie point in a
system.