HP 400D Operating And Servicing Manual page 6

CAUTION
The ground terminal (G) of the INPUT
and OUTPUT
binding posts, the instru-
ment chassis and cabinet, and the third (green) grounding conductor in
the
three -conductor power cable are electrically
connected together at aII times.
When the NEMA connector is used in the proper manner, a ground path is
established between the 400D/H and equipment which is also grounded. Do
not
connect the ground terminal of the instrument to any point which is not
at
ground potential, or a short circuit wiII be created.
If such a
measurement
is necessary, disconnect the NEMA ground in the power cable by using
an
adapter with the grounding
pig
-tail removed, and
the cabinet insulated from
ground. The 400D/n cabinet wiII be at the same potential as the ground
Iead.
Caution must be used if the clip lead is connected to a point which is
more
than a few volts off ground. Because of the potential hazard to personnel,
this method is not
recommended.
One
side of almost all power distribution systems is grounded. Extreme
cau-
tion must be used
if direct measurement of power system voltages is attempted.
If the ground Iead is accidentally connected to the ungrounded side of the Iine,
severe
damage
to the
40 OD/H
is possible because of the short circuit created.
Fower line voltages can be safely measured by using the ungrounded
(upper)
terminal only.
Contacting the grounded power conductor will give a reading
of 0 volts, while contacting the ungrounded lead wiII give full line voltage
reading.
When very low level voltages are being measured, there is a possibility
of
error caused by current flowing in the equipment grounding conductor
and the
ground signal input conductor. This current flow will cause a voltage to
be
developed in the connecting leads which is in series with the signal
under
measurement and wilt be read by the meter.
This source of error can be re-
moved by eliminating the ground toop between the meter and the circuit under
test.
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