4 34405A User’s and Service Guide
Protection Limits
The Agilent 34405A Digital Multimeter pro-
vides protection circuitry to prevent damage
to the instrument and to protect against the
danger of electric shock, provided the Pro-
tection Limits are not exceeded. To ensure
safe operation of the instrument, do not
exceed the Protection Limits shown on the
front and rear panel, and defined below:
Note: The front-panel terminals are shown
above. The rear-panel terminals are identi-
cal. The Front/Rear switch selects the ter-
minal set to be used. Do not operate this
switch while signals are present on the
front or rear terminals. The current-protec-
tion fuse is on the rear panel.
Input Terminal Protection
Limits
Protection Limits are defined for the input
terminals:
Main Input (HI and LO) Terminals. The HI
and LO input terminals are used for voltage,
resistance, capacitance, and diode test
measurements. Two Protection Limits are
defined for these terminals:
HI to LO Protection Limit. The Protection
Limit from HI to LO ("A" in the figure at
left) is 1000 VDC or 750 VAC, which is
also the maximum voltage measurement.
This limit can also be expressed as 1000
Vpk maximum.
LO to Ground Protection Limit. The LO
input terminal can safely "float" a maxi-
mum of 500 Vpk relative to ground. This is
Protection Limit "B" in the figure.
Although not shown on the figure, the Pro-
tection Limit for the HI terminal is a maxi-
mum of 1000 Vpk relative to the ground.
Therefore, the sum of the “float” voltage
and the measured voltage must not exceed
1000 Vpk
Current Input Terminal. The current input
("I") terminal has a Protection Limit of 1.2A
(rms) maximum current flowing from the LO
input terminal. This is Protection Limit "C"
in the figure. Note that the current input ter-
minal will be at approximately the same
voltage as the LO terminal.
Note: The current-protection circuitry
includes a fuse on the front panel. To main-
tain protection, replace this fuse only with a
fuse of the specified type and rating.
12A Current Input Terminal. The 12A cur-
rent input terminal has a Protection Limit of
12A (rms) maximum current flowing from
the LO input terminal. This is Protection
Limit "D" in the figure. Note that the current
input terminal will be at approximately the
same voltage as the LO terminal.
Note: The current-protection circuitry
includes an internal fuse. To maintain pro-
tection, service-trained personnel should
replace this fuse only with a fuse of the
specified type and rating.
IEC Measurement Category II
Overvoltage Protection
To protect against the danger of electric
shock, the Agilent 34405A Digital Multime-
ter provides overvoltage protection for
line-voltage mains connections meeting
both of the following conditions:
The HI and LO input terminals are con-
nected to the mains under Measurement
Category II conditions, defined below, and
The mains are limited to a maximum line
voltage of 300 VAC.
IEC Measurement Category II includes elec-
trical devices connected to mains at an out-
let on a branch circuit. Such devices include
most small appliances, test equipment, and
other devices that plug into a branch outlet
or socket. The 34405A may be used to make
measurements with the HI and LO inputs
connected to mains in such devices, or to
the branch outlet itself (up to 300 VAC).
However, the 34405A may not be used with
its HI and LO inputs connected to mains in
permanently installed electrical devices
such as the main circuit-breaker panel,
sub-panel disconnect boxes, or permanently
wired motors. Such devices and circuits are
subject to overvoltages that may exceed the
protection limits of the 34405A.
Note: Voltages above 300 VAC may be mea-
sured only in circuits that are isolated from
mains. However, transient overvoltages are
also present on circuits that are isolated
from mains. The Agilent 34405A is designed
to safely withstand occasional transient
overvoltages up to 2500 Vpk. Do not use this
equipment to measure circuits where tran-
sient overvoltages could exceed this level.
12 A
rms
10 00VDC
500Vpk
1.2A
rms
1 .25 A / 5 00V FH
LO
CAT II (300V)
75 0VAC
HI
I
V
12 A
Fused
A
C
D
B
34405A users guide.book Page 4 Saturday, September 2, 2006 3:38 PM
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