When measuring conducted
radio interference voltages from active power lines to ground, it is
essential to know the line impedance so that repeatable tests can be
made by more than one laboratory. Artificial line impedances are
specified in MIL-STD-462, VDE, CISPR, C22.4, NACSEM 5100, ANSI C63.2
and other EMI specifications.
The characteristic impedance
of the 5 µH and 50 µH LISNs brackets the mean value of
power line impedance which has been measured by independent
researchers. These two inductance values in parallel with the 50
ohms of the EMI meter fall between the minimum and maximum line
impedance values which have been measured. The mean value would be
represented by a 20 µH inductor in parallel with 100 ohms.
The Solar Electronics LISNs
use a series inductor between the test sample and the power source
to provide the impedance-versus frequency characteristic. A coaxial
connector with d.c. isolation is provided for connection to the
associated frequency selective EMI meter. The power source end of
the inductor is bypassed to ground.
Due to the large
current-carrying capability of some LISNs, it is not always
practical to use a switch for changing inductance values. Instead,
some models are equipped with a high current pin plug-and-jack
combination for quickly connecting and disconnecting a network and
substituting another. This nylon insulated pin plug and jack
arrangement is a safety feature, well isolated from inadvertent
short circuits, providing protection to operating personnel.
Current ratings up to 200
amperes are available in 50 µH styles and 500 amperes in 5 µH
styles. See the chart on the following page.
When measurements are made in
a shielded room, the LISNs intended for FCC applications will also
serve for VDE tests. When operating on an unfiltered power line, the
VDE specifications require a filter consisting of 250 µH inductor
and a capacitor. This filter is included in the 24 ampere LISN,
Type 9348-50-R-24-BNC, and the 50 ampere LISN,
EMI specifications require
one LISN in each ungrounded power lead.
Even though the neutral is considered “ground,” if it is not
connected to chassis inside the unit under test,
the lead must be tested with an LISN. Therefore, use two LISNs in DC
or single phase AC applications, three LISNs for delta-connected
three phase circuits, and four LISNs for ‘Y’ connected three phase