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MicroLim
Line Isolation Monitor
Section 1—Introduction
SECTION 1—INTRODUCTION
What is the MicroLim
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The MicroLim
Line Isolation Monitor (LIM), manufactured by Federal Pioneer, is a
testing device that continually measures the balanced and unbalanced impedance from
line-to-ground on each line of an ungrounded electrical system. The value of the
measured impedance displays on the meter of the LIM as hazard current. Hazard
current is the amount of current that would flow through a low impedance ground fault
on an ungrounded electrical system; hazard current is predicted fault current. The
higher the measured impedance from each line-to-ground, the lower the value of
hazard current.
To perform the measurement, the LIM injects a small test signal of a fixed current and
frequency, then it measures the voltage offset and phase angle offset of the test signal.
The measured and fixed values are analyzed using electrical laws to determine the line-
to-ground impedances of the system to which the LIM is connected. The LIM can
measure impedances that are either pure resistive, pure capacitive, or a combination of
both resistive and capacitive faults. Additionally, the impedances are either equally
balanced or unbalanced between each line and ground. The LIM displays a meter
reading for the line of the system with the lowest impedance to ground, or in other
words, the highest hazard current. For a thorough discussion of LIMs, hazard current,
and isolated power systems, refer to the following sources:
• Healthcare Facilities Handbook by Burton Klein, P.E., published by The National Fire
Protection Association.
• IEEE Recommended Practice for Electrical Systems in Healthcare Facilities, ANSI/IEEE Std.
602-1986, published by The Institute of Electrical and Electronic Engineers, Inc.
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Line Isolation Monitor?
© 1996 Federal Pioneer All Rights Reserved
Bulletin No. 43134-017-02
8/96
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