Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

7.4 Monitoring thresholds
7.4.1 Objectives
Once the monitor is connected to the circuit, it must be programmed to record the desired electromagnetic
phenomena. The process of selecting monitor thresholds is dependent upon the objective of the survey. If the
survey objective is to solve an equipment performance problem, then the monitorÕs threshold settings should
be related to the susceptibility of the equipment. Thus, the monitor should be programmed with magnitudes
for the voltage (and/or current) that will trigger the monitor to produce exception reports for disturbance
events that are expected to exceed the susceptibility limits of the sensitive electronic equipment under inves-
tigation. 
If the objective is to perform a general power quality survey, or to proÞle a single circuit, then the monitorÕs
threshold settings will be dependent upon the limitations of the monitorÕs event storage media, either paper
and/or RAM.
Different manufacturers have adopted different philosophies with regard to programming, data capture, and
display. Instruments can generate erroneous data depending on the measurement systems, grounding, shield-
ing, and hookups. Thus, an understanding of the internal workings of the power monitoring instrument is
critical if the data collected is to have value in the diagnosis and solution to power system variations.
The Þrst point to consider is the triggering level of the power monitoring instrument. Trigger thresholds tell
the monitor to ignore power system variations below the threshold and only trigger on those variations that
exceed the threshold. It is important to remember that missing disturbance recordings do not necessarily
indicate absence of electromagnetic phenomena. It only means that the power system variation did not trig-
ger the monitor. There are several techniques for triggering on various power system variations. These tech-
niques will vary depending on the manufacturerÕs design.
The second point to consider is the method or technique used to report the data collected by the power
monitoring instrument. The display of the data can be a Òhard copyÓ as with a data tape, visual display in a

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
44
format similar to an oscilloscope, or data may be stored on a disk or be transferred to a computer terminal or
PC for further analysis.
All power monitoring instruments are a compromise between cost, portability, and completeness. Instru-
ments are limited by their processing speed, data storage, printing speed, and memory buffers. Given that lit-
erally thousands of threshold excursions can occur in less than a second, these limits may be reached causing
lost data or uncaptured power system variations. Further, instruments that simply indicate a certain distur-
bance occurred cannot accumulate data to represent the number of occurrences, the characteristics, or the
relationship between the different power system variations.
Some power monitoring instruments allow various report formats to be turned on or off. Depending on the
application, these features can be used to make more efÞcient use of the instrument. Graphical instruments
may allow the user to view various waveforms in either visual format or on a hard copy printout. Both for-
mats give a ÒsnapshotÓ of the situation and not a real-time picture as would be available from an oscillo-
scope. However, these snapshots are very convenient for setting up the power monitoring instrument and
understanding the conditions existing on the electrical distribution system. In some cases, snapshots are suf-
Þcient for identifying the source or cause of a power system variation. In many cases, the end user wants to
measure the steady-state conditions. This requires an instrument capable of recording and conveniently
displaying steady state conditions for the complete monitoring period, which could be weeks or months.