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

8.2 Interpreting data summaries
One of the Þrst steps in interpreting the data from a power monitor is to examine a summary of the data
acquired over some time interval. This interval may be anywhere from an hour to a month, but it generally
should be at least one business cycle. See clause 7 for more information on the length of the monitoring
period. Looking at the summary of the data will provide an important overview perspective and quickly
identify more important data to be examined in greater detail.
8.2.1 Summary preparation
The type and detail of summary data should reßect the initial goals and objectives. This is one reason to have
clear goals and to properly set up the power monitor. A summary will typically focus on two items. First,
data should be placed on a timeline to allow quick chronological correlation. Second, data should be catego-
rized by disturbance and time. This summarizes the data based on particular disturbances.
Building a summary may focus on either one or both styles depending on the objectives of the monitoring.
Keep in mind that data being produced does not necessarily indicate a power problem. If many reports are
produced it could be that the thresholds were set too tight and examining each report in detail may be a waste
of time. On the other hand, no disturbances recorded may indicate that thresholds were not set tight enough,
reports were not turned on, or the monitored time did not coincide with the disturbance. 
8.2.2 Summary reality check
The reality check is more of a safeguard technique than an analysis technique, but its importance should not
be underestimated. All power disturbance recording devices are just tools subject to the skill and knowledge
of the user. No matter how careful we are to eliminate wrong data, some may creep in. No matter how cer-
tain we are of an interpretation, it must make sense in the real world. The reality check assures that the
recorded data are reasonable based on the circuit conÞguration and monitor connection method.
A reality check of the summarized data should be performed before attempting to interpret them. This
involves making sure such things as magnitudes are reasonable (i.e., how would an L-N sag to 200 V exist
on a 120 V system?), time stamps are within the monitoring window, waveforms Þt on the graph scale, and
so forth.