What the Eagle™ Records
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Suggested Uses
The power of the histogram is that every cycle is included in the
report. Every cycle during the recording session is reflected in the
count of one of the bins.
If all the counts in a histogram are totaled, then the result is how
many cycles, the recording session lasted (minus any time under
a power outage).
Histograms are presented as a bar graph and a report. The report
is in some ways easier to read than the graph. The absolute
highest and lowest voltages during the recording session are
found by finding the highest and lowest bins with a non-zero
count. At that point, you also know how many cycles the voltage
was at those extremes, and by glancing at the nearby bins, you
know how many cycles the voltage was near those extremes. For
example, if all the bins below 110V are zero, then you immediately
know that there was not even a single cycle of voltage below 110V
anytime during the recording session. If the count at 111V is
1,352,200, then the voltage was at 111V for over 6 hours
(1,352,200 = 6.26 × (60 × 60 × 60)). By totaling the counts for all
the bins in a voltage range (for example, 0 to 150V), you find how
many cycles the voltage was in that range.
More complicated power quality questions can be answered by
exporting the histogram data to a spreadsheet. By dividing each
count by the total of all the counts, the histogram data is
normalized, and can represent a sample probability distribution
function. If a normal, or bell-shaped probability distribution is fit to
this data, a standard deviation is created that can be used to
answer “what high and low limits does the line voltage meet
99.99% of the time?”. A cumulative sum over the data will convert
the distribution function into a sample cumulative probability
function. Correlations between channels can be performed by
comparing the probability functions of channels.
For the voltage histogram, the user is generally interested in the
few cycles that are outside certain limits, not the vast majority of
cycles that are perfectly normal. These few cycles usually
represent power quality issues.