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

8.2.3 Interpreting summaries
Once certain the summarized data is valid, the Þrst round of interpretation can take place. This may not
achieve the desired goal (more investigation may be needed), but it will provide needed information to pro-
ceed in analyzing the data further.
The timeline summary provides a chronological overview of what occurred during the monitoring interval.
This histogram should be compared to load cycles, failure logs, equipment speciÞcations, results from per-
sonnel interviews, or any other information collected.
If the problem involves equipment malfunctions, isolate the disturbances to which the device is sensitive, if
possible. The disturbance/time histogram may quickly show whether any disturbances occurred, and if so,
when they happened. Patterns in time or disturbance characteristics may show the true source of the problem.

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
49
As an example, consider the following scenario:
A workstation intermittently locks up. No consistent failure pattern, such as time of day, is noticed. Wiring
and grounding have been veriÞed. Several lock-ups occurred during the one day monitoring time.
The timeline summary showed the presence of many transients, L-N sags, and N-G voltage increases. Occa-
sionally, an L-N sag and an N-G rise had levels far exceeding the other levels. These excessive levels always
occurred simultaneously and at the same time the workstation locked up. The disturbance/time summary
showed a deÞnite correlation between the high sag/swell disturbances and the lock-ups.
Further investigation showed the existence of a laser printer and a photocopier on the same circuit as the
workstation. These two devices constantly caused small transients, L-N sags, and N-G increases in voltage.
However, when both the printer and copier were used at the same time they caused an L-N sag and an N-G
increase that were much worse. The magnitude of the intermittent N-G increase was found to be the cause of
the lock-ups.