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

7.4.2 Preparation
At Þrst, set up the monitor and let it run in the summary mode for a half hour or so in order to obtain Þrst
order estimates of electrical environment characteristics. It may even be useful to let the monitor run for a
24-hour period in summary mode before Þnal threshold settings are made. One purpose is to keep from
Þlling up the memory with too many exceptions and/or printing-out the entire roll of paper unnecessarily.
This results in superßuous data if any of the settings are too sensitive relative to the magnitude of distur-
bances in the environment.
7.4.3 Electrical environment considerations
Selection of monitor thresholds can be a simple task if the objective of the survey is to monitor an equipment
performance problem in a benign electrical environment (where there are no signiÞcant waveform ßuctua-
tions). The monitorÕs thresholds may be set just below the susceptibility levels of the equipment under test.
The waveform disturbance can then be extracted from the body of waveform ßuctuation records based upon
a time correlation with the equipment malfunction, or when the ßuctuation clearly exceeds the equipmentÕs
susceptibility levels.
NOTEÑThresholds must be set lower than the equipmentÕs susceptibility levels to ensure that the disturbance waveform
is recorded.
Selecting thresholds for an electrically active environment (such as the input to an adjustable-speed drive) is
more difÞcult. If the thresholds are too low, continuous ßuctuations will incapacitate the monitor, possibly
preventing it from capturing more signiÞcant disturbances.
7.4.4 Equipment susceptibility considerations
The best threshold settings are those that relate directly to the susceptibility levels of the sensitive electronic
equipment being investigated. Susceptibility levels for an electronic load may be obtained from the manu-
facturer, or from past surveys performed on that particular electronic load. This information is rarely avail-
able for any speciÞc piece of equipment (the familiar CBEMA curve used in IEEE Std 446-1987 [B12] and
elsewhere is only a consensus guide and as such tends to be conservative.) When speciÞc information is
available, the susceptibility levels do not always match the threshold categories of the monitor. 

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
45
Susceptibility levels derived from generally recognized industry standards (such as CBEMA) often work
well, especially when used in conjunction with the manufacturerÕs speciÞed levels. Subclause 8.2, table II in
FIPS PUB 94 [B6], provides some representative power quality attributes for reference. The table lists envi-
ronmental attributes or disturbances, with values for Òtypical environmentÓ and also Òtypical acceptable lim-
its for computers and power sourcesÓ (with two categories of ÒnormalÓ and ÒcriticalÓ limits). After study of
this type of information (including FCC emission and immunity/susceptibility considerations) the thresholds
might even become Òsecond nature.Ó
Table 3 lists initial thresholds that might be considered as rules of thumb. The thresholds are speciÞed for
120 V equipment in the US, and with general equipment susceptibility considerations as derived from FIPS
PUB 94 [B6] and IEEE Std C84.1-1989 [B1]. The speciÞcations in this table Þt best for normal 120 V loads
that are neither overly sensitive to nor highly tolerant of voltage ßuctuations.
Monitor thresholds shall be set below (more sensitive) equipment susceptibility levels to ensure that distur-
bances are recorded. The aging of equipment, discrepancies between equipment and monitor susceptibility
nomenclatures, and the accuracy of the monitor are factors that could result in the malfunction of equipment
at voltage levels below its expected susceptibility levels.