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

7.5 Monitoring period
7.5.1 Objective
The monitoring period is a direct function of the monitoring objective. Usually the monitoring period
attempts to capture a complete power period, an interval in which the power usage pattern begins to repeat
itself. An industrial plant, for example, may repeat its power usage pattern each day, or each shift. Depend-
ing on the monitoring objective, it may be necessary to monitor as little as one shift.
7.5.2 Baseline power monitoring
Baseline power monitoring is a relatively short process. Its purpose is to document the power proÞle at a spe-
ciÞc site or location. Primary information is steady-state and transient extremes. Other parameters such as
frequency or RFI noise can also be of speciÞc interest. Baseline monitoring is primarily used prior to install-
ing equipment to verify power speciÞcation compliance. The recommended monitoring period is deÞned as
Table 4ÑSuggested threshold settings for 120 V loads 
Conducted phase 
voltage thresh-
olds
Category
Suggested setting
Comments
Sag
108 V rms
Minus 10% of nominal supply voltage
Swell
126 V rms
Plus 5% of nominal supply voltage
Transient
200 V
Approximately twice the nominal phase-to-neutral voltage
Noise
1.5 V
Approximately 1% of the nominal phase-to-neutral voltage
Harmonics
5%
THD
Voltage distortion level at which loads may be affected
Frequency
±Hz
Ñ
Phase imbalance
2%
Voltage imbalance greater than 2% can affect equipment. 
(Three-phase induction motors should be derated when oper-
ated with imbalanced voltages [B11]).
Conducted 
neutral-to-
ground 
differential 
voltage 
thresholds
Swell
3.0 V rms
Typical level of interest for neutral and/or ground problems
Impulsive
transient
20 V peak
Ten to twenty percent of phase-to-neutral voltage
Noise
1.5 V rms
Typical equipment susceptibility level