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

4.4.3.3 Sustained interruptions
The decrease to zero of the supply voltage for a period of time in excess of 1 min is considered a sustained
interruption. Voltage interruptions longer than 1 min are often permanent in nature and require manual inter-
vention for restoration. Sustained interruptions are a speciÞc power system phenomena and have no relation
to the usage of the term outage. Outage, as deÞned in IEEE Std 100-1992, does not refer to a speciÞc phe-
nomenon, but rather to the state of a component in a system that has failed to function as expected. Also, use
Figure 8ÑInstantaneous voltage swell caused by a SLG fault

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
20
of the term interruption in the context of power quality monitoring has no relation to reliability or other con-
tinuity of service statistics.
4.4.4 Voltage imbalance
Voltage imbalance (or unbalance) is deÞned as the ratio of the negative or zero sequence component to the
positive sequence component. The negative or zero sequence voltages in a power system generally result
from unbalanced loads causing negative or zero sequence currents to ßow. Figure 9 shows an example of a
one-week trend of imbalance measured at one point on a residential feeder.
Imbalance can be estimated as the maximum deviation from the average of the three-phase voltages or cur-
rents, divided by the average of the three-phase voltages or currents, expressed in percent. In equation form
voltage imbalance = 100 
´ (max deviation from average voltage)/average voltage [B11]
For example, with phase-to-phase voltage readings of 230, 232, and 225, the average is 229. The maximum
deviation from the average among the three readings is 4. The percent imbalance is 100 
´ 4/229 = 1.7%.
The primary source of voltage imbalance less than 2% is unbalanced single phase loads on a three-phase cir-
cuit. Voltage imbalance can also be the result of capacitor bank anomalies, such as a blown fuse on one phase
of a three-phase bank. Severe voltage imbalance (greater than 5%) can result from single-phasing conditions.