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

7.4.5 Current considerations
Most monitors are capable of monitoring currents; a few monitors will allow monitoring of seven or eight
channels. This feature permits users to diagnose current related equipment performance problems such as
unwanted circuit breaker tripping, and motor, conductor, and transformer overheating. The proliferation of
large, nonlinear loads requires true rms measuring capability and measurement. Setting the current thresh-
olds for these applications usually involves setting the overcurrent thresholds relative to the NEC [B2] limits
or just below the manufacturerÕs nameplate speciÞcations, whichever value is lower.
An important application of current measurements in power system analysis is to help determine the direc-
tion, or origin, of the disturbance. Observing the change in current that occurs simultaneously with the volt-
age disturbance can suggest whether the origin of the disturbance is upstream or downstream from the point
being monitored. This technique can help to determine whether a neutral-ground voltage disturbance is
grounding conductor-related or power-circuit related. For this application, the current thresholds should be
set just above the circuitÕs steady-state current values. It is a good idea to initially monitor for 1 h to charac-
terize transient/inrush current effects on the voltage levels; then set the monitor above the ÒnormalÓ levels
monitored.
7.4.6 Monitor thresholds summary
The following summarizes the monitoring threshold steps:
a)
Determine monitoring objectives.
b)
Monitor in scope mode (if available) to observe steady-state waveform ßuctuations. Set monitor
thresholds just above these values.
c)
Let the monitor run with these sensitive thresholds until it captures about 20 events or 30 min,
whichever occurs Þrst. This will provide a record of background ßuctuations.
NOTEÑBackground voltage ßuctuations, even though below equipment susceptibility levels, may have a
cumulative degrading effect on the load equipment.
d)
Reset the monitor thresholds above (less sensitive than) the ßuctuations recorded using the sensitive
threshold settings. If time permits, repeat the process until no more than one event is recorded in a
30-min period. 

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
46