Case Studies

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To learn more, he pushed the
event table button, checked the
numeric values, and saw distinct
voltage sags on the branch
circuit. (Fig. 6)
The panel was clearly carry-
ing too much load, most likely
from one or more of the facility’s
large motors. That meant the
hub was seeing high voltage
from the grounding problem and
low voltage from the sags, both
of which were causing the hub
to go down.
Conclusion:
Due to the cost and time
required to install a new
receptacle on the branch circuit,
the facility decided to add a
UPS in front of the power
supply to the hub. Since the
remaining equipment on that
line wasn’t disrupted by the
sags, no further actions were
required. The consultant
downloaded a report of the
benchmark power quality
measurements he’d made
and gave it to the facility,
who agreed to set up a regular
preventive maintenance
schedule.
He traced the branch circuit
to the isolated ground recepta-
cle but couldn’t find a
grounding conductor. That was
a little too isolated! Not having
a grounding conductor caused
an open ground condition. That
was not only a problem for the
electronic equipment, but also
a safety hazard to personnel
and equipment. As a temporary
fix, he recommended replacing
the isolated ground receptacle
with a legal, standard recepta-
cle. The standard receptacle
would allow the client to use
conduit as the standard equip-
ment grounding providing
proper grounding for the
branch circuit.
During his examination of
the branch circuit, the consult-
ant also discovered that the
circuit was supplied from a
general-purpose electrical
panel. Since he knew the hub
would require higher quality
power than average equipment,
he thought he should go ahead
and check the panel for any
kinds of sags or swells.
He pressed the Monitor
button (Fig. 4) and set the
recording time for 24 hours.
After the first 15 minutes, he
pushed the trend button while
continuing to record in the
background. (Fig. 5) Shortly
into the cycle he saw a dip in
the line.
2 Fluke Corporation Power Quality Case Study: Network hub failure
Fig. 5 Dip swells
Fig. 6 Dip swells
Fig. 4 Set recording time