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PowerCET Corporation
3350 Scott Blvd., Bldg. 55. Unit 1
Santa Clara, CA 95054 USA
Voice: 408/988-1346 | Fax: 408/988-4869
URL: http://www.powercet.com
E-mail: [email protected]
Power Quality Investigation Report
Grinder Enterprises Inc.
3350 Bot Blvd., Bldg 55
Somewhere Valley, CA
Background / Introduction
Grinder Enterprises has experienced a series of equipment failures thought to be power related. The
repair/replacement of the failed equipment and associated lost production has been substantial.
Grinder’s insurance carrier has indicated that they will no longer cover losses associated with the
electrical service until a survey of the electrical environment is completed and power monitoring
equipment installed on the main electrical service.
PowerCET Corporation was contacted and asked to provide a proposal for both an on-site investigation
and review of the electrical environment and installation of a permanent power quality monitoring
capability for the facility.
The electrical service for the Grinder facility is a 3P4W 208/120 at 1600Amps per phase. The building is
approximately 15,000 sq-ft and contains modern CAD/CAM and CNC machinery.
Photo 1 – Grinder Enterprises facility.
Date: 2/4/2010
Filename: Sample_MachineShopReport.doc
© 2010 by PowerCET Corporation. All rights reserved.
Page 1 of 13
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Photo 2 – Grinder facility, (satellite photo).
Methodology / Technical Approach
1. Visual inspection of failed drive unit shipped to PowerCET Corporation.
2. Initial site visit 3/26/09:
a. Install temporary power monitor to document electrical environment. [Fluke 1750]
b. Inspect electrical distribution and grounding for the facility
c. Conduct spot readings of low-frequency ground current flowing in the ground rod network
in the production area. [Fluke 345]
d. Conduct high frequency readings of the ground rod network in the production area.
[Tektronix THS720 DSO (2-channel) and HF current probes]
3. Follow-up site visit (4/15/09:
a. Retrieve power monitoring data for analysis
b. Conduct additional high frequency readings of ground rod network in production area.
[Tektronix THS720 DSO and Hioki 3145 Noise HiLogger]
4. Follow-up site visit (5/9/09):
a. Inspect installation of new tool ground conductors
b. Document interference levels in ground conductors after corrective action.
Key Findings / Recommendations
Visual Inspection of Failed Drive Unit
Inspection of the failed drive unit revealed a blown fuse on an internal circuit board. There were no
observable burned or blown components on the accessible circuit boards.
Another CNC machine has experienced a failure on start-up. The initial diagnoses (non-Grinder repair
technicians) a bad controller, drive or motor. Prior to replacing any hardware Dennis McGrath had the
unit’s program entries verified. It was discovered that the entry of the motor type was incorrect.
Correcting the entry returned the unit to full operation.
Date: 2/4/2010
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OBSERVATIONS: The visual evaluation of the failed drive and the experience with the program
entry being corrupted is typical of low levels high-frequency (HF) interference causing these types
of failures. The interference is circulating in the loops formed by the building’s electrical ground
and the network of ground rods connected to each machine tool.
Power Monitoring Results
A Fluke 1750 Three-Phase Power Recorder was installed on the main feed for the old 800A main
switchgear. The panel serves the majority of the machine tools as well as supplying the lighting and
administrative loads. The photo below shows the old 800A main, associated distribution and the 1750
installed during the monitoring period.
Photo 3 – Old main service switchgear, now distribution, where Fluke 1750 monitor was installed.
The result of the power monitoring was not remarkable. The following graphics document and
summarized the results of the recorded monitoring data.
Figure 1 shows the average RMS voltage and current levels during the monitoring period. The 5-day
cyclical patter of the current is typical of a single shift business operating Monday through Friday.
Figure 2 is the ITIC voltage tolerance curve for the monitoring data. The dots represent recorded events
based on threshold settings. Note that all the dots are within the normal limit lines.
Figure 3 is the voltage and current waveform from the 104Vrms sag which occurred on 3/28/09 at about
9:47AM. Analysis of the waveform data indicates that it was utility sourced and most likely a fault clearing
on an adjacent feeder. The depth of the voltage sage should not have caused operational problems for
any of the production equipment.
Date: 2/4/2010
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Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Figure 1 – RMS voltage and current time plots for the monitoring period.
Figure 2 – ITIC voltage tolerance curve showing recorded anomalies with in the normal
operational limits of the standard.
Date: 2/4/2010
Page 4 of 13
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Figure 3 – This is the voltage and current waveform recording from the most severe voltage sag
recorded during the monitoring period. The Line-Neutral voltage decreased to 104Vrms which is
not sufficient to cause problems in operating equipment.
Figure 4 (below) summarizes the power and energy for the monitoring period.
Figure 4 – This figure summarizes the power consumption (top) in kW and the accumulated
energy consumption (bottom) in KWh.
Date: 2/4/2010
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RECOMMENDATION: Both the review of the power monitoring data and the analysis of the failed
equipment do not indicate a power quality/reliability problem with the utility supply. The failures
and problems with the equipment appear to be more related with the electronic controls and not
the electrical interface. Based on this data there is insufficient justification to install permanent
power monitoring at this time. Should power monitoring be required in the future it can easily be
installed in the new main service cabinet with remote access to the building’s data network for
automated data collection and analysis.
Wiring / Grounding Inspection
Inspection of the electrical distribution revealed that that the neutral-ground bond in the old service
cabinets was not removed when the electrical entrance was moved to the new outside transformer and
service cabinets. The second bond results in neutral return current being injected into the building's
equipment grounding system and the network of ground rods.
Photo 4 – Old electrical service cabinets.
Photo 5 – Neutral bonding jumper in old
service cabinet.
RECOMMENDATION: Remove neutral-ground bonding jumper, Photo 5, in old service cabinets.
This jumper should have been removed when the new outside transformer and service cabinets
were installed.
CORRECTIVE ACTION: The jumper was removed by Franz Electric during site visit on 4/14/09.
High Frequency Ground Loop Monitoring
All the machine tools have been installed with a supplemental ground rod as has been the tradition in the
industry for ever…Photo 6. Older machine tools without electronic controls tolerated the ground loops
created by the ground rods. The modern CNC equipment with its electronics can, and often does,
experience problems from the network of ground loops resulting from the connection of the supplemental
ground rod.
Figure 5 (below) shows a simple sketch of the existing grounding network as it currently exists in the
facility. Anytime two different grounds are connected a loop is formed…if there is a difference of potential
between the individual grounds current will flow. In addition the physical loop area often impacts the
Date: 2/4/2010
Page 6 of 13
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
amount of interference flowing in the loop as a result of coupling for various interference sources. In the
example below there are 26 individual grounds…taken 2 at a time results in a total of 325 simple ground
loops. [If all possible combinations are taken into consideration the total number of distinctive ground
loops is quite large.]
Photo 6 – Representative example of tool ground rod installation
Figure 5 – Existing ground loop network created by ground rods at each tool.
Date: 2/4/2010
Page 7 of 13
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Figure 6 (below) is a representative example of the type of transient circulating in the ground loop network
described by Figure 5 (above). The peak value is over 400ma of current which is sufficient to cause
equipment mis-operation and memory register problems. This type of signal can be found on most of the
ground rod connections and is generally larger when the associated equipment is operating.
Figure 6 - Representative example of transient event recorded on ground rod conductor
connected to tool on production floor.
Figure 7 (below) is a plot take from the Hioki 3185 that records high frequency conducted interference in
seven frequency bands from 15kHz to 60MHz. The figure document the interference levels at the
difference frequencies.
Higher frequency signals tend to be more troublesome to the electronic equipment and in the figure below
the bulk of the interference resides in the four highest bands…1, 5, 20 and 60MHz.
Date: 2/4/2010
Page 8 of 13
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Figure 7 - Conducted high frequency interference levels recorded on ground rod conductor
connected to tool on production floor.
RECOMMENDATION: Eliminate the ground loop network associated with the installed tools by
disconnecting the existing round rods and installing a ground conductor from the power source
strapped to the respective conduit and terminated where the ground rod wire was connected.
Figure 8 (below) reflects grounding configuration after installation of the new ground conductors
and removal of the ground rod connections. This configuration re-references the equipment to a
single ground source and also decreases the loop area associated with the individual ground
loops.
Figure 9 illustrates the decrease in ground loop area which results in significantly decreased
interference signal levels.
CORRECTIVE ACTION: The ground rods were disconnected from the individual tools and a new
ground conductor was run from the associated electrical panel (Photo 7) , strapped to the conduit
and terminated on the respective equipment at the point where the ground rod had previously
been connected (Photo 8). [NOTE: The equipment grounding for the respective tool is
accomplished via the installed conduit. The external ground wire strapped to the tool’s conduit is
simply a replacement of the ground previously provided by the individual ground rods.]
Date: 2/4/2010
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Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Tools
PG&E
Service
Xfmr
Main CB
Dist CB
Ground
Rod
X
B
A
X
Ground
Rod
C
Dist
CB’s
Ground
Rod
X
Z
Figure 8 – Replacing the ground connection to the ground rod with a grounding conductor
common with the building’s electrical ground and strapping the conductor to the appropriate
conduit eliminates the ground loop and loop area as well.
Figure 9 – Before and after showing the elimination of the ground loop and large loop area.
Date: 2/4/2010
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Photo 7 – New ground conductors strapped
to conduit at branch circuit panelboard.
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Photo 8 – New ground conductor strapped to
conduit that replaced ground rod
connection.
CORRECTIVE ACTION: Figure 10 (below) shows the changes in high frequency signal levels after
the installation of the new grounding conductors—reducing the ground loop area—and removal of
the ground rod connections. Comparing Figure 10 to Figure 7 the change in interference levels is
easily seen. The 20MHz and 60MHz signals have virtually disappeared and only the 5MHz signal
remains and that has been substantially reduced.
It is possible to further reduce the amplitude of the remaining signals through installation of
ferrite cores (a type of filter) on the ground conductors. In Figure 10 on the far right there is a
decrease in signals levels…most apparent on the 5MHz trace…which was caused by clamping
three ferrite cores around the ground conductor being measured. Some decrease is noticeable in
the lower frequency traces, but not as much as with the 5MHz signal…ferrite cores have their
greatest attenuation above 1MHz. The ferrite cores can be added at anytime in the future should
the facility experience unexplained equipment operational problems.
RECOMMENDATION: All future equipment additions MUST be installed with a full-size (same size
as the current carrying conductors) equipment grounding conductor run in the conduit with the
current carrying conductors. In addition the use of supplemental ground rods should be avoided.
Date: 2/4/2010
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Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA
Figure 10 - Conducted high frequency interference levels recorded on ground conductor
connected to tool on production floor. [Note: This data is after removal of ground rod.]
Summary
Inspection of failed equipment indicates that equipment problems appear to be associated with the
electronic interfaces and controls and not the electrical (power) interface.
Power monitoring of the incoming utility supply does not indicate any significant anomalies that would
account for operational or equipment problems. Certainly, the facility will be susceptible to any utility
distribution fault clearing events which could result in lost work in progress, but should not cause failures
and should be infrequent.
The tools were all installed with the traditional supplemental ground rod driven through the floor and
connected to the tool. This forms a ground looped with the buildings electrical ground. As documented in
Figures 6 and 7 there is significant interference circulating in these loops. This type of interference can
(and does) depending on the amplitude of the signal, result in everything from equipment mis-operation to
memory registry corruption and, if of sufficient magnitude, component failure.
Date: 2/4/2010
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The corrective measures recommended and implemented have significantly reduced/eliminated ground
loops, their area and corresponding interference signals flowing through the facility.
Troubleshooting/diagnosing future equipment problems should include (1) verification of configuration
programming and (2) check for blown fuses. Power related failures generally manifest themselves with
visible damage to the input sections of the power supply (electrical interface). High frequency
interference generally causes mis-operation, system lock-up or program errors.
Report prepared by:
Bruce Lonie
President, PowerCET Corporation
Disclaimer
The information contained in this document is provided for educational purposes only as an example
on how to incorporate power monitoring data and other observations into a report format. It is not
intended to provide consulting advice for any specific problem or situation. This is a copyrighted
document and intended for individual use and should not be reproduced or distributed in any form
without specific written permission from PowerCET Corporation.
Date: 2/4/2010
Page 13 of 13
Sample Report
Site: Grinder Enterprises
Somewhere Valley, CA