Investigators needed a number of tools to determine the actual causes of a mysterious electrical problem.
Variable-speed drives (VSDs) turn processing equipment at a plant that makes “mud” (joint compound) for finishing interior drywall. The facility is almost fully automated: It depends on the smooth operation of its electrical system—no hiccups. That hasn’t always been easy to achieve. Here’s an account of a particularly troubling event and how its root cause was uncovered and dealt with.
Experiencing a mysterious event
The power suddenly went off at one of the mud plant’s electrical panels; some outlets went off, others stayed on. Lighting supplied from the same board showed a similar pattern. Because this incident didn’t cause immediate disruption, plant operators were uncertain how long the power was truly off. They only realized there was a problem when they noticed that displays were no longer updating. While the PLCs continued to work, there was no display information to help determine what was happening. The operators eventually discovered that both the server running the SCADA system and the fiber optic switch connecting the PLCs had lost power (both devices are “protected” by a 20-minute UPS).
Power to the circuit was restored before electrical consultants (“the investigators”) arrived at the site. Their preliminary evaluation reveal-ed that a contractor in the adjacent building had, by agreement, fed a ground cable to the ground bar in the affected building. Initial measurements showed that the cable was carrying 2A of ground current. The investigators also discovered that the water pipe—which had numerous cables bonded to it—had eroded due to electrolytic action and was, consequently, leaking.
Conducting a detailed investigation
The first part of the detailed investigation was to measure the ground current on the system. The investigators confirmed, by using an earth ground tester, that there was 2A on the neighboring building’s cable and 1 or 2A on some of the mud plant’s own ground cables. The interconnecting bonds at the ground bar had clearly been installed at different times over many years, resulting in a “rat’s nest” of cables.
The investigators then tried to make measurements of ground current at the building’s ground rods. As is usual in most such cases, understanding a buried ground system is very difficult unless you are prepared to dig and investigate. After a short time, the investigators gave up and decided to look into the ground system at the main breaker outside of the building. They had to suit up in PPE (personal protective equipment) so they could make the measurement. At the same time, while the covers were off the switchgear, they carried out an infrared survey.
The infrared survey showed that the main switchgear and cable connections were running at a completely normal temperature—no hot spots were visible. The investigators then took the opportunity to check the voltage and current harmonics that were present using a three-phase power-quality analyzer. Readings showed very low levels of distortion; the supply looked very clean.
The investigation continued inside the building—at the service panel where the power had been lost. With the help of a current clamp meter, more details on the ground currents were confirmed. The investigators then decided to disconnect the neighbor’s ground connection. When this was done, the ground currents were measured once again, at which point it was found that the disconnected ground cable was not having any effect on the circuits within the problem building.
The investigating team then elected to carry out an infrared survey on the small distribution transfer in the service closet where the ground bar was located. When they removed the front panel of the transformer, the thermal imager showed hot spots on the transformer connection of phase C. This was the circuit that had failed. The loose connection was so bad that during the survey the connection sparked due to a small disconnection. In response, the investigators arranged to power the local panel down so repairs could be made. This, in turn, caused a disruption in the office facility and the production area because the local displays could no longer connect with the SCADA system.
Eliminating future hiccups
Solving problems related to the mysterious power outage at the mud plant was straightforward: All connections in the transformer needed to be tightened. The panel feeding the transformer was also checked and major issues discovered—more than 50% of the connections were loose. While the neutral was connected to ground at the correct point (according to NEC code), the panel was not properly grounded due to a loose connection on conduit. There also were many supplementary grounds on local circuits, again not in line with code. The main breaker in the panel was cracked and, thus, required immediate replacement.
The investigation that ultimately uncovered the causes of the outage and led to their elimination required several pieces of test equipment. Each tool was used to detect a different aspect of the situation:
- The earth ground clamp meter and the current clamp meter both showed high levels of ground current. Most likely, these results were due to the VFDs in both plants being connected to the same transformer.
- The power-quality analyzer showed that the voltage and current harmonics at the service entrance contained very little distortion.
- The thermographic imager indicated hot spots in an internal distribution transformer on one phase, which was the phase that had previously been exhibiting the problem.
Remedial work, including replacing two panels and bringing the associated wiring and grounding up to code, was carried out based on this detailed—and successful—investigation. MT
Frank Healy is a power quality specialist with Fluke. He has advised and consulted on many aspects of instrumentation for electrical engineering, including installation testing, earth/ground testing and power, for more than 20 years. E-mail: email@example.com.