Power Quality Case Study: TWO CASES OF GROUNDING PROBLEMS Dean. A. S. Sempio Corporate Business Group – Technical Services MERALCO, Philippines Email: corpbus.techs.staff@meralco.com.ph
Correction of Grounding Systems Eliminated Nuisance Tripping, Equipment Failures and Shock hazards Introduction The proliferation of computers and electronics are changing the way people look at power supplies and grounding. Factories now use computer controlled equipment to be more productive and efficient; and still have excellent quality control over their products. Many factories that use computers are now aware of problems that come with the use of computers – their production became more sensitive to the characteristics of power and to grounding. In the past, grounding is only associated with safety. The use of computers and other sensitive equipment is changing the way people look at proper grounding methods and practices. Case 1: Computer Production Equipment
Controlled
A factory has upgraded their equipment by installing computer based controls. The computers are ordinary desktop computers that have an interface with the production
equipment. The factory has even installed UPS (uninterruptible power supply) units for reliability and to protect the computers mounted on the equipment. In spite of that, the factory still experiences frequent breakdown of their computer’s hard disk drives – the hard drives that are supposed to last for at least 2 years are being replaced in just a matter of months. The effects of the breakdowns of the hard drives are not that superficial – there is also the loss of production time when the lost programs had to be re-installed in the hard drives. Power Quality Study The Power Quality engineers interviewed the actual users of the equipment and conducted site inspection. They noted the absence of a facility equipment grounding system. Although the factory transformer was grounded
(system grounding), there were no equipment grounding conductors for the production equipment. The production equipment was “grounded” by connecting wires to the nearest metal conductor in the area. A simple representation of the factory “grounding” is shown: Ground Potential Rise (GPR) during transient conditions and the unstable ground potential of the equipment reference ground were the suspected causes of the frequent breakdown of the hard drives. This is due to the equipment “grounding” that is not referenced to the main power supply – A low impedance reference to ground or functional ground of the equipment. Solution The factory decided to install an equipment grounding system that is referenced / connected to the main transformer grounding. The purpose is to have all the “grounds” attain the same potential; when “grounds” of circuits supplied by a particular source have practically zero potential difference with respect to each other then the circuit, control and logic voltages will be stable; ground loops will be eliminated. All of the factory’s production equipment is now connected to equipment grounding that is part of the system grounding. Under the new grounding
setup, the number of incidences of hard disk breakdowns has dropped dramatically. In fact, no hard disks were replaced in the succeeding three years after the new grounding system was installed.
A power quality monitoring equipment was installed at the panel serving one of the production equipment suffering from “nuisance tripping”. The following data was recorded:
equipment as well as the incidence of workers getting electric shocks have virtually disappeared.
The line to ground voltage increased dramatically in one of the circuit conductors that what was later found out to be a fault inside a production equipment. This is also the reason why the equipment body becomes energized. Despite the rise in voltage, no protective devices were activated.
CONCLUSION People are now becoming aware of proper grounding practices with the application of electronics and computers in production equipment. As shown in the case studies, plants need to improve the grounding of their electrical system as they upgrade and automate their production equipment. Proper grounding is no longer just about safety, it is also essential to the proper and continuous operation of sensitive electronic based equipment.
Case 2: Neutral to Ground Voltage A factory is experiencing abnormal line to ground voltages. These abnormal voltages are being blamed for “nuisance tripping” of some production equipment. In some instances, the machine operators get shocked when they come in contact with some parts of their machines or on some metal outcroppings near their equipment. The incidents happen only in certain places within the factory. Power Quality Study The Power Quality engineers interviewed the production workers that come into contact with the equipment that suffer from “nuisance tripping”; they also conducted site inspection. The factory transformer was also checked and it was found out that there was no equipment grounding conductor coming from the transformer. The production equipment “grounding” consists of a wire connected to a panel box embedded in cement.
Solution The absence of proper grounding from the transformer to the equipment explains how the ground voltage “floats” to abnormal levels without tripping the protective devices. Faults in the electrical system can continuously energize equipment if there is no effective grounding that is referenced to the transformer secondary. The factory installed an equipment grounding system referenced / connected to the grounded neutral of the transformer secondary. The “nuisance tripping” of production
Disclaimer Engr. Dean. A. S. Sempio is a member of Corporate Business Group – Technical Services of the MERALCO, Philippines This article is for general information only and is not a substitute for professional advice where the facts and circumstances warrant. The views and opinion expressed above are those of the author and do not necessarily represent the views of the Institute.