Regular grounding measurements can be made without taking equipment offline.
By Jit Patel
Electrical systems must be grounded so that in the event of a lightning strike or utility overvoltage, current will find a safe path to earth. A ground electrode provides the contact between the electrical system and the earth. To ensure a reliable connection to earth, electrical codes, engineering standards and local standards often specify a minimum resistance for the ground electrode.
Poor grounding can contribute to downtime in electrical, cable and telecom utilities. Beyond that, a lack of good grounding is dangerous and increases the risk of equipment failure. Without an effective grounding system, we could be exposed to the risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas. If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people.
Because of the importance of earth grounding, The International Electrical Testing Association specifies ground electrode testing every three years for a system in good condition with average up-time requirements. In addition, these organizations have recommendations and/or standards for grounding to ensure safety:
• OSHA (Occupational Safety Health Administration)
• NFPA (National Fire Protection Association)
• ANSI/ISA (American National Standards Institute and Instrument Society of America)
• TIA (Telecommunications Industry Association)
• IEC (Intl. Electrotechnical Commission)
• CENELEC (European Committee for Electrotechnical Standardization)
• IEEE (Institute of Electrical and Electronics Engineers)
Why Test Grounding Systems?
Over time, corrosive soils with high moisture content, high salt content, and high temperatures can degrade ground rods and their connections. So, although the ground system when initially installed had low earth ground resistance values, the resistance of the grounding system can increase if the ground rods are corroded away.
Grounding testers are indispensable troubleshooting tools to help maintain uptime. With frustrating, intermittent electrical problems, the problem could be related to poor grounding or poor power quality.
That is why it is highly recommended that all grounds and ground connections are checked at least annually as a part of a normal preventive maintenance plan. During these periodic checks, if an increase in resistance of more than 20 percent is measured, the technician should investigate the source of the problem and make the correction to lower the resistance by replacing or adding ground rods to the ground system.
The US National Electrical Code (NEC) gives two principle reasons for grounding a facility:
• Stabilize the voltage to earth during normal operation.
• Limit the voltage rise created by lightning, line surges or unintentional contact with higher-voltage lines.
Current will always find and travel the least-resistance path back to its source, whether it’s a utility transformer, a transformer within the facility or a generator. Lightning, meanwhile, will always find a way to get to the earth.
In the event of a lightning strike on utility lines or anywhere near a building, a low-impedance ground electrode will help carry the energy into the earth. The grounding and bonding systems connect the earth near the building with the electrical system and building steel. In a lightning strike, the facility will be at approximately the same potential. By keeping the potential gradient low, damage is minimized.
If a medium voltage utility line (over 1,000V) comes in contact with a low voltage line, a drastic overvoltage could occur for nearby facilities. A low impedance electrode will help limit the voltage increase at the facility.
A low impedance ground can also provide a return path for utility-generated transients.
Ground Testers and how They Work
There are two types of ground resistance testers: three-point and four-point ground testers and clamp-on ground testers. Both types apply a voltage on the electrode and measure the resulting current.
A three-pole or four-pole ground tester combines a current source and voltage measurement in a “lunch box” or multimeter-style package. They use multiple stakes and/or clamps. Ground testers have the following characteristics:
• AC test current. Earth does not conduct DC well.
• Test frequency that is close to, but distinguishable from the power frequency and its harmonics. This prevents stray currents from interfering with ground impedance measurements.
• Separate source and measure leads to compensate for the long leads used in this measurement.
• Input filtering designed to pick up its own signal and screen out all others.
Clamp-on ground testers are different because they have both a source transformer and a measurement transformer. The source transformer imposes a voltage on the loop under test, and the measurement transformer measures the resulting current. The clamp-on ground tester uses advanced filtering to recognize its own signal and screen out all others.
As an example, the Fluke 1630-2 FC Earth Ground Clamp can measure earth ground loop resistances for multi-grounded systems using a stakeless test method. This test technique eliminates the dangerous and time consuming activity of disconnecting parallel grounds as well as the process of finding suitable locations for auxiliary ground stakes. Earth ground tests can also be performed in places that haven’t been considered: inside buildings, on power pylons, or anywhere there isn’t access to soil.
With this test method, the earth ground clamp is placed around the earth ground rod or connecting cable. Earth ground stakes are not used. A known voltage is induced by one side of the clamp jaw, and the current is measured by the other side of the clamp jaw. The clamp automatically determines the ground loop resistance at this ground rod. This technique is especially useful for multi-grounded systems typically found at utilities, commercial facilities or industrial locations.
The Fluke 1630-2 FC works on the principle that in parallel/multi-grounded systems, the net resistance of all ground paths will be extremely low as compared to any single path (the one under test). So, the net resistance of all the parallel return path resistances is effectively zero. Stakeless measurement only measures individual ground rod resistances in parallel to earth grounding systems. If the ground system is not parallel to earth, then you will either have an open circuit or be measuring ground loop resistance.
Ground Testing Safety
Always use insulated gloves, eye protection and other appropriate personal protective equipment when making connections. It is not safe to assume that a ground electrode has zero voltage or zero amps. To perform a basic ground test (called Fall-of-Potential) on an electrode, the electrode must be disconnected from the building. Newer methods, such as earth ground clamps, allow accurate testing with the electrode still connected.
What is a Good Ground Resistance Value?
There is confusion about what constitutes a good ground and what the ground resistance value needs to be. Ideally, a ground should be of zero ohms resistance.
There is not one standard ground resistance threshold that is recognized by all agencies. However, the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less.
The NEC has stated to “Make sure that system impedance to ground is less than 25 ohms specified in NEC 250.56. In facilities with sensitive equipment it should be 5.0 ohms or less.”
The telecommunications industry has often used 5.0 ohms or less as their value for grounding and bonding.
The goal in ground resistance is to achieve the lowest ground resistance value possible that makes sense economically and physically. UP
About the author: Jit Patel is the product manager for electrical products at Fluke Corp. He has a Bachelor of Science from the University of Wolverhampton and has been with Fluke for the past 20 years beginning as a Technical support engineer based in the UK.