Poor grounding not only contributes to unnecessary downtime, but a lack of good grounding is also 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. The following 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 (International Electrotechnical Commission) »
CENELEC (European Committee for Electrotechnical Standardization) »
IEEE (Institute of Electrical and Electronics Engineers) »
However, good grounding isn't only for safety; it is also used to prevent damage to industrial plants and equipment. A good grounding system will improve the reliability of equipment and reduce the likelihood of damage due to lightning or fault currents. Billions are lost each year in the workplace due to electrical fires. This does not account for related litigation costs and loss of personal and corporate productivity.
Why Test Ground 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 eaten away.
Grounding testers, like the Fluke 1623 and 1625, are indispensable troubleshooting tools to help you 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 your normal Predictive Maintenance plan. During these periodic checks, if an increase in resistance of more than 20 % 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.
What is a Ground and What Does it Do?
The NEC, National Electrical Code, Article 100 defines a ground as: "a conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth." When talking about grounding, it is actually two different subjects: earth grounding and equipment grounding. Earth grounding is an intentional connection from a circuit conductor, usually the neutral, to a ground electrode placed in the earth. Equipment grounding ensures that operating equipment within a structure is properly grounded. These two grounding systems are required to be kept separate except for a connection between the two systems. This prevents differences in voltage potential from a possible flashover from lightning strikes. The purpose of a ground besides the protection of people, plants and equipment is to provide a safe path for the dissipation of fault currents, lightning strikes, static discharges, EMI and RFI signals and interference.
What is a Good Ground Resistance Value?
There is a good deal of confusion as to 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.