Improper grounding accounts for a large percentage of damage and misoperation of sensitive electronic equipment. Multi-grounding renders equipment susceptible to lightning induced transient over-voltages as well as steady-state noise caused by ground loop currents. These problems can be avoided by
- Implementing a single-point grounding system and following the National Electrical Code (NEC) when installing the safety (equipment ground)
- Grounding electrode systems
- Properly designed and selected surge protective devices.
Approximately 90 percent of all grounding electrode systems for structures are driven rods. The NEC requires all driven rods to be a minimum eight feet in the earth and for multiple connected rods a minimum spacing of six feet between rods. The width of the rod has little effect on reducing impedance to earth, however, the depth of the rod and spacing for multiple rods have a significant effect. Frozen soil has about four times the resistivity of 50 degree Fahrenheit soil. Greater distances between multiple rods also decreases the overall impedance to earth. Site Audits often encourage managers to consider installing ten foot or longer rods a minimum of eight feet below the frost line and installing multiple rods at a distance equal to their depth in the soil.
The NEC requires a neutral-ground bond to the enclosure and to an earth ground electrode at the first disconnecting means of power supplied to a structure. This may be a disconnect switch or a main breaker in a power panel. The equipment grounding system for all panels, transformers and equipment must begin at this point. Do not install neutral-ground bonds at subpanels directly supplied power from the first disconnecting means which will result in ground loop currents (neutral current in the equipment grounding system) causing possible equipment malfunction. Do install a neutral-ground bond at the secondary of transformers where the continuity of the neutral conductor has been interrupted to avoid excessive neutral-ground voltage and possible lightning related damage.
All equipment in a structure must be grounded from the neutral-ground bond at the first disconnecting means to ascertain single-point grounding. Do not utilize individual driven rods or connection to a water pipe as the sole or an additional earth ground reference for equipment, this is a code violation, and could prevent circuit breakers from tripping in a fault condition or cause lightning equalization currents to flow through equipment cabinets. Do ascertain that a surge protective device has overcurrent protection installed in the form of a fused disconnect switch or circuit breaker installed in the panel being protected and that it is properly grounded at the ground bar in the panel.
For further information on Grounding, Site Audits or Continuing Education Courses contact Transtector Professional Services team.
