Getting down to earth

By Control Engineering Staff April 1, 2000

W hen a personal computer locks up for no apparent reason, or a load cell stops updating for a short period the tendency is to mumble some harsh words and do what’s necessary to restore operation of the faulty device. Often the cause of the interruption goes undetected and people begin to accept such occurrences as ‘just the way it is;’ but more times than anyone knows the root cause of intermittent interruptions is caused by a poor grounding system.

Grounding and bonding of electrical systems is mainly used to control dangerous overvoltages and is very important for personnel safety. However, increased use of commercially available microprocessor-based products and the urgency to get them operational as quickly as possible increase opportunities to introduce ‘dirty grounds’ into the business and manufacturing environment. Dirty grounds seldom compromise personnel safety, but they sure can create phantom business interruptions.

A good electrical ground for process control and communication circuits requires a resistance to ground of 1/2 ohm or less and that is not always easy to obtain. Actually, the earth is a relatively poor conductor of electricity, but if the area of a path for current is large enough, resistance can be quite low and the earth can become a fairly good ‘conductor.’ Whether examining the effectiveness of existing ground grids or prospecting for good new ground locations, you need specialized instruments, formalized and documented testing procedures, and knowledgeable practitioners.

Also, it is not enough to check the earth resistance at the time of installation. Ground grids are systems, and like any system they require periodic testing and maintenance to keep them functioning at peak performance. Consider three factors that can change earth electrode requirements over time:

Expanding facility size and/or incorporation of different electrical equipment can render formerly suitable low earth resistance standards obsolete;

Increased use of nonmetallic pipes and conduits can change the overall effectiveness of low-resistance ground connections; and

Decreases in the areas water table may result in a drier, high-resistance earth.

When the earth-electrode resistance is not low enough, there are several ways you can improve it.

Improving earth’s resistance

As you might suspect, driving a longer rod deeper into the soil decreases the rod’s resistance. In general, doubling the rod immersion length reduces resistance about 40%. For example, a rod driven 2 ft into the ground and providing a resistance of 88 ohms will provide resistance of about 50 ohms when driven another 2-ft deeper.

Increasing the electrode diameter is another way of lowering resistance, but only a little. Doubling a rod’s diameter reduces the resistance only about 10%. This slight improvement makes increasing a rod’s diameter more beneficial to improve mechanical strength than reducing resistance.

Well-spaced, multiple rods driven into the earth provide parallel paths and is another way to improve the earth’s resistance. In effect this provides two resistances in parallel, however the rule for two resistances in parallel being one-half does not exactly apply. The reduction for two equal-resistance rods is about 60%; three rods, is about 40%; and four rods is about 33%.

When using multiple rods, they must be spaced further apart than the length of rod in the ground. For example, if you drive two rods in parallel and provide 10-ft spacing between the two rods, resistance is lowered about 60%. Increasing the spacing to 20-ft reduction is about 50%.

When you can’t drive ground rods deeper, chemical treatment of the soil is another way of improving earth-electrode resistance, but because chemicals are washed away by rainfall and natural drainage, this can become a high-maintenance solution.

When electronic equipment experiences intermittent interruptions, the best course of action is to find and eliminate the cause, starting with how equipment is grounded.

Dave Harrold, Senior Editor

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