Avoid electrical ground loops
Ground loops (also referred to as "noise") in an electrical system result from unwanted current that flows in a conductor connecting two points that are supposed to be at the same electrical potential, but aren't. When this condition occurs in instrumentation loops, adding electrical current or voltage to, or subtracting it from, the instrument signal is often detrimental to control system perf...
Getting down to earth
Ground loops (also referred to as ‘noise’) in an electrical system result from unwanted current that flows in a conductor connecting two points that are supposed to be at the same electrical potential, but aren’t. When this condition occurs in instrumentation loops, adding electrical current or voltage to, or subtracting it from, the instrument signal is often detrimental to control system performance.
Depending on the nature of the noise, the performance and reliability of different system components can be affected. For instance, traditional or classical 4-20 mA I/O products are immune from all but high frequency noise, such as that produced by variable frequency drives and insulated gate bipolar transistor (IGBT) power switching devices.
Contrary to popular belief, digital data signals used in fieldbus communication architectures are also susceptible to electrical noise.
Minimizing the influences of electrical noise on control system performance and reliability requires:
Using isolated ac power sources;
Establishing a single, common system ground point;
Providing isolations for low-voltage signals (for example, thermocouples);
Minimizing undue influence resulting from stray magnetic fields; and
Selecting appropriate cables and pathways, including adequate cable separation.
Single, common ground
Improper grounding practices, such as grounding cable shield wires at both ends or at the wrong end, are well-documented sources of introducing electrical noise; but are a reoccurring problem in control and instrumentation systems.
Cable shield wires should be grounded at one end, preferably at the power source end. The other end should be taped and protected. (See ‘Proper Shield Grounding Technique’ diagram.) Most faulty grounding system designs are the result of mixing power and grounding sub-systems (such as ac, dc, shields, cabinets, etc.) and/or failing to establish a single, common separate (isolated) ground point on the plant’s ground grid system.
Power and grounding sub-systems should remain separated from one another until the last possible and/or practical connection point. Then, and only then, should the sub-systems be joined. For example, consider a grouping of four control and instrumentation cabinets. The groupings ac-, dc-, shield-, and cabinet-grounds should remain separated throughout the cabinets and connected to an isolated cabinet grouping ground bar. The insulated ground lead from the cabinet grouping ground bar should be routed to a master ground bar then joined by other cabinet groupings. The insulated ground lead from the master ground bar should be routed to an unshared (isolated) point on the plant ground grid.
In large plants, connecting all master grounding points to the same point on the ground grid may not be practical. Assuming the plant ground grid is properly designed, the difference in electrical potential between grid connection points should be negligible, making it permissible to use different ground grid connection points.
Left uncorrected, electrical ground loops can cause corrupt digital and analog signals, and/or quick or slow equipment damage. Finding and correcting the problems can be tedious and time consuming, but there really aren’t any alternatives.
For ordering information about a Control Engineering book on control system power and grounding, search on ‘77782’ at www.controleng.com .