A formal definition of a ground loop that is very general is provided in IEEE Std. 100-1991, IEEE Dictionary as follows: . . . a ground loop is “formed when two or more points in an electrical system that are nominally at ground potential are connected by a conducting path such that either or both points are not at the same potential.” While this is a good general purpose definition, it is not sufficiently specific for use when dealing with signal level circuits and grounding connections. Therefore, a more specific and useful definition as provided in this document is as follows:
Ground Loop (unwanted)
Any conductive path involving “ground” via a grounding or grounded conductor or the earth itself, through which any part or all of the desired signal process current is passed, so that it may be algebraically added to any unwanted current such as “noise” that may also be flowing in the shared ground path.
Ground Loop (desired)
Any number of paralleled conductors and connections involving grounded or grounding conductors of any description, or the earth, and through which it is intended to conduct ac system ground fault or lightning currents, for the purpose of reducing arcing, touch potential hazards, and as an aid to fault clearing.
Ground Loop (benign)
Either of the above two ground loops or a combination of them, where despite the existence of the ground loop, no electrical hazards are created and no signal processes are disrupted, by its existence.
Since we are concerned with the unwanted effects of ground loops on signals, we will mainly use the first of the above definitions.
- There are those that typically employ coaxial cable with only one center conductor for the signal transport process and where the outer braid is grounded at both ends. This includes many kinds of circuits used with computers, process control systems, and similar installations.
- There are those that use a common conductor which is grounded, as a part of the signal return path for one or more signals on a multi-conductor cable. Standard signal protocol, RS-232 usually falls into this category.
Point #5
Use opto-isolators which can provide several kV of isolation for the data path that they are used on. These are available as add-on data transmission protocol converters for most popular forms of data circuits.
This is a very useful retrofit option for data circuits being affected by surges and ground loops. Surge protection devices (SPD) are also recommended to be applied to these circuits if protection from the higher voltages associated with larger currents is needed.
Point #6
Other forms of protocol converters can be applied to standard forms of signal circuits to make them less susceptible to common-mode noise on grounding conductors associated with the signal path. For example, a conversion from RS-232 to RS-422 or RS-485, etc. should be considered in especially noisy environments.
Point #7
Improve the shielding provided for the data signal cables. Place the cables into well and frequently grounded metal conduits or similar raceways.
Point #8
Follow the recommendations for installing signal cables in IEEE Std. 1100, Recommended Practice for Powering and Grounding Sensitive Electronic Equipment.
Equipment interconnected by data signal cables and located on different floors or that is widely separated in a building, may not be able to effectively use some or all of the above solutions, except those involving optical isolation and certain of the protocol conversion techniques. This occurs since the terminating equipment for the signal cables is likely to be powered from different branch circuits, panelboards, and even separately derived ac systems. Therefore, the associated equipment ground references are likely to be at different potential at least some of the time.
While the best solution to the above situation involves either fiber optic or opto-isolation techniques, it is often possible to achieve good performance by providing each of the separate locations with an SRG, and then interconnecting the SRGs with widely spaced apart and multiple grounding/bonding conductors, solid-bottom metal cable trays, wireways, or conduits containing the data signal cables.
An example of using widely spaced grounding/bonding conductors to interconnect two SRG areas is when there is structural building steel available and when it can be used in this role.
Since structural steel columns are installed on standard spacings in a given building, these columns can typically be used for the purpose. Wide spacing is necessary since the conductors involved are inductors and the mutual inductance between such conductors that are not widely spaced, is quite high. This makes several closely spaced conductors appear as a single inductor and not as paralleled inductances, which exhibit lower overall reactance between the items they are being used to interconnect.
Also, each of the above separated equipment areas containing SRGs should be ac powered from a locally installed and SRG referenced isolation transformer as opposed to them being powered from panelboards and feeders from some remotely located power source.
Finally, since separated areas in a building are subject to large potential differences due to lightning discharge currents and some forms of ac system ground faults, the ends of the signal cables should always be equipped with surge protection devices (SPDs).
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