How to 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,
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.
Minimising 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);
Minimising 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
Cable shield wires should be grounded at one end, preferably at the power
source end. The other end should be taped and protected (see diagram).
'A' - is Stripped insulation length on grounded end of shiled wire.
'B' - is Wrapped insulation on -non-grounded end of shield wire.
On 'A' - Preffered length of non-insulated portion of shield wire
is 25mm; 50mm may be used if needed to reach terminations of
the carrier shiled bar.
On 'B' - Shield wire bent back and taped (making sure that all
strands are under the tape) so that no portion of the wire can
touch a signal, ground, or metal point.
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
analogue 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.
Source: Control System Power and Grounding Better Practices, ISBN: 0-7506-7826-7
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