ERICO Practical Guide to Electrical Grounding - Manual (Page 81)

73 Chapter 4: Transients & Other High Frequency “Grounding” & Bonding
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 in this document.
Signals which are transmitted on isolated balanced pairs are
not referenced to ground, and differentially coupled signals
that are referenced to ground are relatively immune to
problems involving the ground reference to which they are
connected. With these circuits we are only concerned with
voltages to ground that are high enough to cause voltage
breakdown of insulation systems or electronic components,
or to saturate the magnetics that may be used to isolate and
couple the signal between the signal cable and the
electronics used to drive or receive the signal on the path.
Unbalanced signals referenced to ground fall into two
general categories:
(1) 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.
(2) 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.
In both of the above examples, if unwanted current flow is
caused in the grounded conductor that also carries signal,
and if there is an overlap between the bandwidth of the
interfering signal and the desired one, then the signal
process is almost certain to be disrupted once the
interference reaches a minimum level of amplitude.
Two principle means of dealing with the above ground loop
problem generally exist as follows:
(1) Change the signal’s protocol using a converter, to
one that does not use the “ground” path for any of
the signal current, or;
(2) Shunt the ends of the cable involved in the ground
loop by effectively bonding the equipment at each
end of the cable to the same SRG. This greatly
reduces the effects of the noise current in the signal
conductor path by providing a myriad of parallel
paths for it to flow in via the low impedance SRG.
However, the desired signal will still stay relatively
evenly divided between the two signal conductors
on the cable and not flow into the SRG. This occurs
because the mutually coupled fields from the
closely coupled supply and return conductors in the
cable and for the signal, act to make this path a
much lower impedance for the signal currents to
travel in than the SRG.
Our recommendation is to properly design and implement
the facility’s grounding system to avoid its unwanted
involvement with the operation of the equipment. This kind
of approach can also eliminate the need to consider
equipment modifications and to engage in costly diagnostic
efforts since most trouble involving common-mode noise is
avoided in the signal circuits. A properly installed SRG
along with good bonding practices is a recommended
method of minimizing common-mode noise problems, so it
becomes a first-line of defense in such cases.
While it may be true that an SRG based design of this kind
is both conservative and somewhat more costly (initially)
than other wiring techniques that are commonly used, our
experience clearly shows that using the SRG approach
produces superior and ultimately, more cost-effective
results due to the lack of later operational problems. In
other words, a conservative design involving an SRG costs
a little more, but avoids lots of very difficult and potentially
expensive problems after the job is done.
RECOMMENDATIONS:
It is generally not possible in complex systems with
interconnected data and signal conductors to avoid all
ground loops. Some approaches that may be used to avoid
the detrimental effects of such ground loops include:
Grounding Book 4/14/99 10/5/99 6:02 PM Page 73 (Black plate)