The gro und loop is probab ly the most difficult circuit noise source to locate. Gro und loops can exist whenever interconnected, non-isolated instru men ts are grounded at more th an one location. Non-isolated simply me an s th at there is no isolation between th e inp ut circuit of the ins trum ent and its ou tput circuit. The input circuit of the inst rum ent is connected to th e outpu t circuit by a mea sur abl e resistance. If an interference pote ntia l exists between the ground points of the input circuit and output circuit, an undesired current begins to flow.
The interference potential that causes current to flow through the ground loop may be due to faults in electrical equi pmen t th at cause leakage curre nts through grou nd. The finite resistance present in the ground plane or in earth ground causes a potential to be developed.
The interface potential could also be produced in the same manner as the potential in a bat tery . This potential , called a galvani c potential , is developed when two dissim ila r met als
come into contact in an electrolytic solution. Interference potentials c an resu lt from thermoelectric potentials developed by the joining of dissimilar metals with a temperature gradient.
Another directly-coupled noise source is leakage curr ent s. Leakage cur ren ts ar e a resu lt of poor insulation that allows current to pass from one lead into another or from a signal lead to ground. When ther e is leak age between source and signal lead s, a noise signal is introduced into the signal circuit similar to those introduced through capacitive and inductive-coupling. Another source of leakage curre nt s is throu gh imp roper ly spaced components within ins tru men ts. During maintenance, if a technician causes a resistor, capacitor, or other circuit component to touch the instrument case or adjacent components, then leakage current path can be introduced into the measurement signal circuit.
Noise cannot be totall y accounted for by th e manuf acture r. The in st ru me nt can be designed with filter circuits to attenuate noises that might originate from within the instrument, but any attempt by the manufacturer to add filter circuits to attenuate noises is based on an
"assumed" amount and type of noise. This is because th e manufacturer is usua ll y unce rtai n of the type of enviro nmen t in which the inst rum ent will be placed. As such, th e user of the ins tru me nt mus t be prepare d to either: (a) evaluat e the extent of noise, which may resu lt in a determination that the existing noise is not significant, or (b) eliminate the causes of unacceptable noise, or (c) prevent the unacceptable noise from interfering with the instrument.
APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309
Noise can be a major source of inaccu racy in me asu rem en t channels. Eli min ati on of this undesirable voltage or current, or at least its reduction to a tolerable level is necessary for pro per process control. Obviously, the best way to redu ce unwanted signals with in an in st ru me nt loop would be to elimi nate the source of th e noise. For example, s ign al leads could be relocated away from power leads or electrical machinery. Often, tho ug h, it is impr acti cal to eliminate th e noise or the ad verse effects caus ed by noise. One is us in g circuit desi gn tha t reduce of the effects of noise. This concentr ates on methods ex te rn al to the instrument's electronic circuits that are used to reduce the magnitude of the noise induced into signal leads. Several such methods are employed in instru menta tion syste ms. The use of shielding and shielded cables can be very effective in reducing the magnitude of noise ind uce d in signal lead s. The use of twis ted pai r cable for signal tra nsm iss ion is also an effective way to limiting interference . In most cases, power leads are also twisted as a mean s of reducing interference.
Other methods used in the industry to reduce noise are:
a. The use of filters (usually capacitors) to pre ven t noise from ent eri ng in st ru ment amplifiers.
b. Periodic insu lati on checks of signal cables to detect path s for lea kag e cur ren ts.
c. Detection and removal of ground loops.
d. Proper grounding of ins trum ent atio n loops.
Therefore, th e problem of noise removal can be atta cke d at two levels. On e is noise elimi natio n; keeping any noise on the inp ut leads from reach ing the amplifier. The other, noise reduction, is minimizing the amount of noise present on the input leads.
The complexity of modern industrial processes often necessitates the monitoring and control of th e pla nt from one control room. To provide this cen tral control, process infor matio n must be tran smi tted over long distances. Many factors mus t be considered when design ing these transmission systems to ensure that reliable and accurate indication and control of the process is achieved.
Noise is an undesirable voltage or current induced in measurement signal leads by an ext ern al source, usua lly adjacent wiring or equipme nt. Noise or interferen ce may take var iou s forms. It may be alt ern at ing cur ren t or voltage of high and low frequencies from utility service, or it may be direct from alarm circuits.
As previously discussed, there are three methods by which noise is introduced into a signal lead . The first method is the capacitive coupling of electrical energy from elect rosta tic fields int o the signal lead. The second method is the inductive coupling of electrical ener gy from
INSTRUMENT TRAINEE TASK MODULE 12309
electromagnetic fields into the signal lead. The th ird method involves th e direct coupling of current into the signal leads through ground loops or leakage currents.
In the process instrumentation industry, there is an effort to standardize signal ranges so that instruments made by one manufacturer are compatible with those made by another manuf acture r. For electronic inst rum enta tion , a ran ge of 4-20 maDC was chosen. Although it is widely accepted by both users and manufacturers of process instruments, other non
standard signal ranges are still widely used.
Generally, signal ran ge s used in the process in du st ry have an elevated zero ra ng e. A signal range with other than 0 maDC or 0 VDC as the minimum signal level was selected because when a "live" zero is used, a distinct difference exists between a minimum signal and a missi ng signal. This provides an immediate ind icatio n of a failure an d mak es locating the cause easier. In addition, an elevated zero will bia s active electronic components into thei r linear range of operation; this improves instrument linearity over the entire span of operation.
The output signal span must be large enough to provide satisfactory resolution and accuracy while minimizing the maximum signal level to allow the use of smaller, lighter components within the in strum ent and to reduce the power requiremen ts of the instr ume nt power supply.
DC cu rre nt signal tran smi ssio n has found th e gr ea tes t acceptance in electronic process control syst ems with the ran ge s of 4-20 maDC a nd 10-50 maDC being most commonly use d. These signal ranges are sufficiently high to eliminate the need for special signal cable and yet are low enough to allow the use of small gauge wire . Cur ren t trans miss ion syste ms are less susceptible to induced noise than voltage transmission because current-controlled devices hav e low input and ou tpu t impedances. For th e noise to develop a significant amo unt of voltage drop across the low impedance, it would have to induce a sizable amount of current.
Con tra st this to th e characteristically high impe danc es of voltage-controlled devices; a much sma ller a mount of induce d current will cau se a significant change in mea sur ed voltage.
However, precautions should still be taken to minimize noise by shielding signal cables and by locating signal cables away from power lea ds an d heavy electrical machinery . Cur ren t transmission systems are more susceptible than voltage transmission systems to interference introduced by leakage currents and ground loop currents.
For process instruments that require voltage inputs, a voltage signal can easily be derived from the current signal by inserting a resistor in series with the signal leads and measuring the voltage developed across the resistor.
DC voltage transmission systems require circuits of higher quality than current systems, especially if the sy stem uses low voltage sign al levels. The signal-to-noise ratio m ust be relati vely large, two or greate r, to obtain satisfacto ry results . Shielding is a mu st in voltage transmissions that extend over long distance.
APPLY PROPER GROUNDING AND/OR SHIELDING OF INSTRUMENT WIRING — MODULE 12309
Shielding is the use of a conducting and/or ferromagnetic (permeable) barrier between a potenti ally disturbing noise source and sensitive circuitr y. Shields are used to protect cables (data an d power) and electronic circuits. They ma y be in the form of met al barr ier s, enclosures, or wrappings around source circuits and receiving circuits.
Shielding attenuates noise signals by two methods: absorption and reflection. In general, electric fields are reflected, while magnetic fields are attenuated by absorption.