File Name: earthing and bonding in hazardous locations .zip
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When asked to quantify what such a ground is, they generally talk about a ground not connected to that dirty, nasty power ground. It seems they expect that noise or problems will somehow arise out of the ground and strike down their equipment. This is basically a question of whether to have a separate, isolated hence somehow clean , instrument ground, or to have it a part of the overall facility grounding grid.
A perfect ground would never vary in potential—zero potential anywhere potential because it is single-ended, unlike voltage, which is double-ended, or a potential in reference to another potential. This theoretical ground would also have zero impedance—current flowing to ground would encounter no resistance, and anything connected would always be at the same ground potential. Unfortunately, we are stuck with the Earth, which is everywhere that our instrument systems are, and our instruments can be significantly geographically distributed.
This earth can be defined at any single point as the zero reference potential point of out instrument system, but it will be different anywhere else. The so-called clean ground, isolated from the rest of the grounds but subject to many of the underground events that the other grounds are subject to can the very source of the problems we wished to avoid.
Under quiescent conditions, such a ground may have less variation in ground potential, but under non-quiescent conditions, it can be subject to considerable variation relative to other ground systems. One should remember that ground is not a sink for noise, but can be a path for it if the ground completes a noise circuit.
Conducted noise flows in complete circuits, and Ohm's Law impedance version applies to conducted noise. The essential trouble with grounds is that they are all different. As part of the course, there was a field demonstration where we were shown two ground rods about 50 feet part. A wire was run between the two ground wires, and a clamp-on ammeter was clamped on the wire.
Low and behold, a current was measured between the two ground rods. The fact that a current was flowing indicated a potential differential between the two grounds—the grounds were not the same. This small epiphany provided the spark for a lifelong interest in grounding. So why are the grounds different? The first difference might be due to different resistivity and, in some cases, different impedances in the earth. This can be due to different soils, non-conductors like rocks and voids, moisture content, salt content, ground water, underground artifacts like pipelines , etc.
It can also be due to the weather and seasonal changes. The current drought is affecting our grounding systems by drying out the soil, shrinking soil away from ground rods, and reducing the level of ground water. The resistivity of the earth is three-dimensional—it has length and width as well as depth. Figure 1 show a resistance mapping of the earth from a side view.
From this one might infer why grounds can be different. The second reason a ground can be different is that there are currents flowing in the earth, some man-made, others caused by nature. Man-made currents can be transient in nature, like ground fault current returning to its source through the ground, or continuous, like the currents induced in the ground by our high-voltage power distribution systems, electric trains, stray currents, or circulating ground currents caused by poor grounding.
Nature is also a big cause of ground currents flowing in the earth. An obvious source is lightning, which causes a large, rapid charge redistribution that results in currents flowing and changing ground potential.
A good analogy of a lightning strike to earth is dropping a large rock in a pond and watching the ripples spread out. This is illustrated in Figure 2. By several estimates, there are about 2, thunderstorms worldwide providing flashes per second not all of these are ground strikes. Clouds become charged and can induce currents in the earth. Think about this happening in a global sense all around us.
Figures 3 and 4 are illustrations of a moving thunderstorm. Lightning can also induce currents in the ground and generate RF interference, as shown in Figure 5. These currents are also called Telluric currents. Some further analogies may help to help better understand grounding. A good analogy for currents flowing and earth potential varying is the use of the ocean. The ocean has flowing currents that we cannot see, and waves varying in height, which can represent varying potential Figure 6.
If we take the ocean analogy a bit further, we can understand why we want to connect all of our grounding systems together to create as much as we can an equipotential grounding plane for our instrumentation systems, electrical systems, communication systems and computer systems. IEEE std. Visualize two ships separated by several waves or troughs, but connected to each other by electrical cables power, signal, etc. We will use the analogy that the water height above a theoretical reference point represents ground potential.
So, the ships, relative to each other, could at any time be on a wave or trough of the same height same potential , on a wave or trough of different heights potential difference , or one ship could be on a wave while the other in a trough larger potential difference. If the two ships are at the same wave height or equipotential plane , the potential would be zero.
To illustrate how much potential difference there can be between two ground points, where the resistance between the points is one ohm and the current from lightning flowing through the earth is 10, amps, the potential difference between those two points is 10, volts, which would exceed the nominal arc overvoltage of wiring of 6, volts.
This is illustrated in Figure 7. This brings us to the good reason to interconnect power and instrument grounds.
What we want is for our instrument systems to ride up and down the varying ground potential with the rest of the facility, like a ship riding up and down on waves. Another important concept in instrumentation grounding is that of a low-frequency, single-point ground, i. These connections should not be daisy-chained and should provide a low-impedance path back to our DC ground.
This ground should be connected to the power grounding grid at only one point. Selection of this point of connection should consider the power distribution system how large ground faults might occur, and how they will return to their source; smaller leaks return using the same path , where the lightning protection system is connected to the grounding grid not too close to the instrument connection , location of cathodic protection systems generally, stay away from them , location of underground pipes stay away , etc.
Figures 8 and 9 show an example DCS grounding scheme and a ground grid not to scale. At higher frequencies, multipoint grounding systems are used because these systems will be grounded anyway through distributed capacitances, and it is better to separately ground them. Before we leave this topic, we should address ungrounded systems.
Instrument systems that are powered typically by VDC systems are not addressed. People do often use ungrounded systems for DC instrument loops where they have an isolated DC power supply or four-wire instrument loop. These loops will function this way and there are applications where this may be necessary.
This question has a life of its own and seems come up time and time again. Shielding is a complex subject that can be very application-dependent. This discussion is primarily directed at shielding in petrochemical plants. The purpose of shielding is to keep noise in or to keep noise out of a circuit, more toward the latter than the former.
Noise can be defined as any unwanted electrical signal in a circuit, while interference is noise that has a detrimental influence on the circuit. To understand shielding and the related question of grounding shields, we must understand how noise gets into a circuit. Noise can also be generated internal to the circuit, in which case, shielding is not much help except to keep it from getting out of the circuit.
There are four basic means that noise is coupled into a circuit: capacitively, inductively, radiated and conducted. The frequency of the noise and the operating frequency of the circuit must be considered. This typically results in different shield grounding methods for low frequency grounded at one end than at high frequency grounded in multiple places. Here, we discuss low-frequency grounding. Shielding capacitively coupled noise: Capacitive or electrostatic coupled noise is coupled through distributed capacitances formed between the source of the noise and the receptor or victim of the noise.
This type of noise is voltage-based Figure We shield against this type of noise coupling by placing a grounded shield at the system reference potential between the noise source and the receptor circuit to provide a conductor for the distributed capacitances to connect to rather than the signal conductors. This normally takes the form of a thin, aluminum-coated Mylar film with a drain wire running the length of the shield around the protected wires Figure 11 Any noise capacitively coupled to the shield is returned to the source via the shield ground connection.
If the shield impedance is zero, the noise does not couple to the protected signal wires since it has been intercepted by the shield at zero potential relative to the signal wire reference. This shield is grounded at one end at the zero potential reference point of the circuit nominally the DC ground reference point.
The shield continuity is maintained through any marshalling and field junction boxes out to the field device, where the shield is folded back and taped or heat-shrinked.
The only common exception is grounded thermocouples, whose shields are commonly grounded at the thermocouple head or the field junction box closest to it. The reason we ground at one end is that our field devices can be a long distance from the receiving element and try as hard as we can, we will still have differences in ground potential.
If we connect at both ends, we can have circulating currents in our shields. This can be particularly detrimental during ground disturbances. Shielding of inductively or magnetically coupled noise: Shielding against inductively or magnetically coupled noise is a bit more difficult.
This noise is coupled or induced by a varying magnetic field similarly to how a transformer works. This noise is current-based—the higher the current at the source, the more noise is coupled at the receptor.
This type of noise is illustrated in Figure For this, the aluminum shield is no barrier and for a metallic shield to be successfully, it must be made of a ferrous material.
However, connecting the shield at both ends can provide some protection against magnetically coupled noise by inducing a counter current on the shield to the noise current coupled to the signal wires.
That said, we still have the problem that if we connect our shields at both ends, we will have circulating currents in our shields from other sources besides our magnetic coupled noise.
In addition, the aluminum shields used in our typical shielded, twisted-pair cables are not designed to be current-carrying conductors and in the extreme case, you may burn through your cable. Some people have been successful in grounding the other end of the shield through a capacitor to provide solidly grounded shield at one end at DC and a varying impedance at frequency, grounded at both ends through the capacitor. If the cable is contained in the same building and both ends share a common equipotential plane, you may be able get away with grounding the shield at both ends, but it is a last resort for process instrumentation systems.
Once in a circuit, this noise behaves like conducted noise. It is good practice to specifically ground your field junction boxes. Ott has a good discussion on this. Conducted noise: Conducted noise is noise that has gotten into a circuit by various means and has been successfully conducted past its entry point.
Often, this type of noise is filtered out by means that include shunting the noise through ground back to its source. A good ground system is a key to doing this successfully. Frequent contributor William Bill L.
An earthing system UK and IEC or grounding system US connects specific parts of an electric power system with the ground , typically the Earth's conductive surface, for safety and functional purposes. Regulations for earthing systems vary considerably among countries, though most follow the recommendations of the International Electrotechnical Commission. Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants. In addition to electric power systems, other systems may require grounding for safety or function. Tall structures may have lightning rods as part of a system to protect them from lightning strikes. Telegraph lines may use the Earth as one conductor of a circuit, saving the cost of installation of a return wire over a long circuit.
AND bonding. By Mark Coles 1. Introduction Electrical connection maintaining various exposed- The IEE's Technical Helpline receives numerous calls conductive-parts and extraneous-conductive-parts at from contractors requesting information on the substantially the same potential'. This article will give an overview of the There are two categories of equipotential bonding : hazards and problems encountered in those LOCATIONS and gives information on the performance Main equipotential bonding requirements of EARTHING and bonding to ensure that Regulation of BS states: In each the potential for gas ignition, from low voltage installation, main equipotential bonding conductors electrical sources and equipment, is reduced. Equipotential bonding connections.
The EN/IEC ‐14 installation directive requires equipotential bonding within hazardous areas of Zones 0 and 1 to prevent the occurrence of sparks capable.
This document details what has proved to beacceptable practice for earthing and bondingof electrical apparatus used in hazardous areas. The subject is not complex, but partiallybecause it is relevant to more than one area ofelectrical expertise a systematic approach tothe subject is desirable. There are numerouscodes of practice which specify how earthingand bonding should be carried out, but thefundamental requirements are independent ofthe geographic location of the installation andhence there should be no significant differencein requirements. This document predominantlydescribes what is acceptable practice in theUnited Kingdom and Europe. If a national codeof practice exists and differs fundamentally fromthis document then it should be questioned.
When asked to quantify what such a ground is, they generally talk about a ground not connected to that dirty, nasty power ground. It seems they expect that noise or problems will somehow arise out of the ground and strike down their equipment. This is basically a question of whether to have a separate, isolated hence somehow clean , instrument ground, or to have it a part of the overall facility grounding grid. A perfect ground would never vary in potential—zero potential anywhere potential because it is single-ended, unlike voltage, which is double-ended, or a potential in reference to another potential.
Beta This is a new way of showing guidance - your feedback will help us improve it. This Technical Measures Document refers to codes and standards applicable to earthing of plant.
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