Automation cabinet. Grounding methods

Grounding techniques in industrial automation systems are very different for galvanically coupled and galvanically isolated circuits. Most of the methods described in the literature relate to galvanically coupled circuits, the share of which has recently decreased significantly due to the sharp drop in prices for isolating DC-DC converters.

3.5.1. Galvanically coupled circuits

An example of a galvanically coupled circuit is the connection of the source and receiver of a standard 0 ... 5 V signal (Fig. 3.95, Fig. 3.96). To explain how to properly ground, consider the option of incorrect (Fig. 3.95) and correct (Fig. 3.96, installation. The following errors were made in Fig. 3.95:

The listed errors lead to the fact that the voltage at the receiver input is equal to the sum of the signal voltage and the interference voltage . To eliminate this drawback, a large copper bar can be used as a grounding conductor, but it is better to perform grounding as shown in fig. 3.96, namely:

The general rule for weakening communication through a common ground wire is to divide the lands into analog, digital, power and protective, and then connect them at only one point. When separating the grounding of galvanically coupled circuits, the general principle is used: grounding circuits with a high level of interference should be carried out separately from circuits with a low level of interference, and they should only be connected at one common point. There may be several grounding points, if the topology of such a circuit does not lead to the appearance of "dirty" ground areas in the circuit, including the signal source and receiver, and also if closed loops are not formed in the grounding circuit, through which the current induced by electromagnetic interference circulates.

The disadvantage of the method of separating ground conductors is low efficiency at high frequencies, when the mutual inductance between adjacent ground conductors plays an important role, which only replaces galvanic couplings with inductive ones, without solving the problem as a whole.

Long conductor lengths also increase the ground resistance, which is important at high frequencies. Therefore, grounding at one point is used at frequencies up to 1 MHz, it is better to ground at several points above 10 MHz, in the intermediate range from 1 to 10 MHz, a single-point circuit should be used if the longest conductor in the ground circuit is less than 1/20 of the interference wavelength. Otherwise, the multipoint scheme [Barnes] is used.

Single point grounding is often used in military and space applications [Barnes].

3.5.2. Shielding of signal cables

Consider the grounding of screens when transmitting a signal over a twisted shielded pair, since this case is most typical for industrial automation systems.

If the interference frequency does not exceed 1 MHz, then the cable must be grounded on one side. If it is grounded from both sides (Fig. 3.97), then a closed circuit is formed, which will work as an antenna, receiving electromagnetic interference (in Fig. 3.97, the interference current path is shown by a dashed line). The current flowing through the screen is a source of inductive interference on adjacent wires and wires inside the screen. Although the magnetic field of the braid current inside the shield is theoretically equal to zero, but due to the technological variation in the manufacture of the cable, as well as the non-zero resistance of the braid, the pickup on the wires inside the shield can be significant. Therefore, the screen must be grounded only on one side, and on the side of the signal source.

The cable sheath must be grounded at the signal source side. If grounding is done from the side of the receiver (Fig. 3.98), then the interference current will flow along the path shown in fig. 3.98 dashed line, i.e. through the capacitance between the cable cores, creating interference voltage on it and, consequently, between the differential inputs. Therefore, it is necessary to ground the braid from the side of the signal source (Fig. 3.99). In this case, there is no path for the interference current to pass. Please note that these diagrams show a differential signal receiver, i.e. both of its inputs have infinite resistance to ground.

If the signal source is not grounded (for example, a thermocouple), then the shield can be grounded from either side, because in this case, a closed loop for the interference current is not formed.

At frequencies above 1 MHz, the inductive resistance of the screen increases and the capacitive pickup currents create a large voltage drop on it, which can be transmitted to the internal conductors through the capacitance between the braid and the conductors. In addition, with a cable length comparable to the interference wavelength (the interference wavelength at a frequency of 1 MHz is 300 m, at a frequency of 10 MHz - 30 m), the resistance of the braid increases (see the "Ground Model" section), which sharply increases the interference voltage on the braid. Therefore, at high frequencies, the cable sheath must be grounded not only on both sides, but also at several points between them (Fig. 3.100). These points are chosen at a distance of 1/10 of the interference wavelength from one another. In this case, part of the current will flow through the cable braid, transmitting interference to the central core through mutual inductance. The capacitive current will also flow along the path shown in fig. 3.98, however, the high frequency component of the interference will be attenuated. The choice of the number of cable grounding points depends on the difference in interference voltages at the ends of the screen, the frequency of the interference, the requirements for protection against lightning strikes, or the magnitude of the currents flowing through the screen if it is grounded.

As an intermediate option, you can use the second screen grounding through the capacitance (Fig. 3.99). At the same time, at high frequency, the screen turns out to be grounded from two sides, at low frequency - from one side. This makes sense in the case when the interference frequency exceeds 1 MHz, and the cable length is 10 ... 20 times less than the interference wavelength, i.e. when it is not yet necessary to carry out grounding at several intermediate points. The capacitance value can be calculated by the formula , where is the upper frequency of the boundary of the interference spectrum, is the capacitance of the grounding capacitor (fractions of Ohm). For example, at a frequency of 1 MHz, a 0.1 uF capacitor has a resistance of 1.6 ohms. The capacitor must be high-frequency, with low self-inductance.

For high-quality shielding in a wide frequency range, a double screen is used (Fig. 3.101) [Zipse]. The inner shield is grounded on one side, the signal source side, to prevent capacitive interference from passing through the mechanism shown in fig. 3.98, and the outer shield reduces high-frequency interference.

In all cases, the screen must be insulated to prevent accidental contact with metal objects and the ground.

Recall that the interference frequency is the frequency that can be perceived by sensitive inputs of automation equipment. In particular, if there is a filter at the input of the analog module, then the maximum noise frequency that must be considered for shielding and grounding is determined by the upper cutoff frequency of the filter's passband.

Since even with proper grounding, but with a long cable, the interference still passes through the screen, it is better to transmit the signal in digital form or through an optical cable to transmit a signal over a long distance or with increased requirements for measurement accuracy. For this, you can use, for example, analog input modules RealLab! series with a digital RS-485 interface or fiber optic converters of the RS-485 interface, for example, type SN-OFC-ST-62.5/125 from RealLab! .

We have carried out an experimental comparison various ways connection of a signal source (thermistor with a resistance of 20 kOhm) through a shielded twisted pair (0.5 turns per centimeter) 3.5 m long. An instrumental amplifier RL-4DA200 with a data acquisition system RL-40AI from RealLab! was used. The amplification channel gain was 390, the bandwidth was 1 kHz. Type of interference for the circuit of Fig. 3.102-a is shown in fig. 3.103.

3.5.4. Cable screens in electrical substations

At electrical substations, on the braid (screen) of the automation signal cable, laid under high-voltage wires at ground level and grounded on one side, a voltage of hundreds of volts can be induced during the switching of the current by the switch. Therefore, for the purpose of electrical safety, the cable braid is grounded on both sides.

To protect against electromagnetic fields with a frequency of 50 Hz, the cable screen is also grounded on both sides. This is justified in cases where it is known that the electromagnetic pickup with a frequency of 50 Hz is greater than the pickup caused by the flow of equalizing current through the braid.

3.5.5. Cable shields for lightning protection

To protect against the magnetic field of lightning, signal cables of automation systems passing through open areas must be laid in metal pipes ferromagnetic material such as steel. The pipes play the role of a magnetic screen [Vijayaraghavan]. Stainless steel cannot be used as this material is not ferromagnetic. Pipes are laid underground, and when above ground, they must be grounded approximately every 3 meters [Zipse]. The cable must be shielded and the shield must be grounded. The grounding of the screen must be very high quality with minimal resistance to ground.

Inside the building, the magnetic field is weakened in reinforced concrete buildings and is not weakened in brick ones.

A radical solution to lightning protection problems is the use of a fiber optic cable, which is already quite cheap and easily connected to the RS-485 interface, for example, through converters of the SN-OFC-ST-62.5/125 type.

3.5.6. Grounding for differential measurements

If the signal source has no resistance to ground, then a "floating input" is formed during differential measurement (Fig. 3.105). The floating input can be statically charged by atmospheric electricity (see also the Ground Types section) or by the input leakage current of an op amp. To drain charge and current to ground, the potential inputs of analog input modules typically contain 1 MΩ to 20 MΩ resistors internally to connect the analog inputs to ground. However, with a high level of interference or a high resistance of the signal source, the resistance of 20 MΩ may not be sufficient, and then it is necessary to additionally use external resistors with resistance from tens of kΩ to 1 MΩ or capacitors with the same resistance at the interference frequency (Fig. 3.105).

3.5.7. Smart Sensors

Recently, the so-called smart sensors, which contain a microcontroller for linearizing the conversion characteristic of the sensor, have been rapidly spread and developed (see, for example, "Sensors for temperature, pressure, humidity"). Smart sensors provide a signal in digital or analog form [Caruso ]. Due to the fact that the digital part of the sensor is combined with the analog part, if the ground is incorrect, the output signal has elevated level noise.

Some sensors, such as those from Honeywell, have a DAC with a current output and therefore require an external load resistance (on the order of 20 kΩ [Caruso]), so the useful signal is obtained in the form of a voltage drop across the load resistor when the sensor output current flows.

cabinets are interconnected, which creates a closed loop in the ground circuit, see fig. 3.69, section "Protective grounding of buildings", "Grounding conductors", "Electromagnetic interference";

analog and digital ground conductors in the left cabinet run in parallel over a large area, so inductive and capacitive pickups from digital ground may appear on analog ground;

the power supply (more precisely, its negative terminal) is connected to the cabinet body at the nearest point, and not at the ground terminal, therefore, interference current flows through the cabinet body, penetrating through the power supply transformer (see Fig. 3.62,);

one power supply is used for two cabinets, which increases the length and inductance of the ground conductor;

in the right cabinet, the ground terminals are not connected to the ground terminal, but directly to the cabinet body. In this case, the cabinet body becomes a source of inductive pickup on all wires passing along its walls;

in the right cabinet, in the middle row, analog and digital grounds are connected directly at the output of the blocks, which is wrong, see fig. 3.95, fig. 3.104.

These shortcomings are eliminated in Fig. 3.108. An additional wiring improvement in this example would be to use a separate ground conductor for the most sensitive analog input modules.

Within a cabinet (rack), it is desirable to group analog modules separately, and digital modules separately, so that when laying wires in a cable duct, the length of the sections of parallel passage of digital and analog ground circuits is reduced.

3.5.9. Distributed control systems

In control systems distributed over a certain area with characteristic dimensions of tens and hundreds of meters, it is impossible to use input modules without galvanic isolation. Only galvanic isolation allows you to connect circuits grounded at points with different potentials.

Cables passing through open areas must be protected from magnetic impulses during a thunderstorm (see the section "Lightning and atmospheric electricity", "Cable screens for lightning protection") and magnetic fields when switching powerful loads (see the section "Cable screens at electrical substations). Pay special attention to the grounding of the cable screen (see section "Shielding of signal cables"). A radical solution for a geographically distributed control system is the transmission of information over an optical fiber or radio channel.

Good results can be obtained by refusing to transmit information by analog standards in favor of digital ones. To do this, you can use the modules of a distributed control system RealLab! NL series from Reallab! . The essence of this approach lies in the fact that the input module is located near the sensor, thereby reducing the length of wires with analog signals, and the signal is transmitted to the PLC via a digital channel. A variation of this approach is the use of sensors with built-in ADCs and digital interfaces (for example, sensors of the NL-1S series).

3.5.10. Sensitive measuring circuits

For highly sensitive measuring circuits in poor electromagnetic environments, the best results are obtained by using a "floating" ground (see section "Types of grounding") in combination with battery supply [Floating ] and fiber optic transmission.

3.5.11. Actuating equipment and drives

The power supply circuits of pulse-controlled motors, servo motors, PWM-controlled actuators must be made with a twisted pair to reduce the magnetic field, and also shielded to reduce the electrical component of the radiated interference. The cable screen must be earthed on one side. The circuits for connecting the sensors of such systems should be placed in a separate screen and, if possible, spatially distant from the actuating devices.

Grounding in industrial networks

An industrial network based on the RS-485 interface is performed by a shielded twisted pair cable with the obligatory use of galvanic isolation modules fig. 3.110). For short distances (about 10 m), in the absence of sources of interference nearby, the screen can not be used. At long distances (the standard allows a cable length of up to 1.2 km), the difference in ground potentials at remote points can reach several units and even tens of volts (see the "Shielding of signal cables" section). Therefore, in order to prevent the current flowing through the shield, equalizing these potentials, the cable shield must be grounded. only one point(no matter which one). This will also prevent a large-area closed loop in the ground circuit, in which high currents can be induced by electromagnetic induction during lightning strikes or the switching of powerful loads. This current, through mutual inductance, induces on the central pair of wires e. d.c., which can damage the port driver chips.

When using an unshielded cable, a large static charge (several kilovolts) can be induced on it due to atmospheric electricity, which can destroy the galvanic isolation elements. To prevent this effect, the isolated part of the galvanic isolation device should be grounded through a resistance, for example, 0.1 ... 1 MΩ (shown by a dashed line in Fig. 3.110).

The effects described above are especially pronounced in Ethernet networks with coaxial cable, when several Ethernet network cards fail during a thunderstorm when grounding at several points (or lack of grounding) during a thunderstorm.

On low bandwidth Ethernet networks (10 Mbps), the shield should only be grounded at one point. On Fast Ethernet (100 Mbps) and Gigabit Ethernet (1 Gbps), shield grounding must be done at several points, following the recommendations in the "Shielding Signal Cables" section

When laying the cable in an open area, you must use all the rules described in the section "Shielding of signal cables"

3.5.12. Grounding at explosive objects

At explosive industrial facilities (see section "Automation of hazardous facilities") when installing ground circuits stranded wire it is not allowed to use soldering for soldering the cores together, since due to the cold flow of the solder, weakening of the contact pressure points in the screw terminals is possible.

The shield of the RS-485 interface cable is grounded at one point, outside the hazardous area. Within the hazardous area, it must be protected from accidental contact with earthed conductors. Intrinsically safe circuits must not be grounded unless the operating conditions of the electrical equipment require it (GOST R 51330.10, section "Shielding of signal cables").

3.6. Galvanic isolation

Galvanic isolation(isolation) of circuits is a radical solution to most of the problems associated with grounding, and its use has become a de facto standard in industrial automation systems.

To implement galvanic isolation, it is necessary to supply energy to the isolated part of the circuit and exchange signals with it. Energy is supplied using an isolation transformer (in DC-DC or AC-DC converters) or using autonomous power sources: galvanic batteries and accumulators. Signal transmission is carried out through optocouplers and transformers, elements with magnetic coupling, capacitors or optical fiber.

The basic idea of ​​galvanic isolation is that the path through which conductive interference can be transmitted is completely eliminated in the electrical circuit.

Galvanic isolation solves the following problems:

reduces the common-mode noise voltage at the input of the differential analog signal receiver to almost zero (for example, in Fig. 3.73, the common-mode voltage on the thermocouple relative to the ground does not affect the differential signal at the input of the input module);

protects the input and output circuits of the input and output modules from breakdown by a large common-mode voltage (for example, in Fig. 3.73, the common-mode voltage on a thermocouple relative to the ground can be arbitrarily large if it does not exceed the insulation breakdown voltage).

To use galvanic isolation, the automation system is divided into autonomous isolated subsystems, the exchange of information between which is carried out using galvanic isolation elements. Each subsystem has its own local ground and local power supply. Subsystems are grounded only for electrical safety and local protection against interference.

The main disadvantage of circuits with galvanic isolation is the increased level of interference from the DC-DC converter, which, however, can be made sufficiently small for low-frequency circuits using digital and analog filtering. At high frequencies, the capacitance of the subsystem to ground, as well as the throughput capacitance of the galvanic isolation elements, is a limiting factor in the merits of galvanically isolated systems. The capacitance to ground can be reduced by using optical cable and reducing the geometric dimensions of the isolated system.

When using galvanically isolated circuits, the concept " insulation voltage" is often misunderstood. In particular, if the insulation voltage of an input module is 3 kV, this does not mean that its inputs can be under such a high voltage under working conditions. In foreign literature, three standards are used to describe the insulation characteristics: UL1577, VDE0884 and IEC61010 -01, but descriptions of galvanic isolation devices do not always refer to them.Therefore, the concept of "isolation voltage" is interpreted in domestic descriptions of foreign devices ambiguously.The main difference is that in some cases we are talking about a voltage that can be applied to isolation indefinitely (working insulation voltage) , in other cases it is probationary voltage (insulation voltage), which is applied to the sample for 1 min. up to several microseconds. The test voltage can be up to 10 times the operating voltage and is intended for accelerated testing during production, since the voltage at which breakdown occurs depends on the duration of the test pulse.

tab. 3.26 shows the relationship between operating and test (test) voltage according to the IEC61010-01 standard. As you can see from the table, concepts such as operating voltage, constant, rms or peak value of the test voltage can vary greatly.

The electrical insulation strength of domestic automation equipment is tested in accordance with GOST 51350 or GOST R IEC 60950-2002 with a sinusoidal voltage with a frequency of 50 Hz for 60 seconds at a voltage indicated in the instruction manual as "insulation voltage". For example, with a test insulation voltage of 2300 V, the operating voltage of the insulation is only 300 V (Table 3.26 RMS, 50/60 Hz,

1 minute.

Today we'll talk about grounding in transformer substation and industrial, the main goals of which are maintenance personnel and stable operation. Many misunderstand the topic of grounding in industrial systems, and its incorrect connection leads to bad consequences, accidents and even costly downtime due to violation and breakdown. Interference is a random variable, which is very difficult to detect without special equipment.

Interference sources on the ground bus

Sources and causes of interference can be lightning, static electricity, electromagnetic radiation, "noisy" equipment, 220 V power supply network with a frequency of 50 Hz, switched network loads, triboelectricity, galvanic couples, thermoelectric effect, electrolytic, conductor movement in a magnetic field, etc. In industry, there is a lot of interference associated with malfunctions or the use of non-certified equipment. In Russia, interference is regulated by standards - R 51318.14.1, GOST R 51318.14.2, GOST R 51317.3.2, GOST R 51317.3.3, GOST R 51317.4.2, GOST 51317.4.4, GOST R 51317.4.11, GOST R 51522, GOST R 50648. For design industrial equipment To reduce the level of interference, they use a low-power element base with minimal speed and try to reduce the length of the conductors and shielding.

Basic definitions on the topic "Common grounding"

Protective earth- connection of the conductive parts of the equipment with the ground of the Earth through a grounding device in order to protect a person from electric shock.
Grounding device- a set of grounding conductors (that is, a conductor in contact with the ground) and grounding conductors.
Common wire - a conductor in the system, relative to which the potentials are measured, for example, the common wire of the PSU and the device.
Signal ground- connection to the ground of the common wire of the signal transmission circuits.
The signal ground is divided into digital land and analog. Signal analog ground is sometimes divided into analog input ground and analog output ground.
force ground- a common wire in the system, connected to protective earth, through which a large current flows.
Solidly grounded neutral b - the neutral of the transformer or generator, connected to the ground electrode directly or through low resistance.
Zero wire- a wire connected to a solidly grounded neutral.
Insulated neutral b - neutral of the transformer or generator, not connected to the grounding device.
Zeroing- connection of equipment with a solidly grounded neutral of a transformer or generator in three-phase current networks or with a solidly grounded output of a single-phase current source.

APCS grounding is usually subdivided into:

1. Protective grounding.
2. Working ground, or FE.

Grounding purposes

Protective grounding is necessary to protect people from damage electric shock for equipment with a supply voltage of 42 V AC or 110 V DC, except for hazardous areas. But at the same time, protective grounding often leads to an increase in the level of interference in the process control system.

Electrical networks with an isolated neutral are used to avoid interruptions in the consumer's power supply with a single insulation fault, since in the event of an insulation breakdown to the ground in networks with a dead-earthed neutral, protection is triggered and the power to the network is interrupted.
The signal ground serves to simplify electrical circuit and cheaper devices and industrial systems.

Depending on the purpose of the application, signal grounds can be divided into basic and screen ones. The reference ground is used for reference and signal transmission in the electronic circuit, and the shield ground is used for grounding the shields. Screen earth is used for grounding cable screens, shielding, instrument cases, as well as for removing static charges from rubbing parts of conveyor belts, electric drive belts.

Types of grounding

One of the ways to mitigate the harmful effect of ground circuits on automation systems is the separate implementation of grounding systems for devices that have different sensitivity to interference or are sources of interference of different power. The separate design of the grounding conductors allows their connection to the protective earth at one point. Wherein different systems grounds are the rays of a star, the center of which is the contact to the protective grounding bus of the building. Due to this topology, dirty ground noise does not flow through the clean ground conductors. Thus, although the ground systems are separate and have different names, ultimately they are all connected to the Earth through a protective earth system. The only exception is "floating" land.

Power ground

Automation systems can use electromagnetic relays, micropower servomotors, solenoid valves and other devices, the current consumption of which significantly exceeds the current consumption of I / O modules and controllers. The power circuits of such devices are made with a separate pair of twisted wires (to reduce radiated interference), one of which is connected to the protective ground bus. The common wire of such a system (usually the wire connected to the negative terminal of the power supply) is the power ground.

Analog and digital ground

Industrial automation systems are analog-digital. Therefore, one of the sources of the analog part is the interference created by the digital part of the system. To prevent the passage of interference through the ground circuits, digital and analog ground are made in the form of unconnected conductors connected together at only one common point. I/O modules and industrial controllers have separate outputs for this. analog ground(A.GND) and digital(D.GND).

"Floating" land

A "floating" ground occurs when the common wire of a small part of the system is not electrically connected to the protective earth bus (that is, to Earth). Typical examples of such systems are battery meters, vehicle automation, aircraft on-board systems, or spaceship. Floating ground is used more often in small signal measurement technology and less often in industrial automation systems.

Galvanic isolation

Galvanic isolation solves many grounding problems, and its use has actually become in process control systems. To implement galvanic isolation (isolation), it is necessary to supply energy by an isolation transformer and transmit a signal to an isolated part of the circuit through optocouplers and transformers, elements with magnetic coupling, capacitors or optical fiber. In the electrical circuit, the path through which the transmission of conducted interference is possible is completely eliminated.

Grounding Methods

The grounding for galvanically coupled circuits is very different from the grounding for decoupled circuits.

Grounding of galvanically coupled circuits

We recommend avoiding the use of galvanically coupled circuits, and if there is no other option, it is desirable that these circuits be sized to
opportunities small and that they are located within the same cabinet.

An example of incorrect grounding of the source and receiver of a standard signal 0 ... 5 V

Here are the following errors:

• heavy load (DC motor) current flows on the same ground bus as the signal, creating a ground voltage drop;
• used unipolar inclusion of the signal receiver, and not differential;
• an input module without galvanic isolation of the digital and analog parts was used, so the power supply current of the digital part, containing interference, flows through the output AGND and creates an additional interference voltage drop across the resistance R1

These errors lead to the fact that the voltage at the input of the receiver Vin equal to the sum of the signal voltage Vout and interference voltage VGrounds = R1 (Ipit + IM)
To overcome this disadvantage, a large copper bar can be used as the grounding conductor, but it is better to make the grounding as shown below.

Need to do:

• connect all ground circuits at one point (in this case, the interference current IM R1);
• connect the ground conductor of the signal receiver to the same common point (in this case, the current Ipit no longer flows through resistance R1, a
  voltage drop across conductor resistance R2 does not add to the output voltage of the signal source Vout)

Example of correct grounding of the source and receiver of a standard signal 0…5 V

The general rule for weakening the connection through a common ground wire is to divide the lands into analog, digital, power and protective followed by their connection at only one point.

When separating the grounds of galvanically coupled circuits, a general principle is used: grounding circuits with a high noise level should be carried out separately from circuits with a low noise level, and they should only be connected at one common point. There may be several grounding points if the topology of such a circuit does not lead to the appearance of "dirty" ground areas in the circuit, including the source and receiver of the signal, and also if closed loops that receive electromagnetic interference do not form in the ground circuit.

Grounding of galvanically isolated circuits

A radical solution to the described problems is the use of galvanic isolation with separate grounding of the digital, analog and power parts of the system.

The power section is usually grounded via a protective earth bus. The use of galvanic isolation allows you to separate the analog and digital ground, and this, in turn, eliminates the flow of interference currents through the analog ground from the power and digital ground. Analog ground can be connected to protective earth through a resistor. RAGND.

Grounding the screens of signal cables in process control systems

An example of an incorrect ( on both sides) grounding the cable shield at low frequencies, if the interference frequency does not exceed 1 MHz, then the cable must be grounded on one side, otherwise a closed loop is formed that will work as an antenna.

An example of incorrect (on the signal receiver side) grounding of the cable screen. The cable sheath must be grounded at the signal source side. If grounding is done on the receiver side, then the interference current will flow through the capacitance between the cable cores, creating an interference voltage on it and, therefore, between the differential inputs.

Therefore, it is necessary to ground the braid from the side of the signal source, in this case there is no path for the passage of the interference current.

Correct shield grounding (additional grounding on the right is used for high frequency signal). If the signal source is not grounded (for example, a thermocouple), then the shield can be grounded from either side, since in this case a closed loop for the interference current is not formed.

At frequencies above 1 MHz, the inductive resistance of the screen increases, and capacitive pickup currents create a large voltage drop on it, which can be transmitted to the internal conductors through the capacitance between the braid and the conductors. In addition, with a cable length comparable to the interference wavelength (the interference wavelength at a frequency of 1 MHz is 300 m, at a frequency of 10 MHz - 30 m), the braid resistance increases, which sharply increases the interference voltage on the braid. Therefore, at high frequencies, the cable braid must be grounded not only on both sides, but also at several points between them.

These points are chosen at a distance of 1/10 of the interference wavelength from one another. In this case, part of the current will flow through the cable braid I Earth, which transmits interference to the central core through mutual inductance.

The capacitive current will also flow along the path shown in Fig. 21, however, the high frequency component of the interference will be attenuated. The choice of the number of cable grounding points depends on the difference in interference voltages at the ends of the screen, the frequency of the interference, the requirements for protection against lightning strikes, or the magnitude of the currents flowing through the screen if it is grounded.

As an intermediate option, you can use second grounding of the screen through the capacitance. At the same time, at high frequency, the screen turns out to be grounded from two sides, at low frequency - from one side. This makes sense in the case when the interference frequency exceeds 1 MHz, and the cable length is 10 ... 20 times less than the interference wavelength, that is, when it is not yet necessary to ground at several intermediate points.

The inner shield is grounded on one side - the signal source side, in order to exclude the passage of capacitive interference along the path shown, and the outer shield reduces high-frequency interference. In all cases, the screen must be insulated to prevent accidental contact with metal objects and the ground. For signal transmission over long distances or with increased requirements for measurement accuracy, it is necessary to transmit the signal in digital form or, even better, via an optical cable.

Grounding of cable screens of automation systems in electrical substations

At electrical substations, on the braid (screen) of the signal cable of the automation system, laid under high-voltage wires at ground level and grounded on one side, a voltage of hundreds of volts can be induced during the switching of the current by the switch. Therefore, for the purpose of electrical safety, the cable braid is grounded on both sides. To protect against electromagnetic fields with a frequency of 50 Hz, the cable screen is also grounded on both sides. This is justified in cases where it is known that the electromagnetic pickup with a frequency of 50 Hz is greater than the pickup caused by the flow of equalizing current through the braid.

Grounding cable shields for lightning protection

To protect against the magnetic field of lightning, the signal cables (with a grounded shield) of the process control system passing through the open area must be laid in metal pipes made of steel, the so-called magnetic shield. Better underground, otherwise ground every 3 meters. The magnetic field has little effect inside a reinforced concrete building, unlike other materials.

Grounding for differential measurements

If the signal source has no resistance to ground, the differential measurement produces a floating input. The floating input can be statically charged by atmospheric electricity or the input leakage current of an op amp. To drain charge and current to ground, the potential inputs of analog input modules typically contain 1 to 20 MΩ resistors internally that connect the analog inputs to ground. However, with a high level of interference or a large signal source, even a resistance of 20 MΩ may not be sufficient, and then it is necessary to additionally use external resistors with a value of tens of kΩ to 1 MΩ or capacitors with the same resistance at the interference frequency.

Grounding Smart Sensors

Nowadays, the so-called smart sensors with a microcontroller inside to linearize the output from the sensor, giving a signal in digital or analog form. Due to the fact that the digital part of the sensor is combined with the analog part, the output signal has an increased noise level if the ground is incorrect. Some sensors have a DAC with a current output and therefore require an external load resistance of the order of 20 kΩ, so the useful signal in them is obtained in the form of a voltage drop across the load resistor when the sensor output current flows.

The load voltage is:

Vload = Vout – Iload R1+ I2 R2,

i.e. it depends on the current I2, which includes the digital ground current. The digital ground current contains noise and affects the voltage across the load. To eliminate this effect, ground circuits must be made as shown below. Here the digital ground current does not go through the resistance R21 and does not introduce noise into the signal at the load.

Proper grounding of smart sensors:

Grounding of cabinets with equipment of automation systems

Installation of APCS cabinets must take into account all the previously stated information. The following examples of grounding control cabinets are divided conditionally on the correct, giving a lower noise level, and erroneous.

Here is an example (incorrect connections are highlighted in red; GND is a pin for connecting a grounded power pin), in which each difference from the following figure worsens the digital part failures and increases the analog error. The following "wrong" connections are made here:

• the cabinets are grounded at different points, so the potentials of their grounds are different;
• the cabinets are interconnected, which creates a closed circuit in the ground circuit;
• conductors of analog and digital grounds in the left cabinet run in parallel over a large area, so inductive and capacitive pickups from digital ground may appear on analog ground;
• conclusion GND the power supply unit is connected to the cabinet body at the nearest point, and not at the ground terminal, therefore, interference current flows through the cabinet body, penetrating through the power supply transformer;
• one power supply is used for two cabinets, which increases the length and inductance of the ground conductor;
• in the right cabinet, the ground terminals are connected not to the ground terminal, but directly to the cabinet body, while the cabinet body becomes a source of inductive interference to all wires running along its walls;
• in the right cabinet in the middle row, analog and digital grounds are connected directly at the output of the blocks.

The listed shortcomings are eliminated by the example of proper grounding of industrial automation system cabinets:

Add. The advantage of the wiring in this example would be to use a separate ground conductor for the most sensitive analog input modules. Within a cabinet (rack), it is desirable to group analog modules separately, digital modules separately, so that when laying wires in a cable duct, the length of the sections of parallel passage of digital and analog ground circuits is reduced.

Grounding in remote control systems

In systems distributed over a certain territory with characteristic dimensions of tens and hundreds of meters, it is impossible to use input modules without galvanic isolation. Only galvanic isolation allows you to connect circuits grounded at points with different potentials. best solution for signal transmission is optical fiber and the use of sensors with built-in ADC and digital interface.

Grounding of actuating equipment and APCS drives

Power supply circuits for pulse-controlled motors, servo motors, and PWM-controlled actuators must be twisted pair to reduce the magnetic field, and shielded to reduce the electrical component of the radiated interference. The cable screen must be earthed on one side. The circuits for connecting the sensors of such systems should be placed in a separate screen and, if possible, spatially distant from the actuating devices.

Grounding in industrial networks RS-485, Modbus

Interface-based industrial network is shielded twisted pair with mandatory use galvanic isolation modules.

For short distances (about 15 m) and in the absence of nearby noise sources, the screen can not be used. At large lengths of the order of up to 1.2 km, the difference in ground potentials at points remote from each other can reach several tens of volts. To prevent current from flowing through the shield, the cable shield should only be grounded at ANY one point. When using an unshielded cable, a large static charge (several kilovolts) can be induced on it due to atmospheric electricity, which can disable the galvanic isolation elements. To prevent this effect, the isolated part of the galvanic isolation device should be grounded through a resistance, for example 0.1 ... 1 MΩ. The resistance shown by the dashed line also reduces the possibility of breakdown due to ground faults or high galvanic isolation resistance in the case of shielded cable. On low bandwidth Ethernet networks (10 Mbps), the shield should only be grounded at one point. For Fast Ethernet (100 Mbps) and Gigabit Ethernet (1 Gbps), the shield must be grounded at multiple points.

Grounding at explosive industrial facilities

At explosive objects, when installing grounding with a stranded wire, it is not allowed to use soldering for soldering the cores together, since due to the cold flow of the solder, weakening of the contact pressure points in the screw terminals is possible.

The screen of the interface cable is grounded at one point outside the hazardous area. Within the hazardous area, it must be protected from accidental contact with earthed conductors. intrinsically safe circuits must not be grounded unless required by the operating conditions of the electrical equipment ( GOST R 51330.10, p6.3.5.2). And they must be installed in such a way that interference from external electromagnetic fields (for example, from a radio transmitter located on the roof of a building, from overhead power lines or nearby high power cables) does not create voltage or current in intrinsically safe circuits. This can be achieved by shielding or removing intrinsically safe circuits from the source of electromagnetic interference.

When laying in a common bundle or channel, cables with intrinsically safe and intrinsically safe circuits must be separated by an intermediate layer of insulating material or grounded metal. No separation is required if metal-sheathed or shielded cables are used. grounded metal constructions should not have breaks and bad contacts between themselves, which can spark during a thunderstorm or when switching powerful equipment. In explosive industrial facilities, electrical distribution networks with an isolated neutral are predominantly used to eliminate the possibility of a spark during a phase-to-earth short circuit and the tripping of protection fuses if the insulation is damaged. For protection against static electricity use the grounding described in the appropriate section. Static electricity can ignite an explosive mixture.

10.17. The input from the ground electrode system to the service building can be carried out with a steel conductor with a diameter of at least 6 mm, a bundle of three galvanized steel wires with a diameter of at least 5 mm each, a power or control cable with aluminum conductors with a cross section of at least 25 mm. Steel conductors are welded directly to the ground electrode. Aluminum cores of power or control cables are connected to a steel bus using a steel-aluminum transition insert, one end of which is pre-aluminized (coated with a layer of aluminum). The adapter insert in place of the grounding device is welded with a non-alloyed part to the connecting bus of the circuit, and with an aluminized part - to the aluminum conductors of the cable. The junction of the cable cores with the transition insert is coated twice with glyptal enamel and enclosed in a cast-iron sleeve filled with bituminous mass.

The following connection technology is used. One end of the steel strip is tinned at a distance of 90 mm, then an elongated aluminum lug is made for the cable of the required section. The tinned strips and the tip are tightened with three bolts and the joint is soldered. The steel strip is welded to the connecting strip of the circuit, and the cable cores are inserted into the tip and pressed with press tongs in 5-6 places. At the end of the docking, the junction of the steel strip and the tip is placed in the MCH-70 cast-iron coupling and poured with bituminous mass.

10.18. In the event that the project does not provide for the laying of grounding buses in buildings, grounding of equipment must be carried out as follows. One continuous conductor from the bundle of grounding conductors coming from the grounding conductor or from the shield of three earths is connected to the grounding bolts of all outer cabinets, forming a ring that closes in front of the connection point of the conductor to the first cabinet; other continuous conductors are connected to the grounding bolts of the power panels, sections of the control panel and remote display.

Grounding of cabinets of one row is carried out in accordance with clause 10.16. Connection of conductors grounding cabinets of one row, as well as conductors coming from transformers of the TS, cable cabinets and other equipment to grounding conductors coming from grounding conductors, is made using bolted die clamps.

10.19. Series connection to the grounding conductor of several grounding cabinets, power panels, console sections and other equipment is prohibited.

10.20. It is forbidden to use heating pipes, rails, sheaths and cable armor for grounding signaling control devices.

Grounding conductors of protective grounding when laying in a building must be isolated from other grounding conductors, cables and metal structures.

Grounding of traffic light bridges, consoles, traffic lights, relay cabinets in areas railways with electric traction and autonomous traction

On sections of railways with electric traction of constant and alternating current

10.21. Grounding of the metal parts of traffic light bridges and consoles, traffic lights and relay cabinets is carried out by connecting them to the middle terminals of the travel choke-transformers.

In cases where there are no choke-transformers nearby, the grounding conductor is connected to the traction rail using a special clip-clip.

The metal equipment of traffic lights on reinforced concrete masts must be interconnected by grounding conductors (Fig. 53 and 54).

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Fig.54. Grounding of equipment of traffic lights on a reinforced concrete centrifuged mast 10 m long

The crossbar of the traffic light bridge or the crossbar of the console is connected to the stairs with a grounding conductor.

The grounding conductor going from the middle output of the travel choke-transformer to a traffic light with a metal mast or a relay cabinet is connected under the nut of one of the bolts for attaching the traffic light to the foundation or under the head of the bolt for attaching the relay cabinet to the base. The grounding conductor going from the middle output of the travel choke-transformer to a traffic light with a reinforced concrete mast, a traffic light bridge or a console, is connected under the nut of the bolt welded to the bottom of the ladder.

When grounding the adjacent relay cabinet and traffic light, the grounding conductor from the middle terminal of the travel choke-transformer is connected under the bolt head of the relay cabinet; The grounding of the traffic light is carried out by a grounding conductor, openly laid between the traffic light and the relay cabinet.

To increase the reliability of grounding of metal structures of traffic light bridges, a second grounding conductor is laid along the rack. One end of this conductor is fixed with a bolt welded to the bridge cross member, and the other goes to the middle terminal of the inductor-transformer. The outlet of the head is welded to the ground conductor. In the presence of two heads, i.e., with twin bridge posts, the outlets of both heads are welded.

The duplication of the grounding of the console is carried out similarly to the duplication of the grounding of the traffic light bridge. In this case, the ground conductor is connected to a bolt welded to the bottom of the console post.

10.22. As a grounding conductor, round steel with a diameter of at least 12 mm should be used in areas with DC electric traction and at least 10 mm in areas with AC electric traction. The ends of the grounding conductor for connection under the bolt must have a ferrule of flat iron or a ring (Fig. 55).

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10.26. In the relay cabinet, the clamps for grounding the arresters must be connected in the shortest possible way to the metal case of the relay cabinet with a copper conductor with a cross section of at least 20 mm.

On sections of railways with autonomous traction

10.27. Relay cabinets are grounded by connecting the metal case of the cabinet to the grounding device of the cable box.

As a connecting wire, the metal sheath and armor of the cable laid between the relay cabinet and the cable box should be soldered together.

A copper grounding wire with a diameter of at least 20 mm is soldered to the junction of the armor and cable sheath and connected to the metal case of the relay cabinet and cable box.

For cables without a metal sheath, this connection can be made with a bundle of three galvanized steel wires with a diameter of 5 mm. The wiring harness is laid in the ground at a depth of at least 30-40 cm and connected to the grounding conductors of the cable box low-voltage earthing switch at a distance of at least 0.4 m above the ground.

Connection should be made by electric or thermal welding or by means of metal clips.

10.28. To equalize and reduce the potentials arising on the current-carrying parts of signaling and track devices of automatic blocking, automatic locomotive and crossing signaling, it is necessary to connect the metal cases of relay cabinets with metal parts of traffic lights or traffic light bridges and consoles with grounding jumpers.

Grounding cable boxes

10.29. For grounding cable boxes, standard grounding devices are used, consisting of one steel rod with a diameter of at least 20 mm, 2.5 m long - a grounding conductor and a grounding conductor welded to it from two galvanized steel wires twisted together with a diameter of 5 mm. To install the ground electrode and lay the ground conductor, a trench must be dug with a depth of at least 0.6 m.

10.30. It is allowed to install a common grounding conductor for grounding low-voltage and high-voltage equipment of power towers of high-voltage signal lines of automatic blocking, equipped with protection that acts to shutdown in case of single-phase ground faults.

With a common grounding conductor, the descents to it from high-voltage (voltage above 1 kV) and low-voltage (up to 1 kV) equipment must be separate and welded to different grounding rods or (in the case of a deep grounding electrode) to one rod, but in different places.

10.31. The grounding conductor is brought to the support along the bottom of the trench, laid along the support and connected to the grounding bolt of the cable box. TO wooden support the grounding conductor is attached with brackets, and to the reinforced concrete conductor - with wire clamps with a diameter of 2.5-4 mm, installed at a distance of 0.5-0.6 m from each other.

10.32. The resistance of grounding devices should not exceed the values ​​given in Table 39.

The existing grounding circuits of computer equipment and automation are usually divided into:

1. Protective earth circuits (PE).
2. Circuits of working grounding (РЗ).

1. Protective earth

The specified type of grounding protects a person from possible damage in case of damage to the insulation of an electrical installation in operation. In existing electrical installations of objects related to automated process control systems, grounding (zeroing) is required to be performed at:

• housings made of metal for the following devices: instrumentation, AC (control devices), switchgear (control devices), lighting devices, signaling devices and protection elements, electric drives for gate valves, etc., electric motors MU (control mechanisms);
• consoles made of metal, as well as boards for any purpose, if electrical devices, devices, other means related to elements of computer technology and automation are mounted on them. At the same time, this requirement applies to opening and / or removable parts of these consoles and boards in cases where they contain any equipment with voltages over 42V for (~) or 110V for const current, as well as auxiliary structures made of metal, the purpose of which is the installation of AC and electrical receivers on them;
• couplings and armor of cables, both power and control, their sheaths made of metal; similar sheaths and metal hoses of conductors (wires and/or cables); pipes for electrical wiring made of steel and other electrical wiring elements made of metal;
• sheaths of conductors made of metal, as well as armor of cables that make up circuits, "U" in which does not exceed 42V for (~) or 110V for const current, which are located on single structures made of metal, together with conductors, elements whose structures, made of metal, need to be grounded or neutralized.

Some grounding conductors are not required for the following network elements:

• means and devices used for automation, which are mounted on already grounded metal structures, if there is a stable electrical contact between their cases and these structures;
• removable and opening parts of fences, consoles, etc. in those cases when equipment with a voltage of not more than 42V for (~) or 110V for const current is mounted on them; · housings of electrical receivers that are connected to the network through special separating pipes, or have double insulation. Such receivers must not be connected to the earthing system. According to the requirements of the PUE (clause 1.7.70), neutral conductors in the considered electrical installations (grounding) can be:
• trays made of metal, as well as metal boxes;
• cable sheaths made of Al;
• pipes protecting electrical wiring made of metal;
• conductors used for similar purposes such as copper or steel strips, etc.;
• for TN systems, “0” working conductors are used for these purposes, except when it comes to branches going to single-phase power consumers. Zeroing of the latter is carried out along the zero (3rd) protective conductor.

Grounding elements

All connections of grounding conductors are allowed to be made only by welding, soldering, bolting, using special flags and clamps.
In cases where protective conductors made of non-ferrous metals are connected to grounding nodes, they must be terminated with special lugs, and flexible copper jumpers must be double-sided.
When using connections with bolts, it is mandatory to use spring washers (lock washers as an option).

Types of protective grounding of process control systems

Products such as electrical receivers, consoles and shields are equipped with grounding nodes, to which protective conductor is connected directly, and the support frames, which have multi-section shields, are connected with strip steel passing through the grounding nodes of all frames. When it comes to grounding electrical receivers subject to vibrations, a flexible copper jumper is used.

Grounding of technical means

Protective grounding of automated process control systems is usually started from the main, which is connected to the existing grounding conductor available in the facility's power supply system. Protective grounding mains (both SVT and SA) are connected to protective grounding at a single point, which should be located as close as possible to the ground electrode itself. In a single grounding node with a neutral wire TN-C (TN-C-S, TN-S), the protective grounding line of the process control system is connected. The specified node is located on the power boards SVT or SA.
If this switchboard (RS) is far enough away from the transformer substation with a dead-earthed neutral, then a 4-wire circuit is used in the indicated area (three phases and one working "0" conductor, TN-C). Starting from the distribution board, already 5-wire (three phase, TN-c and zero protective, TN-S).
The shield itself must be equipped with re-grounding. This requirement stems from the need to reduce fluctuations in the potential of the shield itself relative to the ground, which are due to changes in the current flowing through TN-C between the transformer substation and the switchboard.

Grounding for ICU

In any technical means of automated process control systems, it is mandatory to have ICU equipment ( information technologies). This includes:

• equipment that performs a basic function (input, search, display, storage, etc.), or management of messages and data;
• equipment, the supply voltage of which does not exceed 600 V.

In general, ICUs include the following types(types) of equipment that, to a greater or lesser extent, are used for the operation of the entire process control system:

• computing devices used as part of a PC or in conjunction with them (both in separate cases and without them);
• terminal equipment;
• terminals;
• PC, etc.

2. Working ground

Other name specified system"zero system" of technical means used in automated process control systems. In addition, in a number of information sources, working grounding is also referred to as functional, physical, logical, informational, circuit, etc.

The null system includes only two elements: grounding conductors and the ground electrode itself. The presence of a personal grounding conductor for this system is necessary due to the occurrence of large spreading currents. The latter can occur during short circuit, during electric welding, etc. This creates significant potential differences between individual points of the grounding device, as well as significant fluctuations in the potentials of certain points of natural and / or artificial grounding conductors in relation to the earth.

The operation of any electrical equipment leads to the emergence of high-power magnetic fields, which are sources of interference in the lines intended for the transmission of information that connect the SVT with electric drives, technological units, local control systems, etc. The power of the signals mentioned above is only a fraction of a watt, and the voltage value is from several V to several tens of mV and even less. This explains the fact that the generated interference is comparable in terms of its performance with useful signals, which can lead to serious distortions of the latter. Therefore, protection from these interferences is essential. And the qualitative solution of grounding issues is one of the most important methods for protecting process control systems and communication lines.

see also.