Installation inspection of PLC design

This article takes Siemens S7-200plc as an example to explain the installation, inspection and maintenance of programmable logic controllers. Of course, the content described in this chapter has certain guiding significance for most PLCs.
Special attention should be paid to the fact that when installing and removing various modules and related equipment of the programmable logic controller, the power supply must be cut off first. Failure to do so could result in equipment damage and personal injury.
1. General guidance for the installation of programmable logic controllers
The following introduces the general method of PLC design and installation:
•When wiring the PLC, ensure that all electrical appliances comply with the national standards and regional electrical standards. Keep in touch with regional authorities in a timely manner to determine which standards meet your particular needs.
• To use the wire correctly. The PLC module uses wires of l.50mm2~0.50mm2.
•Do not overtighten the screws of the connector, the maximum torque should not exceed 0.36Nm (Newton meter).
•Try to use short wires (up to 500 meters of shielded wires, or 300 meters of unshielded wires), and wires should be used in pairs as much as possible. Use a neutral or public wire to pair with a hot wire or signal wire.
• Separate the AC line and the DC line of the high-current fast switch from the signal line of the small current.
• Correctly identify and divide the terminals of the PLC module, and leave a buffer coil at the end of the line.
• For lightning surges, install appropriate surge suppression equipment.
•Do not connect the external power supply in parallel with the DC output point as an output load, which may cause reverse current impact output, unless a diode or other isolation barrier is used during installation.
•Control equipment may fail under unsafe conditions, resulting in misoperation of the controlled equipment. Such misoperation could result in serious personal injury and serious equipment damage. Consider using an emergency shutdown function independent of the PLC, an electromechanical overload protection device, or other redundant protection.
2. Guidelines for grounding and circuit reference points when using isolated circuits
(1) The following points should be followed for grounding and circuit reference points when using isolated circuits:
•Should Choose a reference point (0V) for each installed circuit. These different reference points may be connected together. This connection may cause unexpected currents, which may cause logic errors or damage the circuit. The cause of different reference potentials is often due to ground points being separated too far in physical area. When devices that are far apart are connected by communication cables or sensors, the current generated between the cable and the ground will flow through the entire circuit. even at short distances, the load current of large equipment can also change between it and the ground potential, or directly generate unpredictable current through electromagnetic action. Power supplies with improperly selected reference points may create destructive currents in the circuit between each other, resulting in damage to equipment.
•When several CPUs with different ground potentials are connected to a network, an isolated RS-485 repeater should be used.
•PLC products have installed isolation components at specific points to prevent unwanted current generation during installation. When you plan to install, you should consider where these isolation elements are and where they are not. At the same time, you should also consider the isolation between the related power supplies and the isolation of other equipment, and where the reference points of the related power supplies are.
• It is best to choose a ground reference point, and use isolation components to destroy useless current loops that may produce unpredictable currents. Keep in mind that new circuit reference points may be introduced during temporary connections, such as when a programming device is connected to the CPU.
•When grounding on site, you must always pay attention to the safety of grounding, and operate the isolation protection equipment correctly.
• In most installations, the best noise suppression can be obtained if the sensor’s power supply M terminal is connected to the ground.
The above is an overview of the isolation characteristics of PLC0, but some characteristics may be different for special products. Please refer to each PLC system manual, from which you can find out which isolation components and their isolation levels are included in the circuit of your product. Isolation components with a level less than 1500V AC can only be used as functional isolation, not as a safety isolation layer.
(2) Isolation characteristics of PLC
The following isolation characteristics of PLC can be used as a reference.
•The CPU logic reference point is similar to the M point provided by the DC sensor.
•The CPU logic reference point is similar to the M point provided by the CPU input power supply powered by DC power supply.
•CPU communication port and CPU logic port (except DP port) have the same reference point.
•The analog input and output are not isolated from the CPU logic, and the analog input adopts differential input and provides a low-voltage public mode filter circuit.
•The isolation between logic circuit and ground is 500V AC.
•The isolation between DC digital input and output and CPU logic is 500V AC.
• The isolation between the points of the DC digital I/O group is 500V AC.
• The isolation between relay output, AC output and input and CPU logic is 1500VAC.
• The isolation between the points of the relay output group is 1500V AC.
•AC power line and neutral line and ground, CPThe isolation between U logic and all I/Os is 1500V AC.
3. Power supply installation
(1) AC input PLC installation guidelines
The following items are general guidelines for AC AC wiring installation.
• Use a single-pole switch to isolate the power supply from the CPU, all input circuits and output (load) circuits.
• Use an over-current protection device to protect the CPU’s power supply, output points, and input points. It is also possible to add fuses to each output point for wider protection.
•When you use the Micro PLC 24V DC sensor power supply, you can cancel the external overcurrent protection of the input point, because the sensor power supply has a short circuit protection function.
•Connect all ground wire terminals of PLC with the nearest grounding point to obtain the best anti-interference ability. It is recommended that all be connected to a single conductive point (aka one point ground) using a 1.50mm2 wire.
•The DC sensor power supply of the local unit can be used as the input of the local unit, extended DC input and extended relay coil power supply. This sensor power supply has short-circuit protection function.
• In most installations, the best noise suppression can be obtained if the sensor’s power supply M terminal is connected to the ground.
(2) DC Input PLC Installation Guidelines
The following items are general guidelines for wiring in DC isolated installations.
• Use a single-pole switch to isolate the power supply from the CPU, all input circuits and output (load) circuits.
• Use over-current protection devices to protect CPU power supply, output points, and input points. It is also possible to add a fuse to each output point for overcurrent protection. When using the Micro 24V DC sensor power supply, the external overcurrent protection of the input point can be canceled, because the sensor power supply has a current limiting function inside.
•Ensure that the DC power supply has sufficient impact resistance to ensure that a stable voltage can be maintained when the load changes suddenly. At this time, an external capacitor is required.
• In most applications, the best noise suppression can be obtained by connecting all DC power supplies to the ground. A parallel circuit of resistors and capacitors is connected between the common end of the ungrounded DC power supply and the protection ground. Resistors provide a path for electrostatic discharge and capacitors provide a path for high frequency noise, their typical values ​​are IM Ω and 4700pF.
•Connect all grounding terminals of PLC with the nearest grounding point to obtain the best anti-interference ability. It is recommended that all grounding terminals be connected to an independent conductive point with a 1.5mm2 wire (also known as one-point grounding).
• Between the 24V DC power supply circuit and the equipment, and between the 120/230VAC power supply and the hazardous environment, safety electrical isolation must be provided.
4. Use of Suppression Circuit
(1) General Guidance for Suppression Circuit Use
Suppression circuits should be added to inductive loads to suppress the voltage rise when the power is turned off. The following method can be used to design a specific suppression circuit. The effectiveness of the design depends on the actual application, so the parameters must be adjusted to suit the specific application. All device parameters must be kept in line with the actual application.
(2) Protection of DC transistor output module
PLC DC transistor output internally contains a zener diode that can adapt to various installations. For inductive loads with large inductance or frequent switching, external suppression diodes can also be used to prevent strikes through the internal diode.
You can also use an external Zener diode to form a suppression circuit. If the external DC voltage is 24V, the breakdown voltage of the selected Zener diode should be 8.2V and the power should be 5W.
In this way, when the transistor turns from on to off, since the freewheeling diode provides a current channel for the release of inductance energy, no high voltage will be formed at both ends of the inductance. For the suppressor circuit with Zener diode, due to the voltage characteristic of Zener diode, it can also suppress the generation of high voltage across the inductor. Therefore, the transistor will not be damaged. See Figure F-1-1.

(3) Protection of relay output module
There are two main aspects to the protection of the relay output module. One aspect is the protection of the relay contacts, so that the inductance will not generate high voltage to the contacts of the relay when the power is cut off. Another aspect is the protection of the power supply, so that the power supply that provides voltage for the relay is not affected by high voltage. The main way to suppress high voltage is to connect an RC absorbing circuit in parallel at both ends of the inductive load. In addition to using RC to absorb the AC power supply, a varistor can also be connected in parallel to eliminate the voltage impact.
The reference value of DC load RC suppression circuit is R=12Ω, C=0.5μF/A ~1μF/A.
When the AC voltage of the AC load is 115V/230V, for each static load of 10VA, the reference value of the RC suppression circuit is R=0.5×US Ω, C=0.002μF/A ~0.005μF/A. If the varistor is connected in parallel, the working voltage of the varistor is 20% higher than the normal power supply voltage. See Figure F-1-2.