PLC design installation check - Solutions - Huaqiang Electronic Network

Electronic scale crystal oscillator 3.2*2.5mm 3225 16M (16.000MHZ) 12PF 10PPM 20PPM 30PPM

Crystal oscillator

F380J106MMA original authentic, spot stock brand NICHICON

This article provides a detailed guide on the installation, inspection, and maintenance of programmable controllers, using the Siemens S7-200 PLC as an example. The information is also applicable to most PLC systems. It is crucial to always turn off the power before installing or removing any modules or equipment to avoid damage and potential injury.

When wiring the PLC, ensure that all electrical components meet local and national standards. Always consult with local authorities to determine the appropriate requirements for your specific location. Use wires within the range of 1.50 mm² to 0.50 mm². Avoid over-tightening connector screws—never exceed 0.36 Nm torque. Try to use short wires, up to 500 meters for shielded cables or 300 meters for unshielded ones. Pair signal wires with neutral or ground wires where possible.

Keep AC lines and high-current DC lines separate from low-current signal lines. Correctly identify and label all PLC module terminals, and leave buffer coils in place. Install surge suppression devices to protect against lightning strikes. Avoid connecting external power supplies in parallel with DC output points unless you have a diode or isolation barrier in place to prevent reverse current flow.

If the control device malfunctions, it could lead to serious accidents or equipment damage. Consider implementing an emergency stop function independent of the PLC, or use electromechanical overload protection as a backup.

When using isolated circuits, select a common reference point (0V) for each circuit. If multiple reference points are used, ensure they are properly connected to avoid unexpected currents. This can cause logic errors or damage to the system. Use isolated RS-485 repeaters when connecting CPUs with different ground potentials. PLCs often include isolation components to prevent unwanted currents during installation. Be aware of these components and consider the isolation between power supplies and other equipment.

Choose a stable ground reference point and use isolation components to break unwanted current loops. Temporary connections, such as programming devices, may introduce new reference points. When grounding in the field, always follow safety protocols and operate isolation devices correctly. In most cases, connecting the sensor’s M terminal to ground improves noise suppression.

The isolation characteristics of PLCs vary by model. For example, the CPU logic reference point is similar to the M point of DC sensors. Analog inputs and outputs are typically not isolated from the CPU logic. Isolation levels between different parts of the PLC range from 500V AC to 1500V AC, depending on the component.

For power supply installation, use a single-pole switch to isolate the power supply from the CPU, input, and output circuits. Include overcurrent protection for the CPU and I/O points. If using a 24V DC sensor power supply, it may already have built-in short-circuit protection. Connect all ground terminals to a single point using 1.5 mm² wire for optimal interference immunity.

When working with DC power supplies, ensure they can handle sudden load changes. Adding an external capacitor can help maintain voltage stability. Most DC power supplies should be grounded for better noise rejection. A resistor and capacitor network can be used between the ungrounded DC power supply and the ground to suppress static and high-frequency noise.

To protect inductive loads, use suppression circuits. For DC transistor outputs, Zener diodes or external suppression diodes can be added. For relay outputs, RC snubber circuits or varistors are commonly used to suppress voltage spikes. These components help protect both the relay contacts and the power supply.

Following these guidelines ensures safe and reliable operation of your PLC system, reducing the risk of failure and improving overall performance.