Cable Management for Power, Control, PLC Power, and I/O Lines
- Power lines, control lines, PLC power lines, and I/O lines should be routed separately. Isolation transformers should connect to the PLC and I/O via dual-insulated cables. Separate the routing of PLC I/O lines from high-power lines. If they must share the same cable tray, bundle AC and DC lines separately. Ideally, route them in separate trays to maximize spatial separation and minimize interference.
Placement of PLC in Relation to Noise Sources
- PLC should be kept far from strong interference sources like welders, high-power silicon rectifiers, and large machinery, not installed in the same cabinet as high-voltage devices. Inside the cabinet, the PLC should be at least 200mm away from power lines. Inductive loads like high-power relays or contactor coils within the same cabinet should have RC snubber circuits in parallel.
Separation of Input and Output Lines
- Separate routing for PLC inputs and outputs is recommended. Discrete and analog signals should also be laid out separately. For analog signal transmission, use shielded cables with one or both ends grounded, ensuring the grounding resistance is less than 1/10 of the shield resistance.
- Do not use the same cable for AC and DC output lines. Keep output lines as far as possible from high-voltage and power lines, avoiding parallel runs.
I/O Wiring
Input Connections
- Inputs should not be too long, but if the environment has low interference and voltage drop is minimal, they can be somewhat longer.
- Do not use the same cable for input and output lines; they should be separated.
- Use normally open contacts for connections to inputs to make ladder diagrams consistent with relay logic, aiding readability.Output Connections
- Output wiring can be either independent or common. Different groups can use different types and voltage levels, but within the same group, only one type and voltage level should be used.
- Short-circuiting loads connected to PLC outputs can damage the printed circuit board, as output components are encapsulated on it and connected to terminal blocks.
- When using relay outputs, the size of inductive loads impacts relay lifespan. Therefore, choose loads wisely or use isolation relays.
- PLC output loads can generate interference, so protective measures like freewheeling diodes for DC outputs, RC snubbers for AC outputs, and bypass resistors for transistor or triac outputs should be implemented.
Correct Grounding and Ground System Improvement
- Good grounding is essential for reliable PLC operation, preventing accidental voltage surges. Grounding serves two purposes: safety and interference suppression. An effective grounding system is vital for countering electromagnetic interference in PLC control systems.
- Ground lines in PLC systems include system ground, shield ground, AC ground, and protective ground. A chaotic grounding system causes uneven potential distribution at grounding points, leading to ground potential differences and ground loop currents, which disrupt system operation. For instance, cable shields should be grounded at one point; if grounded at both ends (A, B), ground potential differences can lead to current through the shield, significantly increasing during anomalies like lightning. Additionally, the shield, ground wire, and earth can form a closed loop, where under changing magnetic fields, induced currents can interfere with the signal through coupling between the shield and the core. Incorrect handling of system and other grounds can result in ground loop currents, unevenly distributing potential across ground lines, affecting the logic and analog circuits within the PLC. PLCs have low tolerance for logic voltage interference; uneven ground potential distribution can disrupt logic operations, data storage, causing data corruption, program crashes, or system hangs. Uneven analog ground potential can reduce measurement accuracy, causing severe signal distortion and misoperations.
- Safety Ground or Power Ground
- Connecting the power line's ground end and cabinet to ground serves as safety grounding. If there's a power leak or the cabinet becomes live, the current safely dissipates into the earth, protecting personnel.
- System Ground
- The PLC controller is grounded to share potential with controlled devices, known as system grounding. The grounding resistance should not exceed 4Ω, typically connecting the PLC system ground with the negative of the switch-mode power supply in the control cabinet as the control system ground.
- Signal and Shield Ground
- Signal lines generally need a single reference ground. Shielded cables in environments prone to conducted interference should be grounded at one point, either at the site or in the control room, to prevent ground loops. When the signal source is grounded, the shield should be grounded at the signal side; if not, at the PLC side. For signal lines with joints, connect and insulate the shield securely, avoiding multipoint grounding. When connecting multiple measurement points' shielded twisted pairs to a multi-core twisted pair cable with overall shielding, ensure all shields are connected and insulated, selecting a single grounding point.
Mitigating Interference from Variable Frequency Drives (VFDs)
- VFD interference mitigation typically includes:
- Using isolation transformers to block most conducted interference from the power supply.
- Using filters which have strong anti-interference capabilities and can prevent the device's own interference from being conducted back to the power supply. Some filters also absorb voltage spikes.
- Implementing output reactors between the VFD and motor to reduce electromagnetic radiation from the line during energy transfer, thus preventing interference with other equipment's operation.