[2025 Control Systems] Can DI and DO signal cables in a SIS system share the same multi-core cable?

II. Conditions permitting the use of shared cables 1. Systems of the same voltage level that are not intrinsically safe and meet SIL1 requirements: When both DI and DO signals are 24VDC DC signals and the systems in question are not intrinsically safe, multi-core cables can be used provided that the SIL1 safety integrity requirements are met. For example, in some simple control loops, there is a lighted button on the control panel; the button represents a DI, the light represents a DO, and both are operated at 24V. At this time, using a single cable can reduce the number of cables required, lower project costs, improve construction efficiency, save space, and facilitate management and maintenance. However, it is necessary to ensure that the mutual interference between signals remains within acceptable levels, and appropriate protective measures should be taken, such as using shielded cables and ensuring that the shielding layer is properly grounded. 2. Requirements for independence and functional isolation: If DI/DO signals are used for SIL1-level functions and share detection elements with the Basic Process Control System (BPCS), isolation through signal distributors or the use of independent sensing points is required to ensure the independence and integrity of the SIS system.

III. Situations where shared cables are prohibited 1. High safety integrity levels (SIL≥2): For measuring instruments and final elements with a safety instrument function of SIL2 or higher, they must be installed separately from the BPCS, and the corresponding signal cables must also be laid independently. This is because systems with high safety integrity levels require extremely high levels of reliability and independence; using shared cables can lead to system failures and safety incidents, failing to meet the strict requirements for system independence and reliability.

2. For AC/DC hybrid or different voltage levels, the AC power cables and signal cables should be laid separately to avoid electromagnetic interference； Signal cables of different voltage levels should also be laid separately to prevent high-voltage signals from interfering with low-voltage signals. If the signal voltage levels of DI and DO are different, for example, DI is 24VDC while DO is 220VAC, then signals with different levels or frequencies need to be isolated. Using common cables can cause the high-voltage signals to interfere with the low-voltage signals, which may even lead to equipment damage or safety hazards.

3. Mixing of intrinsically safe systems with non-intrinsically safe systems: The cables of intrinsically safe circuits must be run in separate conduits or equipped with shielding, and must maintain a sufficient distance from non-intrinsically safe cables to prevent energy interference and ensure compliance with the explosion protection requirements of the intrinsically safe system.

IV. Models, naming, and application areas of shielded cables 1. Models and naming of shielded cables 1.1 RVVP series: Woven shielded flexible wire and cable with copper cores, polyvinyl chloride insulation, and polyvinyl chloride sheathing. “R” denotes a flexible cable, “V” denotes PVC insulation, “P” denotes shielding, and “VP” denotes a shielded flexible cable. 1.2 KVVP series: Woven shielded control cable with copper core, PVC insulation, and PVC sheath. ““K” denotes the control cable, “V” denotes PVC insulation, and “P” denotes shielding. 1.3DJYP2VR: Copper core, polyethylene insulation, PVC sheath; copper-plastic composite tape winding for partial shielding. 1.4 DJYP2VP2: Copper core, polyethylene insulated, PVC sheathed, copper-plastic composite tape wrapped split screen and main screen. 1.5 DJYP2VP2R: Copper core, polyethylene insulated, PVC sheathed, copper-plastic composite tape wrapped screen – main screen and sub-screens. 1.6 JYVP: Control cable for instruments, with copper wire braided shielding and polyethylene insulation as well as flame-retardant polyvinyl chloride sheathing. 1.7. KJYVPR: Control flexible cable for instruments, with copper wire braided shielding and polyethylene insulation as well as flame-retardant polyvinyl chloride sheathing.

2. Application areas of shielded cables 2.1. Industrial environments: such as factories and industrial plants, server rooms, etc., where they are used in automated equipment (such as PLC control systems, servo motors, etc.) to prevent interference generated by high-power electrical devices (such as frequency converters, high-power motors) from affecting signal transmission； It is also used for sensor signal transmission to ensure the accuracy of high-precision measurements. 2.2. Communication systems: Suitable for communication systems with high requirements regarding interference, such as aerospace monitoring centers and military communications, to ensure stable signal transmission. 2.3. Data Center: Horizontal subsystems used in data centers to enhance network speed and efficiency, ensuring the stable operation of large volumes of data traffic and business applications. 2.4. Low-voltage dedicated networks: systems such as surveillance, access control, and fire protection, which enhance the performance and reliability of the systems. 2.5. Transmission of confidential information: In fields such as military, government, and finance, this is to prevent information from being leaked or altered. 2.6. Other complex electromagnetic environments: In situations such as lightning, high voltage, and radio interference, it isolates external electromagnetic fields to maintain the quality of signal transmission.

V. Engineering Practices and Supplementary Requirements 1. Shielding and grounding measures: If it is necessary to share cables for special reasons, shielded cables with separate shielding layers should be used and grounded at one end to avoid ground loop interference； The AC power supply and signal wires in the cable tray should be separated by a metal partition. 2. Safety lifecycle management requires compliance with relevant standard requirements throughout the entire lifecycle of the safety instrumented system, including stages such as design, installation, commissioning, acceptance, and maintenance. During the acceptance testing phase, Factory Acceptance Testing (FAT) and Site Acceptance Testing (SAT) should be conducted to ensure the reliability of the shared cables and the safety of the system ； During the maintenance phase, it is necessary to regularly test the cable’s insulation resistance and shielding integrity. 3. Design redundancy and fault margin: For SIL2 and higher level systems, certain hardware fault margin (HFT) requirements must be met. The design of shared cables should utilize redundancy and other approaches to fulfill these requirements, thereby enhancing the reliability and availability of the system.

VI. Reasons why DI/DO cannot share multi-core cables and reasons why they can share them 1. Reasons why they cannot share them 1.1 Risk of signal interference DI signals are typically used to monitor device status, switch positions, etc., while DO signals are used to drive actuators such as valves and solenoid valves. The DO signal can generate electromagnetic interference when driving loads, especially when it drives inductive loads such as solenoid valves; at the moments of switching on and off, induced currents and electromagnetic interference are produced, and these interferences can affect the accuracy of the DI signal, leading to incorrect operations of the DI signal.

1.2 Safety Integrity Requirements For systems with a high safety integrity level (SIL≥2), extremely high requirements are placed on reliability and independence; the use of shared cables can lead to system failures and safety incidents, failing to meet the strict requirements for system independence and reliability at SIL2 and higher levels.

1.3 Different voltage levels and frequencies: If the voltage levels of the DI and DO signals differ – for example, DI is 24VDC while DO is 220VAC – signals with different levels or frequencies need to be isolated. Using the same cable can cause high-voltage signals to interfere with low-voltage signals, which may even lead to equipment damage or safety hazards.