Cabling, Grounding and Noise
Cable types and connectors, shielding and grounding, routing, fiber, and an installation checklist.
On this page
- Cable types by network
- RS-485 and fieldbus cable in detail
- Ethernet cable: categories, construction and channel length
- Connectors
- Routing and separation
- Shielding and grounding
- Variable frequency drives: the biggest noise source
- Fiber optic cable
- Environment: flexing, oil, heat and ingress
- Testing and certifying cable
- Installation checklist
- Common mistakes
- FAQ
Cable types by network
Every network standard names its cable, and for good reason: the impedance, capacitance, shielding and twist are part of the design. General-purpose wire is the most common cause of a network that “should” work but does not.
| Network | Cable | Impedance | Connector | Notes |
|---|---|---|---|---|
| RS-232 | Shielded multicore, low capacitance | n/a | DB9, DB25, terminals | Under 15 m at 9 600 baud |
| RS-485 / Modbus RTU | Shielded twisted pair, 24 AWG, low capacitance (Belden 9841 or equivalent) | 120 Ω | Terminals, DB9, M12 | Daisy chain; third conductor for the reference |
| PROFIBUS DP | Violet cable, one shielded twisted pair (type A) | 150 Ω | D-sub 9 with terminator switch, M12 B-coded | Strip length and shield contact matter |
| DeviceNet | Thick (round, 15 AWG power pair) or thin (18 AWG) cable with signal and power pairs | 120 Ω | Open style, M12 or mini | Trunk and drop lengths limited by speed |
| CANopen | Shielded twisted pair, 120 Ω | 120 Ω | D-sub 9, M12 A-coded | Terminate at both ends |
| AS-Interface | Yellow profiled flat cable (data and power), black for auxiliary power | n/a | Piercing connectors, M12 | Polarity is fixed by the profile |
| IO-Link | Standard unshielded 3-wire M12 sensor cable | n/a | M12 A-coded | 20 m maximum |
| Ethernet 100BASE-TX | Shielded twisted pair, 4-pair (Cat5e) or 2-pair (industrial Cat5) | 100 Ω | RJ45, M12 D-coded | Solid for fixed, stranded for flexing |
| Gigabit Ethernet | Cat5e or Cat6, 4 pairs, shielded in industry | 100 Ω | RJ45, M12 X-coded | All four pairs used |
| PROFINET | Type A (fixed), B (flexible), C (continuous flexing) cable in green sheath | 100 Ω | RJ45 IP20, IP67 push-pull, M12 | Four-core for 100 Mbit/s |
| EtherNet/IP | As industrial Ethernet, often in a yellow or blue sheath | 100 Ω | RJ45, M12 | Follow ODVA cable guidelines |
| EtherCAT | Standard Cat5e or better | 100 Ω | RJ45, M8, M12 | 100 m between nodes |
| 4–20 mA, 0–10 V analog | Shielded twisted pair, instrument cable | n/a | Terminals | Shield at one end (the receiving end); twist reduces pick-up |
RS-485 and fieldbus cable in detail
| Property | Why it matters | Typical for RS-485 |
|---|---|---|
| Characteristic impedance | Must match the terminators; a mismatch reflects signals | 120 Ω |
| Capacitance | Rounds the edges; limits speed and length | About 40 pF per meter or lower for the pair |
| Twist | Makes noise cancel in the pair | Tight, uniform twist, about 1 turn per 2 cm |
| Conductor size | Resistance over long runs; strength | 24 AWG (0.25 mm²) common; 22 AWG for long runs |
| Shield | Blocks electric-field noise | Foil plus drain wire, or braid |
| Third conductor | The common reference | In a three-conductor cable, or a second pair |
| Temperature and flex rating | Cable stays intact in service | PVC for fixed, PUR for flex and oil |
A Cat5e Ethernet cable is often used for short RS-485 runs: one twisted pair for A and B (with the standard 100 Ω impedance, acceptable at moderate speeds) and another for the reference. It works at 9 600 baud over modest distances, but a proper 120 Ω RS-485 cable is better for long or fast runs. Never split a twisted pair across two different pairs: the noise rejection is lost.
Ethernet cable: categories, construction and channel length
| Category | Bandwidth | Used for | Notes |
|---|---|---|---|
| Cat5e | 100 MHz | 100 Mbit/s and 1 Gbit/s | The industrial minimum |
| Cat6 | 250 MHz | 1 Gbit/s, 10 Gbit/s to 55 m | Larger diameter; harder to terminate |
| Cat6A | 500 MHz | 10 Gbit/s to 100 m | Backbones and vision systems |
| Cat7 and higher | 600 MHz and up | Special use | Needs non-RJ45 connectors for full performance |
| Construction | Meaning | When to use |
|---|---|---|
| UTP | Unshielded twisted pair | Office; clean control rooms only |
| F/UTP, FTP | Overall foil shield | Industrial default for panels |
| S/FTP, SF/UTP | Braid and foil shields | Heavy noise; near drives |
| Solid conductors | One copper strand per conductor | Fixed installation: lower loss, but breaks if flexed |
| Stranded conductors | Several strands | Patch cords and flexible runs; about 20 percent higher attenuation |
| Drag-chain / continuous-flex (PROFINET type C) | Special stranding and PUR jacket | Cable carriers, robots |
Worked example: the 100 m channel
- The standard 100 m limit is for a channel with up to 90 m of fixed (solid) cable and up to 10 m of patch cords. Stranded patch cords have about 20 percent more attenuation, so each meter of patch cord counts as 1.2 m of solid cable.
- With 10 m of patch cords the fixed cable may be no longer than 100 − 1.2 × 10 = 88 m.
- With 20 m of patch cords (flexible runs at both ends) the limit falls to 100 − 1.2 × 20 = 76 m.
- A flexible industrial cable has higher attenuation again: check the manufacturer’s maximum length (often 50 to 80 m for a continuous-flex cable).
- Beyond this, add a switch or a media converter at the limit (a switch is a repeater that restarts the 100 m), or use fiber.
Connectors
| Connector | Used for | Rating | Notes |
|---|---|---|---|
| RJ45, IP20 | Panel Ethernet, inside the cabinet | IP20 | Use latching boots; do not use office plugs where vibration is present |
| RJ45 push-pull (IP67) | Ethernet on the machine, outside the cabinet | IP65/67 | Common on PROFINET field devices |
| M12 D-coded, 4 pin | 100 Mbit/s Ethernet (PROFINET, EtherNet/IP, EtherCAT) | IP67 | Screw thread; keyed so it cannot be mixed up with A-coded |
| M12 X-coded, 8 pin | Gigabit Ethernet | IP67 | Shielded; keyed |
| M12 A-coded, 4 or 5 pin | Sensors, IO-Link, DeviceNet, CANopen | IP67 | Power and signal |
| M12 B-coded, 5 pin | PROFIBUS | IP67 | Marked violet |
| M8 | Small sensors, EtherCAT in tight spaces | IP67 | Fine thread |
| D-sub 9 (DB9) | RS-232, PROFIBUS, CAN | IP20 | Screw-locked; the shield is on the shell |
| Screw or spring terminals | RS-485, analog, power | IP20 | Ferrules on stranded wire; check the strip length |
- Crimping and termination quality is a leading cause of intermittent faults: use the right tool and the connector made for that cable; follow the manufacturer’s stripping dimensions.
- Keep the shield intact through the connector with a 360-degree contact.
- Mark the ports and use connector colors to keep networks from being mixed up (for example green for PROFINET, yellow for EtherNet/IP).
- Strain relief and bend radius. A sharp bend or a pulled connector damages the pairs; use at least 8 times the cable diameter for fixed cable, more for flexing.
- Cap or plug unused ports, especially M12, to keep out dust and water.
Routing and separation
| Cable group | Examples | Treat as |
|---|---|---|
| Power | Mains, motor and VFD output cables, contactors, heaters, welders | Noise sources |
| Control | 24 V DC digital I/O, relay and solenoid wiring | Mild noise sources |
| Signal and communication | Ethernet, fieldbus, RS-485, analog signals, encoders | Sensitive |
- Separate the groups with metal dividers, or keep at least 200 mm between power and communication cable when they run parallel for a long distance; more near a drive output.
- Cross at 90 degrees when different groups must cross.
- Use metal trays and conduits, bonded to the panel at both ends; a closed steel conduit is an excellent screen.
- Do not run communication cable in the same bundle as the motor cable. Do not coil excess length next to power wiring.
- In the cabinet, put the drive and its filter at one side, the PLC and communication modules at the other, and keep the wiring from touching.
- Avoid sources: fluorescent light ballasts, welders, large contactors, transformers and radio equipment.
Shielding and grounding
A shield blocks electric-field noise by carrying it to ground before it reaches the signal wires. It works only if it is connected properly. The rules differ with frequency: at low frequencies (50/60 Hz hum) grounding at one end avoids a ground loop; at high frequencies (drives, radio, fast digital edges) the shield must be grounded at both ends, with a 360-degree clamp rather than a “pigtail”, and the two grounds must be bonded together so the shield does not carry the equalizing current.
| Cable | Shield practice | Why |
|---|---|---|
| Analog signals (4–20 mA, 0–10 V) | Ground at one end, normally the receiving (PLC) end | Avoids hum from a ground loop at low frequency |
| RS-485 and fieldbus | Ground at each node through a short connection (or one end plus capacitors), with the panels bonded | Follow the network’s rules; PROFIBUS grounds the shield at every connector |
| Ethernet, industrial | Shield connected at both ends through the RJ45 or M12 housing | High-frequency noise; equipotential bonding between panels |
| VFD motor cable | Shield grounded at both ends with 360-degree clamps (drive and motor) | Carries the common-mode current back to the drive |
Ground loops and equipotential bonding
Two panels with a few volts between their grounds, joined by a signal cable with its shield (or a common conductor) connected at both ends, let current flow through the cable and cause hum and bit errors. Remedies: run a heavy equipotential bonding conductor beside the cable, so that most of the current takes it (often a 16 mm² or larger copper strap); use isolated ports; or break the connection with fiber. Bonding the panels is the right fix for noise; isolation is the right fix for safety and for unbonded systems.
Variable frequency drives: the biggest noise source
A VFD switches its output between two voltage levels thousands of times a second, with very fast edges. The long motor cable acts as an antenna, and the cable capacitance gives a high-frequency common-mode current that returns through ground, the shield and any other nearby conductor, including communication cables.
Worked example: the capacitance of a motor cable
- A shielded motor cable has a capacitance of about 200 pF per meter between each conductor and the shield. A 50 m run has about 200 × 50 = 10 000 pF = 10 nF to ground on each conductor.
- Each switching edge charges and discharges this capacitance, so the drive’s output carries a spike of current at every transition (a few amperes for a fast edge on a high bus voltage). That is the noise the shield and the bonding must carry.
- Doubling the cable length doubles the capacitance and the current: keep the motor cable as short as the installation allows, and fit output filters (dV/dt or sine filters) on long runs.
| Measure | What it does |
|---|---|
| Shielded motor cable, 360° bonded at both ends | Contains the noise and returns it to the drive |
| Drive and filter on a bare metal panel | A good high-frequency ground plane |
| Line reactors, EMC (RFI) filters | Reduce noise on the supply side |
| Output (dV/dt or sine) filters | Slow the edges on long motor cables |
| Ferrites | Reduce common-mode current on a cable (a help, not a cure) |
| Separate routing | Motor cable apart from communication cable |
| Lower carrier frequency | Fewer, less intense switching events, at the cost of motor noise |
Fiber optic cable
Optical fiber carries light, not current, so it is immune to electrical noise, lightning and ground differences, and is the right choice between buildings, across drives and for long runs. Its limits are cost, careful termination and bend radius.
| Type | Core / wavelength | Typical reach at 100 Mbit/s to 1 Gbit/s | Use |
|---|---|---|---|
| Multimode, OM1/OM2 | 62.5 or 50 µm, 850 nm and 1300 nm | 2 km at 100 Mbit/s, about 275 to 550 m at 1 Gbit/s | Within a plant or building |
| Multimode, OM3/OM4 | 50 µm, 850 nm | About 300 to 550 m at 1 Gbit/s; far more at 10 Gbit/s | Backbones |
| Single-mode | 9 µm, 1310 nm and 1550 nm | 10 km and beyond | Between buildings and sites |
| Plastic optical fiber (POF) | 980 µm core, 650 nm | Tens of meters | Sensors and short links in noisy places |
Worked example: a link budget
- A single-mode transmitter sends -15.0 dBm; the receiver needs at least -31.0 dBm. The budget is 16.0 dB.
- The losses on a 10 km link: fiber 10 × 0.35 = 3.5 dB, four connectors at 0.5 dB = 2.0 dB, four splices at 0.1 dB = 0.4 dB. Total 5.9 dB.
- Received power -15.0 − 5.9 = -20.9 dBm; margin -20.9 − (-31.0) = 10.1 dB. Keep at least 3 dB for aging, dirt and later repairs: this link has plenty.
- For 500 m of multimode at 850 nm (about 3 dB per km, two connectors): 1.5 + 1.0 = 2.5 dB, well within a typical 8 to 10 dB budget.
- The commonest fiber fault is a dirty connector: clean with a lint-free wipe or a click-cleaner before every mating, and never look into a fiber or a transmitter (invisible infrared).
- Media converters and SFP modules join copper and fiber; match the wavelength and mode on both ends and use the right pair of fibers (transmit to receive).
- Bend radius is about 10 times the cable diameter (or 30 mm for bare fiber) when installed.
- Fiber is not unbreakable: protect it from crushing and from being pulled by its connectors.
Environment: flexing, oil, heat and ingress
| Condition | Choose | Avoid |
|---|---|---|
| Continuous flexing (drag chain, robot) | Cable rated for continuous flex (PROFINET type C, PUR jacket, fine stranding) | Solid-core or office patch cords |
| Oil, coolant, chemicals | PUR jacket, oil-resistant (check the specific fluid) | PVC |
| Heat | Rated cable (for example 90 °C or higher; PTFE for very hot places) | Standard PVC near hot surfaces |
| Cold | Low-temperature rated PUR | Standard PVC (stiff, cracks) |
| Outdoor and UV | UV-resistant jacket, gel-filled or armored for burial | Indoor cable |
| Washdown, food | IP67/IP69K connectors and cables, stainless housings | Open terminals |
| Hazardous areas | Approved cable and glands for the zone; intrinsically safe segments | Anything not certified |
| Rodents, crushing | Armored cable or conduit | Bare cable on the floor |
Ingress ratings: IP20 is finger-safe in a cabinet; IP65 is dust-tight and protected against water jets; IP67 survives temporary immersion; IP69K survives high-pressure steam cleaning. The connector, the cable gland and the device must all meet the rating.
Worked example: voltage drop on a DeviceNet-style trunk
- A thick-cable trunk of 100 m, with a conductor resistance of about 5.4 mΩ per meter, carries 3.6 A to the devices at the far end.
- The 24 V loop goes down one conductor and back on the other: drop = 3.6 A × 2 × 0.0054 Ω/m × 100 m = 3.89 V (about 16.2 percent of 24 V).
- DeviceNet specifies a maximum drop of 5 V per conductor and the devices need at least 11 V; this trunk is fine. Where many outputs switch at once, measure the voltage at the last device under full load.
Testing and certifying cable
| Test | Tool | What it finds |
|---|---|---|
| Continuity and wire map | Simple cable tester | Opens, shorts, crossed or split pairs |
| Length and TDR | Cable tester with time-domain reflectometer | Where a break is, and the length |
| Certification | Certifier (Fluke DSX and similar) | Attenuation, crosstalk, return loss to the category; a printed report for the customer |
| Resistance across the pair (RS-485) | Multimeter, power off | About 60 Ω with two terminators |
| Insulation resistance | Megohm meter | Damaged cable, water ingress (not on live networks) |
| Fiber power and loss | Optical power meter and light source, or OTDR | Link loss, where the loss occurs |
| Bus test (PROFIBUS) | PROFIBUS tester | Signal level, reflections, terminator, address list |
| Live signal | Oscilloscope, protocol analyzer | Noise, reflections, errors: see Sniffers, Analyzers and Serial Tools |
Installation checklist
- Drawings and labels: every cable has a number at both ends and appears on the drawings.
- Right cable for each network, in the right jacket for the environment.
- Routes checked against the separation rules; no communication cables beside motor leads.
- Connectors terminated with the right tool; shield contact complete; strain relief in place.
- Terminators fitted and counted (RS-485, PROFIBUS, CAN: two per segment).
- Shield and bonding done as the network requires; the equipotential bond fitted.
- Length and node limits within the specification, with margin.
- Test results recorded for every cable (continuity at least, certification where required).
- Spare capacity: spare conductors and conduit space.
- Unused ports capped.
- As-built documents updated and a copy left in the panel.
Common mistakes
| Mistake | What you see | Fix |
|---|---|---|
| General-purpose wire for RS-485 or PROFIBUS | Errors at higher baud rates or long runs | Use the specified cable |
| Communication cable bundled with motor cable | Random errors when the drive runs | Separate by 200 mm or more; use conduit |
| Shield connected at one end only on a noisy site | Radio-frequency noise gets in | Bond both ends with 360° clamps, add equipotential bonding |
| Shield grounded at both ends with no bonding | Hum, ground-loop current | Add a bonding conductor, or isolate |
| Solid-core cable in a moving machine | Intermittent faults, then a break | Use continuous-flex cable |
| RJ45 plug crimped on stranded cable meant for solid | Intermittent connection | Match plug and cable; test |
| Pairs split in an RJ45 (wired 1-2, 4-5 with the pairs wrong) | Works at 10 Mbit/s but not 100 or 1000 | Wire to T568A or B; certify |
| Bend radius ignored on fiber | High loss, dead link | Re-route; use the bend-insensitive type |
| Dirty fiber connector | High loss, intermittent link | Clean, inspect |
| No labels | Hours lost tracing | Label both ends |
Related
- Industrial Communication: the overview of this guide.
- Serial Communication: RS-232, RS-422 and RS-485: the electrical layer, terminators and bias.
- Industrial Ethernet: EtherNet/IP, PROFINET and EtherCAT: the protocols that run on this cable.
- Troubleshooting Industrial Networks: when a cable test is the next step.
- I/O Wiring and Signals: analog and discrete wiring in the PLC guide.
- Electronics: shielding and grounding basics.
FAQ
Can I use office Ethernet cable on a machine?
For fixed runs inside a clean cabinet, yes (a shielded one is better). For flexing, oil, vibration or noise, use industrial cable and connectors.
Should the shield be grounded at one end or both?
For low-frequency hum on analog signals, one end. For drives and high-speed networks, both ends with 360-degree clamps and an equipotential bond. Follow the network standard.
How far from power cable should communication cable be?
At least 200 mm when running side by side for a long distance, crossing at 90 degrees where they meet, and in separate metal compartments if you can. Near a VFD output, go further.
Why does an Ethernet link work at 10 Mbit/s but not at 100?
A damaged cable pair or a split pair: the lower speed uses lower frequencies and tolerates faults that 100 Mbit/s does not. Replace or certify the cable.
When do I choose fiber?
Between buildings, across large ground differences, next to heavy electrical equipment, for lengths above 100 m, and wherever lightning is a risk.
References
- ANSI/TIA-568.2-D, Balanced Twisted-Pair Telecommunications Cabling and Components Standards; ISO/IEC 11801, Generic cabling for customer premises.
- PROFIBUS & PROFINET International, PROFINET Cabling and Interconnection Technology and PROFIBUS Installation Guideline.
- ODVA, Media Planning and Installation Manual for EtherNet/IP (PUB00148).
- IEC 61918, Industrial communication networks: Installation of communication networks in industrial premises; IEC 61000-5-2, Earthing and cabling (EMC).
- Ott, H. W., Electromagnetic Compatibility Engineering, Wiley.
- NFPA 79, Electrical Standard for Industrial Machinery; IEC 60204-1.
Last reviewed: October 2026. Speeds, lengths, node counts and menu names are typical: confirm against the specification and the manufacturer’s documentation for the devices you use.